Heritability of heat tolerance in a small livebearing fish, Heterandria formosa

Author's personal copy Ecotoxicology (2011) 20:535–542 DOI 10.1007/s10646-011-0624-2 Heritability of heat tolerance in a small livebearing fish, Heterandria formosa Cathleen M. Doyle • Paul L. Leberg Paul L. Klerks • Accepted: 21 February 2011 / Published online: 4 March 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Climate change is expected to result in an increased occurrence of heat stress. The long-term population-level impact of this stress would be lessened in populations able to genetically adapt to higher temperatures. Adaptation requires the presence of geneticallybased variation. At-risk populations may undergo strong declines in population size that lower the amount of genetic variation. The objectives of this study were to quantify the heritability of heat tolerance in populations of the least killifish, Heterandria formosa, and to determine if heritabilities were reduced following a population bottleneck. Heritabilities of heat tolerance were determined for two lines of each of two source populations; two bottlenecked lines (established with one pair of fish) and two regular lines. Heat tolerance was quantified as temperature-atdeath (TAD), when fish acclimated at 28°C were subjected to an increase in water temperature of 2°C/day. Mid-parent/mean offspring regressions and full-sib analyses were used to estimate the heritability of TAD. Heritability estimates from parent/offspring regressions ranged from 0.185 to 0.462, while those from sib analyses ranged from 0 to 0.324, with an overall estimate of 0.203 (0.230 for the regular lines, 0.168 for bottlenecked ones). Fish from the bottlenecked line from one source population (but not C. M. Doyle
P. L. Leberg
P. L. Klerks (&) Department of Biology, University of Louisiana at Lafayette, Box 42451, Lafayette, LA 70504-2451, USA e