American Journal of Botany 98(12): 2049–2063. 2011.

PHYLOGENETIC AND POPULATION GENETIC ANALYSES OF

DIPLOID LEUCAENA (LEGUMINOSAE; MIMOSOIDEAE) REVEAL
CRYPTIC SPECIES DIVERSITY AND PATTERNS OF DIVERGENT
ALLOPATRIC SPECIATION1

Rajanikanth Govindarajulu2,5, Colin E. Hughes3,4, and C. Donovan Bailey2,4

2Department of Biology, P. O. Box 30001 MSC 3AF, New Mexico State University, Las Cruces, New Mexico 88001 USA;
3Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland; and 4Department of Plant
Sciences, South Parks Road, University of Oxford, Oxford OX13RB UK

• Premise of the study: Leucaena comprises 17 diploid species, five tetraploid species, and a complex series of hybrids whose
evolutionary histories have been influenced by human seed translocation, cultivation, and subsequent spontaneous hybridiza-
tion. Here we investigated patterns of evolutionary divergence among diploid Leucaena through comprehensively sampled
multilocus phylogenetic and population genetic approaches to address species delimitation, interspecific relationships, hybrid-
ization, and the predominant mode of speciation among diploids.
• Methods: Parsimony- and maximum-likelihood-based phylogenetic approaches were applied to 59 accessions sequenced for
six SCAR-based nuclear loci, nrDNA ITS, and four cpDNA regions. Population genetic comparisons included 1215 AFLP loci
representing 42 populations and 424 individuals.
• Results: Phylogenetic results provided a well-resolved hypothesis of divergent species relationships, recovering previously
recognized clades of diploids as well as newly resolved relationships. Phylogenetic and population genetic assessments identi-
fied two cryptic species that are consistent with geography and morphology.
• Conclusions: Findings from this study highlight the importance and utility of multilocus data in the recovery of complex evo-
lutionary histories. The results are consistent with allopatric divergence representing the predominant mode of speciation
among diploid Leucaena. These findings contrast with the potential hybrid origin of several tetraploid species and highlight the
importance of human translocation of seed to the origin of these tetraploids. The recognition of one previously unrecognized
species (L. cruziana) and the elevation of another taxon (L. collinsii subsp. zacapana) to specific status (L. zacapana) is con-
sistent with a growing number of newly diagnosed species from neotropical seasonally dry forests, suggesting these communi-
ties harbor greater species diversity than previously recognized.

Key words: allopatric speciation; cryptic species; Leguminosae; Leucaena; Leucaena cruziana; Leucaena zacapana; phylog-
eny; population genetics.

Patterns of diversification among species can be explained recognized in land plant evolution, in part because it violates
by a wide variety of evolutionary mechanisms. Geographic iso- assumptions associated with bifurcating species trees, but more
lation leading to divergence among populations is generally importantly because of the evolutionary novelty introduced by
considered to be the most common mode of speciation (e.g., such events (e.g., Rieseberg, 1995; Rieseberg et al., 2003;
Grant, 1971), but reticulate evolution and polyploidy following Linder and Rieseberg, 2004). As a result, much research fo-
hybridization between divergent populations can also prompt cused on recovering the evolutionary history of plant lineages
sudden reproductive isolation and speciation in plants (e.g., seeks to distinguish between divergent and reticulate mecha-
Rieseberg, 1997; Mavárez et al., 2006). Hybridization is well nisms and to quantify their relative contributions to the genera-
tion of species diversity. At the same time, few studies have
investigated speciation in relation to geography, making it dif-
1 Manuscript received 3 June 2011; revision accepted 12 October 2011.
ficult to assess the relative frequency of allopatric vs. sympatric
Earlier fieldwork that laid the foundations and provided much of the speciation or the extent to which speciation is associated with
material for this research benefitted from the support of numerous ecological differences (ecological speciation) (Barraclough et al.,
colleagues in Mexico and notably, J. L. Contreras, H. Ochoterena, M. 1998; Barraclough and Vogler, 2000).
Sousa, and S. Zárate, as well as ongoing support from the Instituto de Here, we investigated patterns of diversification among dip-
Biología of the Universidad Nacional Autonoma de México. The authors loid members of the mimosoid legume genus Leucaena, which
also thank the Oxford Forestry Institute and A. Sing for providing seed of currently comprises 17 diploid species, five tetraploid species,
Leucaena, J. Pannell for helpful discussions, and L. Urban for comments and a potentially complex series of putative hybrids (Hughes,
on the manuscript. Components of this project were completed while 1998a) whose evolutionary histories have been influenced by
C.D.B. was on sabbatical at the Department of Plant Sciences, Oxford. This
research was supported by funds from NSF DEB0817033 & EF0542228
human translocation of seed, cultivation, and subsequent spon-
(C.D.B.), the Leverhulme Trust (C.E.H.), the Royal Society (C.E.H.), and taneous hybridization (Hughes et al., 2002, 2007). All species
the United Kingdom Department for International Development (C.E.H.). of Leucaena are small to medium-sized trees growing predomi-
5 Author for correspondence (e-mail:raj2010@pitt.edu) nantly in the seasonally dry tropical forests of Mexico and Cen-
tral America and extending north into the dry subtropics of
doi:10.3732/ajb.1100259 northern Mexico and Texas, and south into the seasonally dry
American Journal of Botany 98(12): 2049–2063, 2011; http://www.amjbot.org/ © 2011 Botanical Society of America
2049
2050 American Journal of Botany [Vol. 98

forests on both sides of the northern Andes as far south as Peru Schleinitzia novoguineensis were chosen as outgroups based on the results of
(Hughes, 1998a). Seeds and pods of a subset of Leucaena spe- previous studies (e.g., Hughes et al., 2003). Multiple alleles derived from the
same accession are indicated by a numerical suffix (e.g., 1, 2...). For a few ac-
cies are widely used as a minor food plant in south-central Mex- cessions, DNA extractions and silica gel dried materials became depleted dur-
ico (Hughes, 1998b; Hughes et al., 2007), and one species, L. ing the study. These were replaced by DNA from either the same individual tree
leucocephala has been extensively introduced throughout the trop- or another individual from the same population depending on availability.
ics as a fast-growing agroforestry and forage tree (Brewbaker,
1987; Hughes, 1998b) and is now a pantropically naturalized, PCR, DNA sequencing, and alignment —A total of four cpDNA regions and
invasive weed (Hughes and Jones, 1999). seven potentially independent nuclear-encoded loci were sequenced from each
Previous phylogenetic studies of Leucaena, primarily apply- accession. Chloroplast regions sequenced included the two trnK introns flank-
ing data from cpDNA and nrDNA ITS, have failed to fully re- ing the matK gene (primers trnK1L-849R and 1908F-trnK2R, Lavin et al.,
2000), the intron between psbA-trnH (primers psbAF and trnHR; Sang et al.,
solve the relationships among diploid species, conclusively 1997), and the rpl32-trnL spacer (primers trnL and -rpl32-F, Shaw et al., 2007).
identify the parentage of several tetraploid species (Hughes et al., The nuclear-encoded loci included nrDNA ITS and six anonymous SCAR-
2002), or investigate the potential occurrence of homoploid based markers referred to as 23L, 28, A9, A2, PA1213, and A4A5 specifically
hybridization among diploids. Sequence data from a set of con- developed for Leucaena phylogenetics (Bailey et al., 2004).
served low-copy nuclear genes identified specifically for le- PCR reactions included 1× PCR buffer (10 mmol/L Tris-HCl, 50 mmol/L
gume phylogenetics (Choi et al., 2006), as well as a selection of KCl, 2.5 mmol/L MgCl2), 100 µmol/L of each dNTP, 0.5 µmol/L of each for-
ward and reverse primer, 35 mmol/L betaine, 1.5 U of Taq polymerase and 1 µL
other low-copy nuclear genes, have proved insufficiently vari- of genomic DNA in a 25-µL reaction. PCR amplifications began with a 3 min
able within Leucaena to be especially useful (R. Govindarajulu, denaturation at 94°C, followed by 35 cycles of 15–30 s denaturation at 94°C,
unpublished data; Bailey et al., 2004). To overcome these dif- 30–90 s annealing at 57–60°C (see Appendix S1 in Supplemental Data with the
ficulties, we developed a set of anonymous nuclear-encoded online version of this article) for primers and annealing temperatures), and
loci identified using a sequence-characterized amplified region 60–90 s extension at 72°C; followed by a final extension for 7 min at 72°C. All
(SCAR) technique that has considerable potential for address- of the sequences for chloroplast regions and most of the sequences for SCAR
based loci were generated by direct sequencing of PCR amplified products.
ing evolutionary questions in Leucaena (Bailey et al., 2004). However, accessions that yielded polymorphic reads from the direct sequenc-
These new SCAR markers and an AFLP-based population ing were cloned following previously published methods (Hughes et al., 2002).
genetic approach are used here to analyze evolutionary rela- As many as 10 colonies were sequenced for each cloned sample to recover
tionships among the diploid species of Leucaena and their pat- discrete variation consistent with the observed polymorphisms in the directly
terns of diversification with three main objectives. First, diploid sequenced PCR product.
species of Leucaena are the fundamental units from which
polyploids were likely derived, suggesting that a “diploids first” Sequence assembly and phylogenetics —Individual loci were aligned in the
approach (e.g., Brown et al., 2002; Beck et al., 2010) to resolve program CLUSTAL_X (Thompson et al., 1997) and manually adjusted by eye
in the program WinClada (Nixon, 2002). For parsimony analyses, indels were
the evolutionary history of diploids is needed to provide foun- scored as gap characters using the simple gap coding method of Simmons and
dations for comprehensive downstream assessment of polyploid Ochoterena (2000) implemented in the program SeqState ver. 1.4 (Müller,
origins. The relationships and origins of the polyploid species 2006). Parsimony analyses were performed with the program NONA (Goloboff,
are investigated in the accompanying paper (Govindarajulu 2000) spawned from WinClada (best tree search, 100 random replications,
et al., 2011 in this issue). Second, it has been unclear to what extent holding 10 trees per rep, and applying max* and 1000 strict consensus boot-
hybrid origins of several polyploid Leucaena (Hughes et al., strap replicates, each comprising 100 mults holding 10 trees each). The best
fitting maximum likelihood (ML) tree (model GTR+Γ) and 500 ML bootstrap
2002, 2007) might extend to the origin and diversification of analyses (model GTR+CAT) were performed using the program RAxML
diploids. To examine these questions, we used densely sampled (Stamatakis et al., 2008). Phylogenetic analyses included investigation of individ-
gene tree and species tree phylogenetic approaches to explore ual gene trees and simultaneous analysis of concatenated matrices. First, parsimony
levels of hybridization and relationships among diploid popula- analyses were run on each data partition to assess potential gene tree/species
tions. Third, this framework also served as a molecular test of tree problems and the utility of each locus to resolve relationships within Leu-
the morphologically based species boundaries established by caena. For each gene tree, the positions of multiple accessions from each taxon
and individual sequences from heterozygous individuals were characterized as
Hughes (1998a). Resulting diploid species relationships were monophyletic, “unresolved” (if they were unresolved relative to one another),
then used to guide relevant population genetic comparisons to or polyphyletic. Attention was particularly paid to “polyphyletic” sequences
further investigate hybridization among diploids and to identify from individual accessions because these could indicate potential hybridization
genetically distinct and isolated population systems correspond- or gene tree/species tree issues. A series of simultaneous analyses were per-
ing to species. formed including and excluding accessions that were polyphyletic in the indi-
vidual analyses to evaluate their influence on the topology of the inferred
species tree. However, these accessions did not have any major impacts on sup-
MATERIALS AND METHODS port or resolution among diploid species. For the simultaneous analyses pre-
sented here, sequences from heterozygous accessions were fused (using IUPAC
DNA extractions—A combination of previously extracted DNA samples ambiguity coding to score polymorphisms) irrespective of whether they were
(Bailey et al., 2004; Hughes et al., 2002, 2007) and newly obtained silica-dried monophyletic, unresolved, or polyphyletic in the individual analyses to ensure
or fresh leaf materials were used for both the DNA sequencing and AFLP stud- matching of terminals across data partitions prior to concatenation of matrices.
ies. DNA recovery applied the chemistry and machinery presented in Alexan-
der et al. (2007), except that 10 mmol/L Tris-HCl in 70% ethanol was used in AFLP studies—Sampling —A total of 424 diploid individuals were subject to
place of the 70% ethanol column wash step. DNA quality and quantity were AFLP analysis. Samples for this component included the diploid accessions
evaluated by visualizing 3 µL of each sample alongside a 100-bp DNA mass used in the phylogenetic study (see above) and DNA extracted from at least 10
ladder (NEB-N3231: New England Biolabs, Beverly, Massachusetts, USA) on individuals from each of 42 populations of diploid Leucaena species (Appendix
1% agarose gels. 1). The latter were derived from greenhouse grown seeds collected from each
of 10 different trees per wild population (Hughes, 1998b).
Phylogenetic studies using DNA sequence data—Sampling —Two or more
representatives from each of the 17 diploid species and all infraspecific taxa AFLP analysis—Five positive control samples were included on each 96-
previously recognized for three of these species of Leucaena were sampled, well plate (L. collinsii subsp. zacapana 57/88/06, L. collinsii subsp. collinsii
making 59 ingroup accessions (Appendix 1). Desmanthus fruticosus and 51/88/06, L. lempirana 5/91/05, L. multicapitula 81/87/06, and L. salvadorensis
December 2011] Govindarajulu et al.—Cryptic diversity and allopatry 2051

99/90/02). Restriction ligation reactions (RLs) and preselective amplifications grid cell were generated from a total of 1652 georeferenced herbarium speci-
followed a modified Vos et al. (1995) AFLP approach marketed by Applied men records of diploid Leucaena (examined by Hughes, 1998a) using the pro-
Biosystems (“Plant Mapping Protocol” – P/N 402977 rev. E; Foster City, Cali- gram BRAHMS ver. 6.60 (Filer, 2008) and plotted in the program DIVA ver.
fornia, USA). Approximately 50 ng of gDNA was restriction digested using 7.1.7 (Hijmans, 2010). Sympatry was mapped using a variety of grid cell sizes,
EcoRI and MseI (New England Biolabs) and ligated with EcoRI and MseI but cell size was found to have little impact on the results, and a 5-km grid cell
adaptors using T4 DNA Ligase (New England Biolabs) at 37°C for 12–16 h. size (25 km2) was selected based on estimated pollinator dispersal distances
The RLs for each sample consisted of an 11-µL reaction containing 1× T4 Li- (see also Hughes et al., 2007).
gase buffer (NEB), 50 mmol/L NaCl, 0.05 mg/mL BSA, 1 pmol/L MseI Adapter
Pair, 10 pmol/L EcoRI Adapter Pair, 1 U MseI, 5 U EcoRI, and 67 U of T4 li-
gase (NEB). The RLs were subsequently diluted to a final volume of 200 μL RESULTS
with 0.1× Tris-EDTA (TE). Each sample was subject to preselective amplifica-
tion with a single selective base on each primer (EcoRI-A and MseI-C) and
three independent selective primer amplifications (5′FAM- EcoRI-AC/MseI- Phylogenetic analyses of DNA sequences— Extensive PCR,
CTA, 5′FAM-EcoRI-AT/MseI-CTG and 5′JOE -EcoRI-AA/MseI-CTA). Prese- cloning, and sequencing recovered at least one allele for 636 of
lective and selective amplifications included 1.5 mmol/L MgCl2, 0.1 mol/L the 649 sequence/accession combinations for the 11 sequenced
Tris-HCl pH 8.3, 0.5 mol/L KCl, 0.25 µmol/L of each primer, and ca. 2 U Taq regions and 59 ingroup samples. In contrast, presumed primer
in a 20-µL reaction containing 4-µL of dilute RL or preselective amplified prod- site divergence (Bailey et al., 2004) limited the successful PCR
uct, respectively. Cycling conditions followed the ABI Plant Mapping Protocol.
Selective products were capillary electrophoresed on an automated ABI 3100
and sequencing in the outgroups Desmanthus fruticosus and
sequencer (Applied Biosystems) with the Genescan-500 ROX standard Schleinitzia novoguineensis to just the cpDNA/ITS/PA1213
(Applied Biosystems). and cpDNA/ITS loci, respectively. Diploid gene trees lacking
appropriate outgroup sequences were rooted with L. cuspidata
AFLP data analysis—The program GeneMapper 4.0 (Applied Biosystems) for gene tree comparisons. This rooting derives from L. cuspi-
was used to identify and score loci between 75 and 500 bp. The total number of data being the only diploid resolved as sister to other Leucaena
alleles amplified in an accession was compared to the mean and standard devia- in a previous nuclear-based phylogeny of the group (e.g.,
tion of the number of fragments amplified for all individuals for that population Hughes et al., 2002). Separate analyses of each nuclear-encoded
to identify those that failed to amplify well. In practice, those falling below
a standard deviation typically failed to amplify more than few alleles per
DNA sequence region and the combined cpDNA matrix were
reaction. run to evaluate potential gene tree/species tree conflicts. The
The data matrix was subjected to distance-based and model-based analyses length of each alignment, number of accessions lacking se-
to identify distinct genetic clusters among populations. First, principal coordi- quence coverage, number of gap characters, percentage parsi-
nate analyses (PCO) based on Euclidean distances were generated in the pro- mony informative characters, and the tree statistics for each data
gram MVSP ver.3.13m (Kovach Computing Services, Pentraeth, UK). The matrix are presented in Table 1. Assessments of well-supported
PCO analyses display the clustering pattern of genotypes indicating genetic
differentiation among populations and have proved very useful in helping to
nodes confirmed that all data partitions (Appendix S2A–F,
identify hybridization at a variety of taxonomic levels when compared to tree- see online Supplemental Data), except A4A5 and nrDNA ITS
based and network-based approaches (Reeves and Richards, 2007). Second, for (online Appendix S2G, H), were generally congruent with each
subsequent comparisons of specific interest, the number of uniquely supported other and with results from previous studies (e.g., Hughes et al.,
genetic clusters was estimated using the model based Bayesian statistical analy- 2002, 2007). However, additional sampling for A4A5 and
sis of Pritchard et al. (2000). The scoring of AFLP profiles for the program nrDNA ITS revealed deep gene tree/species tree problems
structure ver. 2.3.1 treated unobserved alleles as missing data (Evanno et al.,
2005). Structure analyses included 10 000 burn-ins and MCMC replicates for
manifest by occurrences of highly supported divergent alleles
each run, 10 replicate runs for each K value (K = 1–10). The admixture model from single accessions, more than two alleles in many acces-
and allele frequencies were treated independently. Finally, the number of sions (R. Govindarajulu, unpublished data), and well-supported
unique genetic clusters was tested by using ΔK calculation as applied by Evanno incongruence relative to the other loci and one another. These
et al. (2005). two loci were excluded from the simultaneous analysis.
Gene trees constructed from separate analyses of chloroplast
Geographic distributions and sympatry—To investigate geography and and nuclear DNA sequences (Appendix S2) were generally
quantify sympatry in relation to species delimitation and potential reticulate consistent with results from previous studies using two loci
evolutionary history, we mapped the geographic distributions of (1) the taxa in
each of three major diploid clades, (2) pairs of well-supported recently derived
(Hughes et al., 2002) and, where support was recovered, these
sister species (sensu Barraclough and Vogler, 2000), and (3) levels of sympatry resolved three major diploid clades within Leucaena. From
observed across the native range of Leucaena irrespective of phylogenetic rela- the 295 sequence/accession combinations that are possible for
tionship. Coordinate files for plotting distributions and number of species per the five nuclear-encoded loci and 59 ingroup accessions, we

Table 1. Summary of data assembled for chloroplast and nuclear gene regions. The concatenated matrix includes all data matrices except A4A5 and
ITS.

Gene region Length (bp) Gap char No. of PIC % PIC IC L CI RI

cpDNA 2597 104 176 6.5 1 178 0.50 0.76

23L 874 75 160 16.8 0 315 0.60 0.83
28 742 72 108 13.2 0 286 0.44 0.75
A9 1235 156 432 28 3 1159 0.47 0.74
PA1213 810 84 121 13.5 1 326 0.53 0.80
A2 705 34 97 12.9 6 179 0.69 0.90
A4A5 778 77 326 38 NA 825 0.54 0.90
ITS 537 40 328 55 NA 895 0.54 0.79
Concatenated matrix 6960 525 1100 15.8 NA 2355 0.54 0.81
Notes: PIC, parsimony informative characters; IC, number of accessions without sequence coverage; L, tree length; CI, ensemble consistency index; RI,
ensemble retention index.
2052 American Journal of Botany [Vol. 98

Fig. 1. Best maximum likelihood (ML) tree recovered from simultaneous analysis of five nuclear loci and cpDNA data analyzed with RAxML. Branch
support values represent ML and parsimony derived bootstrap values, respectively.
December 2011] Govindarajulu et al.—Cryptic diversity and allopatry 2053

Fig. 2. Summary of results for Leucaena lanceolata s.l. (A) PCO scatter plot for all accessions analyzed using AFLPs. (B) Plot of the geographic
distribution of accessions representing divergent groups recovered from phylogenetic analysis (Fig. 1) and PCO (Fig. 2A). (C) Plot of the mean likelihood
estimates calculated for K = 1–10 in structure (Pritchard et al., 2000). (D) ΔK plot calculated according to Evanno et al. (2005).

identified just four cases were two alleles recovered from the With the exception of multiple accessions representing L.
same accession resolved in divergent positions, contradicting lanceolata, ML and parsimony-based simultaneous analyses
the monophyly of the species (L. collinsii subsp. zacapana resolved multiple accessions of each diploid or infraspecific
[PA1213; accession 18/84], L. magnifica [PA1213; 19/84], L. taxon into well-supported monophyletic groups (Fig. 1). In
lanceolata var. lanceolata [23L; 134/92], and L. salvadorensis contrast, the polyphyletic placement of L. lanceolata accessions
[A9; 34/88]). Each case was restricted to a single locus for the from Oaxaca, Veracruz, and western Chiapas (913, 134/92,
respective accession, and the inclusion or exclusion of these 43/85, 50/87, 51/87) relative to those from northwestern coastal
accessions and sequences had no impact on supported nodes in Mexico (46/85, 44/85, 90/92) contradicts the current division of
the combined phylogenetic analysis (R. Govindarajulu, unpub- L. lanceolata into two infraspecific varieties proposed by
lished data). Hughes (1998a).
Both the ML and parsimony-based simultaneous analyses of
the concatenated matrix recovered robustly supported relation- AFLP-based assessments— The potential for interspecific
ships among lineages and provided stronger support and resolu- hybridization among diploid species exhibiting few genetic
tion within clades 2 and 3 (Fig. 1) than individual gene trees crossing barriers (Sorensson and Brewbaker, 1994) and prob-
(Appendix S2). The modest support for a few nodes within lems with the current delimitation of L. lanceolata (see above)
clade 1 may be attributable to limited character data supporting prompted our investigation of species limits using extensive
these nodes or conflicting signal resulting from the potential sampling of individuals and a population genetic approach. The
inclusion of hybrid accessions or species. An assessment of un- complete matrix included 1215 loci scored from 363 samples
ambiguously optimized character distributions along branches that amplified well for all three selective primer combinations.
in the parsimony tree suggests that poorly supported nodes are An extensive series of PCO analyses was carried out using the
subtended by short branches relative to other branches in the diploid phylogeny as the framework for relevant focused com-
combined phylogeny (Appendix S3), further reducing concerns parisons. These similarity-based comparisons included clade
about the inclusion of hybrid terminals. to clade analyses (clade 1 to 2, clade 1 to 3, and clade 2 to 3),
2054 American Journal of Botany [Vol. 98

Fig. 3. Summary of results for Leucaena collinsii s.l. (A) PCO scatter plot for all accessions analyzed using AFLPs. (B) Distribution of accessions
representing divergent clades recovered from phylogenetic analysis (Fig. 1) and divergent clusters recovered from PCO (Fig. 3A). (C) Plot of the mean
likelihood estimates calculated for K = 1–10 in structure (Pritchard et al., 2000). (D) ΔK plot calculated according to Evanno et al. (2005).

followed by infraclade comparisons removing the most divergent ered in the phylogenetic analysis (Fig. 1), and the clusters differ
taxon(a) sequentially, and ultimately the analysis of accessions by six fixed alleles in the AFLP data set. Leucaena lanceolata
of each species in isolation from other species. These compari- var. lancolata accession 1577 showed evidence of possible hy-
sons also recovered distinctive clusters of taxa (online Appen- bridization between these two lineages, as evidence by an ad-
dix S4) that were largely consistent with the phylogenetic mixed background in results from structure and its
hypothesis. In all 27 comparisons only four of 363 accesssions intermediate position in PCO plots (Fig. 2). However, 1577 be-
(L. esculenta 2143 [Appendix S4E], L. lanceolata 1577 [Ap- haved erratically in each comparison that it was included (Fig.
pendix S4A, D, N, U], L. pueblana 125_92 [Appendix S4B, E, 2 and Appendices S4A, D, N, U), even resolving between clades
L], L shannonii 1_91_03 [Appendix S4P]) failed to resolve 1 and 3 in broad comparisons. The cause of these anomalous
with their respective taxon in one or more result, revealing po- placements is unclear.
tential hybrid backgrounds or otherwise unique genotypes. This Separate PCO analyses of multiple populations represent-
result along with limited evidence for hybrids in the phyloge- ing L. collinsii subsp. collinsi and L. collinsii subsp. zacap-
netic analyses suggests that we recovered few individuals with ana, which formed strongly supported monophyletic sister
mixed backgrounds indicative of homoploid hybridization groups in the phylogeny (Fig. 1), resolved these taxa into
among diploid species. divergent clusters (Fig. 3A). These were also recovered from
For L. lanceolata and L. collinsii, more extensive geographic structure (ΔK = 2, Fig. 3C, D), and they differed by nine
sampling in the AFLP study revealed levels of genetic diversity fixed alleles. None of these results identified potential ad-
potentially consistent with previously overlooked species-level mixed individuals.
diversity. AFLP data from eight populations and 39 accessions Genetic diversity consistent with potential overlooked spe-
of L. lanceolata were analyzed to investigate the polyphyly of cies was also recovered in PCO analyses of available material
L. lanceoata observed in the combined phylogenetic analysis of L. lempirana (Appendix S4Y), L. macrophylla (Appendix
(Fig. 1). PCO analyses resolved L. lanceolata accessions into S4W), L. salvadorensis (Appendix S4Z), and L. trichodes (Ap-
two distinct groups (44/85, 46/85) and (134/92, 43/85, 51/87, pendix S4X). The single populations of L. trichodes sampled
2166, 2171, 1577) along axis 1 (Fig. 2A) and are further sup- from the east and west sides of the Andes in Venezuela and
ported by results recovered from structure (ΔK = 2; Fig. 2C, Ecuador, respectively, differed by 36 fixed allelic differences;
D). These groups correspond to the two divergent clades recov- samples from one population each for the subspecies for L.
December 2011] Govindarajulu et al.—Cryptic diversity and allopatry 2055

Fig. 4. Geographic distribution of 1652 herbarium records representing diploid accessions of Leucaena from clades 1–3.

macrophylla differed by 12 fixed allelic differences; two popu- volcanic axis across Mexico, taxa from clade 2 restricted to in-
lations of L. lempirana from closely adjacent valleys in north- land areas of south-central Mexico in the central Mexican high-
ern Honduras differed by seven fixed allelic differences; and lands and valleys south of the volcanic axis, while taxa belonging
populations of L. salvadorensis from northern Nicaragua dif- to clade 1 occur along the western coast of Mexico into south-
fered by three fixed allelic differences from those in southern central Mexico, Central America, and northern South America
Honduras. However, all these comparisons are based on sparse (Fig. 4). The geographically widespread clade 1 species L. mac-
sampling across the geographic range in both the phylogenetic rophylla generates a slight (Appendix S5), and in some areas
and population genetic approaches and may thus be biased. superficial (as a result of the scale), overlap in distribution be-
tween clade 1 and clade 2 taxa (Fig. 4).
Geographic distributions— Mapping of 1652 georeferenced An assessment of sympatry irrespective of phylogenetic rela-
wild diploid herbarium specimen records using the three differ- tionship identified very few regions with potential overlap among
ent approaches shows a clear pattern of allopatry among diploid noncultivated wild diploid accessions, with just 30 of the 1104
species. First, the geographic distributions of the three major grids (5-km grid cells) containing populations of two diploid spe-
clades, with minor exceptions, are almost entirely allopatric cies (Fig. 5; Appendix S5) and none containing more than two.
(Fig. 4) with taxa from clade 3 restricted to northeast Mexico Finally, geographic distributions of each well-supported pair
and the southern United States, areas east and north of central of sister species are also generally allopatric. For example,

Fig. 5. Geographic representation of sympatry among all 1652 diploid accessions of Leucaena. Each symbol on the map represents one of 1104 grid
cells that had one or more occurrences of Leucaena; either a single species or two species occurred in that 25-km2 area.
2056 American Journal of Botany [Vol. 98

Fig. 6. Geographic distributions of well-supported sister species pairs recovered from phylogenetic analysis (Fig. 1). (A) Leucaena greggii and L.
retusa. (B) L. multicapitula and L. salvadorensis. (C) L. matudae and L. pueblana. (D) L. lanceolata s.s. and L. macrophylla.

L. greggii/L. retusa (Fig. 6A), L. multicapitula/L. salvadorensis Species diversity—Inclusion of multiple accessions of all spe-
(Fig. 6B), L. collinsii subsp. collinsii/L. collinsii subsp. zacap- cies in the phylogenetic analyses and population genetic assess-
ana (Fig. 3B), and L. matudate/L. pueblana (Fig. 6C) occupy ments, along with detailed distribution maps for all taxa, provide
strictly allopatric extant distributions. Only the pair of sister an excellent basis for the re-evaluation of boundaries among
species L. lanceolata (s.s.) (see below) and L. macrophylla are diploid species of Leucaena. For 16 of the 17 species recognized
found in partial sympatry (Fig. 6D). by Hughes (1998a), we found high support for monophyly of mul-
tiple accessions in the phylogenetic analyses that is congruent with
results from population genetic assessments, morphology, and
DISCUSSION geographic isolation among wild populations, supporting the
species circumscriptions presented in the monographic treat-
Previous investigations of phylogenetic relationships among ment of Hughes (1998a). These results are consistent with a vari-
species of Leucaena and its close relatives (e.g., Luckow, 1997; ety of species concepts, including the phylogenetic species
Hughes et al., 2002, 2003) have run into common and recurrent concept (sensu Nixon and Wheeler, 1990; Davis and Nixon, 1992),
problems associated with limited available variation at some monophyletic species concept (e.g., Donoghue, 1985), and bio-
universally applied loci (e.g., Shaw et al., 2007) and limited logical species concept (Dobzhansky, 1935; Mayr, 1942).
concerted evolution with nrDNA ITS (e.g., Álvarez and These data and analyses also provide strong evidence for pre-
Wendel, 2003). The application of the SCAR-based approach viously overlooked cryptic species within L. collinsii and L.
of Bailey et al. (2004) for locus development, specifically lanceolata. The two divergent and well-supported monophyl-
implemented for studies involving closely related species, has etic clades of accessions representing L. lanceolata (Fig. 1),
proven useful in resolving highly supported divergent diploid which are congruent with population genetic differences (Fig. 2),
species relationships in Leucaena. This finding is also consis- contradict current and historical species delimitations. How-
tent with the application of (Thorogood et al., 2009) and at least ever, these lineages occupy distinct and disjunct geographic
one extension of the method (González, 2010) in other plant distributions (Fig. 2), providing further evidence for two dis-
groups. Thus this approach, along with conserved orthologous tinct species. Similarly, accesssions of L. collinsii subsp. zacapana
markers (e.g., Fulton et al., 2002; Choi et al., 2006; Lohithaswa are geographically isolated from L. collinsii subsp. collinsii
et al., 2007), has helped bridge the gap between sole reliance on (Fig. 3), and the two subspecies formed well-supported mono-
standard phylogenetic markers (e.g., nrDNA ITS and certain phyletic groups (Fig. 1) for which fixed allelic differences were
cpDNA markers) and the coming accessibility of massive detected in the population genetic analyses (Fig. 3). These find-
marker sets derived from second generation sequencing ap- ings along with morphological and chromosome differences
proaches in nonmodel systems (e.g., Gompert et al., 2010; support recognition of these as distinct species (see taxonomic
M. M. Koopman [Eastern Michigan University] et al., unpub- treatment later), increasing the number of recognized diploid
lished manuscript). species of Leucaena from 17 to 19.
December 2011] Govindarajulu et al.—Cryptic diversity and allopatry 2057

In addition to these clear-cut examples of previously under- in plants and animals (e.g., Mayr, 1942), entails geographic isola-
estimated species diversity, population genetic assessments re- tion between populations and subsequent divergence into distinct
veal population structures and fixed allelic differences indicative lineages. In contrast, sympatric and parapatric speciation is in-
of strongly differentiated and geographically structured varia- ferred for species with overlapping or proximate distributions
tion among populations of several other species of Leucaena, during speciation, which require the development of reproduc-
including L. lempirana, L. macrophylla, L. salvadorensis, and tive isolating mechanisms as part of the speciation process (e.g.,
L. trichodes. This variation is especially notable within L. trichodes Kondrashov, 1986). Although few studies have tested the rela-
where populations from opposite sides of the Andes in Venezu- tionship between geography and speciation (e.g., Barraclough
ela and Ecuador differ by 36 fixed allelic differences, reflecting and Vogler, 2000; Savolainen et al., 2006; Papadopulosa et al.,
the likely lack of gene exchange and degree of isolation across 2011), where such data are available for non-island systems, cla-
the Andes (e.g., Dick et al., 2003). However, much denser sam- dogenesis has been found to be predominantly associated with
pling in both the phylogenetic and population genetic analyses allopatry, with sympatry interpreted as a consequence of postspe-
would be needed to confidently distinguish whether these pat- ciation range movements (Perret et al., 2007). In contrast, recent
terns are the result of sparse sampling or cryptic evolutionary studies that focused on island and island-like systems have begun
divergence among populations that could merit recognition of to question predominant views on the relative importance of allo-
additional species. patric and sympatric mechanisms of speciation (Barluenga et al.,
The discovery of overlooked species diversity prompted by 2006; Savolainen et al., 2006; Papadopulosa et al., 2011).
densely sampled (complete or near-complete taxon sampling and Contemporary geographic distributions of diploid species of
multiple accessions of species) molecular phylogenetic analyses Leucaena show a high degree of allopatry consistent with allo-
that reveal robustly supported reciprocally monophyletic clades patric divergent speciation as the predominant mechanism un-
that coincide with other evidence from geography, ecology, and derlying diploid species diversification in Leucaena (e.g., Grant,
morphology is increasingly common. Taking examples of le- 1971; Barraclough and Vogler, 2000). Allopatry is evident at
gumes from neotropical seasonally dry tropical forests, recent three levels: (1) the early divergence of major clades whose
novelties delimited in similar ways include Caesalpinia oyame taxa remain largely allopatric (Fig. 4), (2) overall patterns of
(Sotuyo et al., 2007; Sotuyo and Lewis, 2007), Mimosa jaenensis wild populations of diploid species irrespective of their phylo-
(Särkinen et al., 2011), Coursetia greenmanii (de Stefano et al., genetic relationships (Fig. 5), and (3) four of the five recently
2010), Coursetia caatingicola (de Quieroz and Lavin, 2011), and derived well-supported pairs of sister species (Figs. 3, 6).
Poissonia eriantha (Pennington et al., 2011). This steady addi- Furthermore, artificial crossing experiments in Leucaena
tion of new species across different genera suggests that species have shown a low degree of reproductive isolation between
diversity of neotropical seasonally dry forests may have been sig- most diploid taxa. The results of 65 diploid-diploid crosses,
nificantly underestimated. representing 12 of the 19 species analyzed here, show a high
degree of crossability (77%) (Sorensson and Brewbaker, 1994).
Hybridization and speciation among diploid Leucaena— At Although crossability remains to be tested among the remain-
the diploid level, the evolutionary significance of homoploid ing diploids, currently available data show that crossability is
hybridization and introgression in species diversification has retained among distantly divergent lineages across the whole
remained controversial (e.g., Anderson and Stebbins, 1954). diploid phylogeny. The overwhelming absence of diploid-dip-
The role of hybridization in the formation of polyploid species loid hybrids in the context of genus that retains crossability is in
and lineages is widely recognized (e.g., Doyle et al., 1990; line with geographical isolation and allopatry underlying dip-
Wendel et al., 1995); however, there is growing evidence that loid species diversification in the genus.
homoploid hybridization and introgression have also been im-
portant in many plant and animal groups (e.g., Baack et al., Biogeography— Species of Leucaena are concentrated in the
2005; Kane et al., 2009). In Leucaena, the important outcomes seasonally dry tropical forest (SDTF) biome, a vegetation type
of hybridization in terms of multiple polyploid taxa are clearly that occupies a wide but highly disjunct distribution across the
established (Hughes et al., 2002, 2007; Govindarajulu et al., neotropics (Pennington et al., 2000, 2009) and is characterized
2011 in this issue), and a variety of studies have demonstrated by erratic moisture availability, long periods of seasonal
the potential for hybridization among diploids in experiments drought, a general absence of grasses and natural fire distur-
that show high artificial crossability between species (e.g., So- bance, high levels of endemism (β diversity), and an abundance
rensson and Brewbaker, 1994) and as a result of human trans- of succulent plants including Cactaceae that has prompted its
location and cultivation (Hughes et al., 2007). Nonetheless, our designation as the “succulent biome” (Schrire et al., 2005). A
results derived from individual gene tree hypotheses, combined predeliction for this type of vegetation suggests that Leucaena
species tree hypotheses, and an AFLP-based population genetic shows a pattern of phylogenetic niche conservatism (sensu
approach revealed little evidence for hybridization or introgres- Donoghue, 2008) to SDTFs, with only minor incursions of a
sion between wild-collected diploid individuals or populations, few lineages into mid-elevation seasonal pine–oak forests
suggesting that reticulation has been of little importance in the (L. trichandra and L. macrophylla), more-mesic less-seasonal
historical diversification of diploid Leucaena. Furthermore, the lowland forests (L. multicapitula), and subtropical dry matorral
predominantly allopatric geographic distributions (Figs. 4–6) (L. greggii and L. retusa). This distribution pattern suggests
appear to confirm that there are few opportunities for diploid hy- that diversification of diploid Leucaena was determined more
brids to arise in wild populations. Thus, the available evidence by geographic than ecological isolation, in line with the pre-
suggests that diploid Leucaena are predominantly derived from dominance of allopatry among species.
divergent, rather than reticulate, mechanisms of speciation. In common with phylogenies of other woody SDTF clades,
Such divergent modes of speciation are partitioned into allo- the three major clades within Leucaena occupy distinct and
patric or sympatric-parapatric mechanisms. Allopatric specia- largely disjunct geographical areas in northeast Mexico and
tion, long considered the most common mechanism of speciation Texas, inland south-central Mexico, and Pacific coastal Mexico,
2058 American Journal of Botany [Vol. 98

Central America, and northern South America (Fig. 4). This TAXONOMIC TREATMENT
pattern of strong geographical structuring in the diploid Leucaena
phylogeny, and across phylogenies for other dry forest groups Leucaena zacapana— The results presented here strongly
(Lavin, 2006), has been attributed to limited dispersal and im- support raising L. collinsii subsp. zacapana to species rank,
migration across the fragmented SDTF biome (Lavin, 2006; distinct from L. collinsii. Multiple accessions of each taxon
Pennington et al., 2009), a pattern potentially accentuated by form well-supported monophyletic sister clades (Fig. 1), and
the resilient ecology of dry forests and phylogenetic niche con- these groups were also recovered in all the population genetic
servatism (Pennington et al., 2010). analyses (e.g., Fig. 3A). These two population systems are fur-
Coalescence of sequences of nuclear loci and the resultant re- ther distinguished by nine fixed AFLP allelic differences and
ciprocal monophyly of multiple accessions of Leucaena species is by cytological studies that suggest L. collinsii subsp. colllinsii
also consistent with patterns observed for other seasonally dry is 2n = 52, while L. collinsii subsp. zacapana is 2n = 56 (Cardoso
tropical forest lineages and indicative of long persistence of en- et al., 2000; Schifino-Wittmann et al., 2000), and by genome
demic populations and allopatric speciation in geographically size measurements (Govindarajulu et al., 2011 in this issue). In
isolated and evolutionarily persistent dry forest patches (Barra- addition, a suite of quantitative morphological differences
clough, 2010; de Stefano et al., 2010; Pennington et al., 2010). (Hughes, 1998a) and geographically disjunct and isolated dis-
tributions of these two lineages (Fig. 3B), further support rec-
Conclusions— We find little evidence for contemporary or ognition as two distinct species.
historical hybridization among wild-collected diploids and, as a As recognized here Leucaena collinsii is restricted to the
result of limited reticulation and the utility of the markers used, central depression of Chiapas in Mexico and adjacent fringes of
recover a well-resolved phylogenetic species-level hypothesis. the Departamento of Huehuetenango in Guatemala, between
Population genetic structure and phylogenetic resolution iden- 400 and 900 m a.s.l., while L. zacapana is a narrowly restricted
tify two additional morphologically cryptic species that are endemic in the Motagua Valley system in Guatemala between
supported by a variety of data. Last, the pattern of diversifica- 100 and 800 m above sea level (fig. 44 in Hughes, 1998a). The
tion across neotropical seasonally dry forests is consistent with intervening mountains of central and northwestern Guatemala
a general mechanism of divergent allopatric speciation in the rising to between 2000 and 3000 m a.s.l., effectively isolate
formation of most diploid species of Leucaena. these two species.
Diploid Leucaena represent the majority of species in a The seasonally dry tropical forests of the Motagua Valley
genus known to be complicated by human translocation, poly- system in southeastern Guatemala are known to harbor a num-
ploidy, and hybridization (e.g., Harris et al., 1994; Hughes and ber of endemic dry forest plant species (e.g., in legumes Cal-
Harris, 1998; Hughes et al., 2002, 2007). Through the applica- liandra carcera Standl. & Steyermark, Mimosa canahuensis
tion of newly available data, dense sampling strategies, and Standl. & Steyermark, Aeschynomene eriocarpa Standl. &
complementary phylogenetic and population genetic ap- Steyermark). However, compared to some other neotropical
proaches, we have clarified the evolutionary diversification of seasonally dry valleys such as the central depression of Chiapas
diploids. There is little evidence to suggest that reticulate evo- and the Tehuacán Valley in south-central Mexico, or the Marañón
lutionary processes have played a significant role in the diversi- Valley in northern Peru, current estimates of endemism for Mot-
fication of diploid Leucaena. agua are modest. Thus, it seems at first sight somewhat surprising
The recognition of L. cruziana and L. zacapana as cryptic that there are two endemic species of Leucaena, L. magnifica, a
species is in keeping with a renaissance of species discovery narrowly restricted endemic only known from the Guatemalan
being driven in part by the development of new tools for DNA Department of Chiquimula (Hughes, 1998a), and L. zacapana
barcoding and concerns related to loss of biodiversity (e.g., from this one valley system. These findings suggest that levels
Savolainen et al., 2005; Smith et al., 2008). Furthermore, the of endemism in the Motagua valley may be underestimated.
recovery of cryptic species-level diversity in Leucaena is con- Leucaena zacapana (C. E. Hughes) R. Govindarajulu &
sistent with densely sampled phylogenies revealing geographi- C. E. Hughes comb. et stat. nov. Leucaena collinsii subsp.
cally structured genetic variation with patterns of coalescence zacapana C. E. Hughes, Kew Bull. 46(3): 553, 1991. Type:
among conspecific accessions for a growing number of neotro- Guatemala. Zacapa: Estanzuela in dry thorn forest, 1 Mar 1988,
pical seasonally dry forest plant groups (Sotuyo and Lewis, Hughes 1102 (holotype: FHO!; isotypes: K! MEXU!).
2007; Pennington et al., 2009, 2010, 2011; de Stefano et al., As circumscribed here, L. zacapana corresponds directly to
2010; de Quieroz and Lavin, 2011; Särkinen et al., 2011). L. collinsii subsp. zacapana presented by Hughes (1998a). Dis-
Finally, a general pattern of allopatric divergence among tinguishing features, illustrations, and specimen citations lists
diploid Leucaena species is in line with historical opinion previously presented under L. collinsii by Hughes (122–128
suggesting that this mechanism is the “null” model of spe- and fig. 43 in Hughes, 1998a) can be directly applied and are
ciation in most sexual lineages (e.g., Coyne and Orr, 2004). not further elaborated on here.
However, this stands in contrast to a growing body of evi-
dence that gene flow can be common during and after specia- Leucaena lanceolata s.l.—None of the previous circumscrip-
tion in many groups (e.g., Chapman and Burke, 2007; tions of the variable and widely distributed L. lanceolata has
Papadopulosa et al., 2011) as well as abundant evidence for proved satisfactory. These treatments range from the recognition
allopolyploid origins of several Leucaena species (Hughes of a single taxon (McVaugh, 1987) to division into either nine
et al., 2002, 2007). Reconciling this pattern of strictly allopatric separate species (Britton and Rose, 1928) or two infraspecific
diploid speciation with known allopolyploid speciation in taxa (Zárate, 1994; Hughes, 1998a). While several distinct mor-
Leucaena reinforces the idea that human translocation and phological variants are apparent across the range of L. lancoleata
cultivation have been critical in creating artificial sympatry s.l., these are often geographically localized and based on rela-
and opportunities for hybridization (Hughes et al., 2007; tively minor quantitative differences in leaves and pods, while
Govindarajulu et al., 2011 in this issue). overall patterns in these traits show no clear-cut discontinuities
December 2011] Govindarajulu et al.—Cryptic diversity and allopatry 2059

that are congruent with geography (figs. 58, 59 in Hughes, 1998a). Leucaena brandegeei Britton & Rose, N. Amer. Fl. 23: 128.
Notably, populations from the Pacific coast of eastern Michoacán 1928. Type: MEXICO. Baja California Sur: nr La Mesa,
and inland populations from Oaxaca show overlapping patterns Cape region, 31 Oct 1902, T.S. Brandegee s.n. (holotype:
of pod size and indumentum spanning the boundaries between NY!; isotypes: US!UC!).
the two infraspecific taxa, L. lanceolata var. lanceolata and L. Leucaena palmeri Britton & Rose, N. Amer. Fl. 23: 123.
lanceolata var. sousae recognized by Zárate (1994) and Hughes 1928. Type: MEXICO. Sonora: nr Alamos, 26°59′N,
(1998a). We identify two robustly supported lineages that coin- 108°57′W, 20 Sep 1890, Palmer 718 (holotype: NY!;
cide with the geographical disjunction between populations from isotype: US!).
western and northwestern Mexico (western Guerrero and Micho- Leucaena pubsecens Britton & Rose, N. Amer. Fl. 23: 122.
acán to Sonora and Baja California) and those from Pacific 1928. Type: MEXICO. Sinaloa: nr Mazatlán, 23°14′N,
coastal Oaxaca, southwestern Chiapas, spanning the Isthmus of 106°24′W, 1925, J.G. Ortega 5988 (holotype: NY!; iso-
Tehuantepec to Veracruz (Fig. 2B). Geographically structured ge- types: GH!US!).
netic variation of this sort is a common feature of seasonally dry Leucaena sinaloensis Britton & Rose, N. Amer. Fl. 23: 124.
tropical forest plants (e.g., Lavin, 2006; Pennington et al., 2009) 1928. Type: MEXICO. Sinaloa: vicinity of Palmar,
and provides a robust basis for delimitation of two species, here 22°13′N, 105°36′W, 15 Apr 1910, Rose et al., 14650 (ho-
recognized as L. lanceolata (the typical northwestern lineage) lotype: NY!; isotype: US!).
and L. cruziana Britton & Rose for the Oaxaca, Veracruz, and Leucaena sonorensis Britton & Rose, N. Amer. Fl. 23: 122.
western Chiapas lineage. 1928. Type: MEXICO. Sonora: Sierra de Alamos, nr
Recognition of L. cruziana as a species distinct from L. lan- Alamos, 26°58′N, 108°57′W, 14 Mar 1910, Rose et al.,
ceolata is congruent with results from analysis of plastid and 12821 (holotype: NY!; isotype: US!).
nuclear DNA, AFLP data, and geography, and strongly sup- Leucaena nitens M. E. Jones, Contr. West Bot. 15: 136. 1929.
ported by fixed differences indicative of isolation and consistent Type: MEXICO. Sinaloa: nr Mazatlán, 23°14′N,
with the phylogenetic species concept (see above). Re-exami- 106°24′W, 20 Nov 1926, Jones 22465 (holotype: POM!;
nation of the somewhat complex and overlapping patterns of isotypes: MO!US!).
variation in quantitative leaf and pod traits in the light of these For additional material examined, see online Appendix S6.
new results, suggests that this new division is satisfactory in Leucaena cruziana Britton & Rose, N. Amer. Fl. 23: 123.
comparison to previous classifications. The narrower circum- 1928. Type: MEXICO. Veracruz: Barranca de Panoaya,
scription of L. lanceolata proposed here to include only popula- 19°18′N, 96°25′W, Dec 1919, Purpus 8387 (holotype: NY!;
tions from western Guerrero to Sonora including Baja California isotypes: GH!UC!US!).
(Fig. 2B), creates a morphologically more coherent species un- Leucaena rekoi Britton & Rose, N. Amer. Fl. 23: 122. 1928.
complicated by the variability in pod traits and especially pod Type: MEXICO. Oaxaca, nr Pochutla, close to the Pa-
vestiture found in coastal Michoacán and parts of Oaxaca, cific coast, 15°44′N, 96°28′W, 28 Sep 1917, Reko 3632
which was highlighted as problematic by Hughes (1998a). Leu-
(lectotype, flowering shoot and leaves only: US!).
caena lanceolata, as circumscribed here, generally has leaves
Leucaena purpusii Britton & Rose, N. Amer. Fl. 23: 123.
with 3–5 pairs of pinnae, 4–6 pairs of leaflets per pinna and
1928. Type: MEXICO. Veracruz: Rim of barranca at Re-
leaflets <20 mm wide and pods <18 cm long and < 22 mm wide,
while L. cruziana has leaves with 2–3(−4) pairs of pinnae, 3– mudadero, 19°15′N, 96°34′W, Jan 1926, Purpus 10607
4(−5) pairs of leaflets per pinna, leaflets 20–35 mm wide, and (holotype: NY!; isotype: US!).
pods (16–)20–37 cm long and (16–)20–32 mm wide, although Leucaena lanceolata var. sousae (S. Zárate) C. E. Hughes,
there are no clear-cut discontinuities in any of these traits. Contr. Univ. Michigan Herb. 21: 288. 1997. Leucaena
The revised synonymy for L. lanceolata listed below is less lanceolata subsp. sousae S. Zárate, Anales Inst. Biol.
extensive than suggested by Hughes (1998a), but still includes Univ. Auton. México, BOT. 65: 117. 1994. Type: MEX-
five species described by Britton and Rose (1928), who tended ICO. Oaxaca: 17 km WNW of Puerto Escondido, Distr.
to pigeon-hole the minor variants they observed among the lim- Juquila, 15°57′N, 97°13′W, 21 Oct 1976, Sousa 6390
ited material available to them, as distinct species. (holotype: MEXU!; isotype: UC!).
Under this new division, pod indumentum, which was previ- Three of the nine species previously recognized by Britton
ously used as one character to distinguish typical L. lanceolata and Rose (1928) and subsequently treated as conspecific with
var. lanceolata from L. lanceolata var. sousae, but for which L. lanceolata by Zárate (1994) and Hughes (1998a)—L. rekoi,
there were several notable and problematic exceptions (Hughes, L. cruziana and L. purpusii—as well as the subsequently de-
1998a), is confirmed to be an unreliable and labile character for scribed L. lanceolata subsp. sousae (Zárate, 1994; Hughes,
species delimitation. Pods vary from densely velutinous to gla- 1998a), have type localities (from Pochutla, Oaxaca; Barranca
brous (when pods are often lustrous or glossy) within and de Panoaya, Veracruz; Remudadero, Veracruz; and Puerto Es-
among populations of both L. cruziana and L. lanceolata, just condido, Oaxaca, respectively) that fall within the distribution
as it does within several other species of Leucaena (e.g., L. di- of the southeastern lineage. The three earlier names by Britton
versifolia, L. lempirana, L. trichandra) (Hughes, 1998a). and Rose (1928) were published simultaneously in their North
Leucaena lanceolata S. Watson, Proc. Amer. Acad. Arts 21: American Flora, but doubt has been cast over the identity of L.
427. 1886. Type: MEXICO. Chihuahua: Batopilas, Hacienda rekoi (Zárate, 1994), because the type collection is a mixed
San Miguel, SW Chihuahua, 27°53′N, 108°26′W, Sep 1885, gathering of leaves and flowers from Leucaena, which Zárate
Palmer 6 (holotype: GH!; isotype: NY!UC!US!). (1994) suggests are doubtfully distinguishable from L. macro-
Leucaena microcarpa Rose, Contr. U. S. Natl. Herb. 5: 141. phylla, and fruits of Caesalpinia (Coulteria) velutina (Britton
1897. Type: MEXICO. Baja California Sur: nr Mira- & Rose) Standl. Of the two remaining names, which are both
flores, 23°21′N, 109°47′W, 13 Oct 1890, Brandegee 186 based on types from geographically closely adjacent localities
(holotype: US!; isotype: UC!). in Veracruz, we are choosing to use the name L. cruziana,
2060 American Journal of Botany [Vol. 98

which appears before L. purpusii, albeit on the same page in de Quieroz, L. P., and M. Lavin. 2011. Coursetia (Leguminosae) from
Britton and Rose (1928), to recognize the southeastern lineage eastern Brasil: The monophyly of three caatinga-inhabiting species is
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Donoghue, M. J. 1985. A critique of the biological species concept
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Appendix 1. Plant material used for phylogenetic study and AFLP analyses. Seed lot number is provided if DNA was extracted from a seedling raised from a seedlot.
Herbarium vouchers are all at FHO, with duplicates variously deposited at CAS, EAP, K, MEXU, NY, US, and MO. For complete locality and GenBank
information, see Appendix S7 (in Supplemental Data with the online version of this article).

(A) Phylogenetic study Hughes CE 1421, 4_91 -Erandique, Lempira, Honduras. Hughes CE
1654, 128/92 -Tierra Colorada, Oaxaca, Mexico. Hughes CE 1682,
137/92 -La Trinitaria, Chiapas, Mexico. Hughes CE 1701, 140/92 -San
Taxon, Voucher, Seed lot (where applicable, e.g., 51/81) -Locality.
Marcos, San Marcos, Guatemala. Hughes CE 2121, -Matatlan, Oaxaca,
Mexico. L. trichodes (Jacquin) Bentham, Hughes CE 775, 2/86/07
L. collinsii Britton & Rose, Hughes CE 1187, 51/88 -Huehuetenango, -Trujillo, Venezuela. Hughes CE 997, 61/88 -Manabi, Ecuador. L.
Guatemala. Hughes CE 527, 52/88 -Chiapas, Mexico. L. cruziana, Hughes zacapana (C.E. Hughes) R. Govindarajulu & C.E. Hughes, Hughes CE
CE 1672, 134/92 -Matias Romero, Oaxaca, Mexico. Hughes CE 2180, -El 1096, 57/88 -Chiquimula, Guatemala. Hughes CE 1120, 56/88 -Zacapa,
Limon, Palmasola, Veracruz, Mexico. Hughes CE 559, 43/85 -Oaxaca, Guatemala. Hughes CE 299, 18/84 -Progreso, Guatemala.
Mexico. Hughes CE 913, -Veracruz, Mexico. Hughes CE 835, 51/87
-Oaxaca, Mexico. Hughes CE 872, 50/87 -Oaxaca, Mexico. L. cuspidata
Standley, Hughes CE 1580, -Tolantongo Cardonal, Hidalgo, Mexico. (B) AFLP analyses
Hughes CE 1583, 89/92 -Mina San Miguel, Hidalgo, Mexico. Hughes CE
1586, 88/92 -Jacala, Hidalgo, Mexico. L. esculenta (Sessé & Mociño ex
DC.) Bentham, Hughes CE 2114, -Miahuatlán, Oaxaca, Mexico. Hughes Taxon, Voucher, Seed lot (where applicable, e.g., 51/81) -Locality, -Number of
CE 2143 -Huajuapan de León, Oaxaca, Mexico. Bailey & Ochoterena 216, individuals analyzed per population.
-Mexico, Mexico. Hughes CE 894, 47/87 -Guerrero, Mexico. Hughes CE
903, 48/87/01 -Michoacan, Mexico. Desmanthus fruticosus Rose, Hughes L. collinsii Britton & Rose, Hughes CE 527, 45/85 -Narcisco Mendoza,
CE 1532, 109/92 -La Paz,Baja california, Mexico. L. greggii S. Watson, Chiapas, Mexico -8, Hughes CE 1187, 51/88 -Chacaj, Huehuetenango,
Hughes CE 1057, 82/87 -Nuevo León, Mexico. Hughes CE 695, 19/86 Guatemala -13. L. cruziana Britton & Rose, Hughes CE 1672, 134/92
-Nuevo León, Mexico. Hughes CE 695, 20/86/02 -Nuevo León, Mexico. -Matias Romero, Oaxaca, Mexico -1, Hughes CE 559, 43/85 -San jon,
Hughes CE 695, 21/86/07 -Nuevo León, Mexico. L. lanceolata S. Watson, Oaxaca, Mexico -10, Hughes CE 2166, -Cerro Gordo, Veracruz, Mexico
Hughes CE 1577, 90/92 -Alamos, Sonora, Mexico. Hughes CE 631, 46/85 -1, Hughes CE 2171, -La Mancha, Palmasola, Veracruz, Mexico -1,
-Michoacán, Mexico. L. lempirana C.E. Hughes, Hughes CE 1411, Hughes CE 835, 51/87 -Puerto Angel, Oaxaca, Mexico -11. L. cuspidata
6/91/03 -Negrito, Yoro, Honduras. Hughes CE 1447, 5/91 -Aguan Valley, Standley, Hughes CE 1851, 83/94 -Camarones, Hidalgo, Mexico -9,
Yoro, Honduras. L. macrophylla subsp. istmensis C.E. Hughes, Hughes Hughes CE 1586, 88/92 -Jacala, Hidalgo, Mexico -1, Hughes CE 1583,
CE 580, 47/85 -Oaxaca, Mexico. L. macrophylla subsp. macrophylla 89/92 -Mina San Miguel, Hidalgo, Mexico -1. L. esculenta (Sessé &
Bentham, Hughes CE 1179, 55/88 -Guerrero, Mexico. Hughes CE 2076, Mociño ex DC.) Bentham, Hughes CE 2114, -Miahuatlán, Oaxaca,
-Coxcatlan, Puebla, Mexico. L. magnifica (C.E Hughes) C.E. Hughes, Mexico -1, Hughes CE 2143, -Huajuapan de León, Oaxaca, Mexico -1,
Hughes CE 1089, 58/88 -Chiquimula, Guatemala. Hughes CE 412, 19/84 Hughes CE 894, 47/87 -San Martín Pachivia, Guerrero, Mexico -10,
-Chiquimula, Guatemala. L. matudae (S. Zárate) C.E. Hughes, Hughes CE Hughes CE 903, 48/87 -Michoacán, Mexico -8. L. greggii S. Watson,
2153, -San Miguel Tecuixiapan, Guerrero, Mexico. Hughes CE 879, 49/87 Hughes CE 695, 19/86 -El Barrial, Nuevo León, Mexico -6. L. lanceolata
-Guerrero, Mexico. L. multicapitula Schery, Hughes CE 1024, 86/87 S. Watson, Hughes CE 603, 44/85 -Escuinapa, Sinaloa, Mexico -7,
-Penas Blancas, Guanacaste, Costa Rica. Hughes CE 1025, 81/87 -Los Hughes CE 631, 46/85 -Playa Azul, Michoacán, Mexico -10, Hughes
Santos, Panama. Schleinitzia novo-guineensis (Warb.) Verdc., Chaplin, CE 1577, -Alamos, Sonora, Mexico -1. L. lempirana C.E. Hughes,
57/84 -Munda, Soloman Islands. L. pueblana Britton & Rose, Hughes Hughes CE 1447, 5/91 -Valle del Aguán, Yoro, Honduras -16, Hughes
CE 1648, 125/92 -Lower Tehuacan Valley, Oaxaca, Mexico. Hughes CE CE 1411, 6/91 -Cuyamapa, Yoro, Honduras -9. L. macrophylla subsp.
2089, -Cuicatlan, Oaxaca, Mexico. Hughes CE 2140, -Santo Domingo macrophylla Bentham, Hughes CE 2076, -Coxcatlan, Puebla, Mexico -1,
Tonala, Oaxaca, Mexico. L. pulverulenta (Schlechtendal) Bentham, Hughes CE 1179, 55/88 -Vallecitos, Guerrero, Mexico -10, Hughes CE
Hughes CE 1051, 83/87/02 -Tamaulipas, Mexico. Hughes CE 1058, 2156, -Grutas de Cacahuamilca, Taxco, Guerrero, Mexico -1, Hughes CE
84/87 -Texas, USA. Hughes CE 1593, -Xilitla, San Luis Potosí, Mexico. 2164, -Cerro El Encinal, Iguala, Guerrero, Mexico -1. L. macrophylla
Hughes CE 1611, -Huejutla de Reyes, Hidalgo, Mexico. Hughes CE 1866, subsp. istmensis C.E. Hughes, Hughes CE 580, 47/85 -San Isidro Llano
-Misantla, Veracruz, Mexico. L. retusa Bentham in Gray, Rajanikanth Grande, Oaxaca, Mexico -10. L. magnifica (C.E Hughes) C.E. Hughes,
& Bailey 2009, 23/09/02 -Eddy Co, New Mexico, USA. Bendeck s.n., Hughes CE 412, 19/84 -El Rincón, Chiquimula, Guatemala -10, Hughes
23/86 -Coahuila, Mexico. L. salvadorensis Standley ex Britton & Rose, CE 1089, 58/88 -El Carrizal, Chiquimula, Guatemala -8. L. matudae
Hughes CE 1407, 7/91 -San Juan de Limay, Esteli, Nicaragua. Hughes (S. Zárate) C.E. Hughes, Hughes CE 2153, -San Miguel Tecuixiapan,
CE 742, 17/86 -Choluteca, Honduras. Hughes CE 746, 34/88 -Choluteca, Guerrero, Mexico -1, Hughes CE 879, 49/87 -Mezcala, Guerrero,
Honduras. L. shannonii Donnell Smith, Hughes CE 1417, 1/91 -Santa Mexico -9, Hughes CE 2148, -San Juan Tetelcingo, Guerrero, Mexico
Caterina Mita, Jutiapa, Guatemala. Hughes CE 1676, 135/92 -Cintalapa -1. L. multicapitula Schery, Hughes CE 1025, 81/87 -Los Santos,
de Figueroa, Chiapas, Mexico. Hughes CE 1714, 141/92 -Santa Rita, Panama -13. L. pueblana Britton & Rose, Hughes CE 1648, 125/92
Yoro, Honduras. Hughes CE 507, 53/87 -Champoton, Campeche, Mexico. -Tehuacan Valley, Oaxaca, Mexico -1, Hughes CE 2089, -Cuicatlan,
L. trichandra (Zuccarini) Urban, Hughes CE 1106, 53/88 -Guatemala, Oaxaca, Mexico -1, Hughes CE 2140, -Santo Domingo Tonala, Oaxaca,
Guatemala. Hughes CE 1130, 54/88 -Huehuetenango, Guatemala. Mexico -1, Hughes CE 2077, -Teotitlán del Camino, Oaxaca, Mexico -1,
December 2011] Govindarajulu et al.—Cryptic diversity and allopatry 2063

Hughes CE 2092, -Dominguillo, Oaxaca,Mexico -1, Hughes CE -3, Hughes CE 2166, -El Zamorano, Francisco Morazan, Honduras -1.
2139, -Santo Domingo Tonala, Oaxaca, Mexico -1. L. pulverulenta L. trichandra (Zuccarini) Urban, Hughes CE 1654, 128/92 -Oaxaca,
(Schlechtendal) Bentham, Hughes CE 1051, 83/87 -Tamaulipas, Mexico -1, Hughes CE 1682, 137/92 -Chiapas, Mexico -1, Hughes CE
Mexico -4, Hughes CE 1058, 84/87 -Texas, USA -3 L. retusa Bentham 1421, 4/91 -Erandique, Lempira, Honduras -10, Hughes CE 1106, 53/88
in Gray, Bendeck s.n., 23/86 -Coahuila, Mexico -9. L. salvadorensis -Los Guates, Guatemala -5, Hughes CE 1130, 54/88 -Huehuetenango,
Standley ex Britton & Rose, Hughes CE 742, 17/86 -La Garita, Guatemala -1, Hughes CE 1708, -Erandique, Lempira, Honduras -1,
Choluteca, Honduras -8, Hughes CE 746, 34/88 -Choluteca, Honduras Hughes CE 1709, -Erandique, Lempira, Honduras -1, Hughes CE 1710,
-9, Hughes CE 36/88, 36/88 -La Garita, Choluteca, Honduras -9,
-Erandique, Lempira, Honduras -1. L. trichodes (Jacquin) Bentham,
Hughes CE 1407, 7/91 -Esteli, Nicaragua -9, Hughes CE 98/90, 98/90
-La Garela, Choluteca, Honduras -9, Hughes CE 1211, 99/90 -Namali, Hughes CE 775, 2/86 -Cuicas, Trujillo, Venezuela -9, Hughes CE 997,
Choluteca, Honduras -4. L. shannonii Donnell Smith, Hughes 61/88 -Manabi, Ecuador -11, Hughes CE 1418, 3/91 -Copan, Honduras
CE 1417, 1/91 -Jutiapa, Guatemala -10, Hughes CE 1676, 135/92 -2. L. zacapana (C.E. Hughes) R. Govindarajulu & C.E. Hughes,
-Chiapas, Mexico -1, Hughes CE 1714, 141/92 -Yoro, Honduras -1, Hughes CE 299, 18/84 -Puerto de Golpe, El Progreso, Guatemala -1,
Hughes CE 1399, 2/91 -La Puerta, Chontales, Nicaragua -4, Hughes Hughes CE 1120, 56/88 -Vallecitos, Zacapa, Guatemala -8, Hughes CE
CE 239, 22/83 -Comayagua, Honduras -9, Hughes CE 282, 26/84 299, 15/83 -Puerto de Golpe, El Progreso, Guatemala -10, Hughes CE
-Comayagua, Honduras -1, Hughes CE 507, 53/87 -Campeche, Mexico 1096, 57/88 -El Carrizal, Chiquimula, Guatemala -13.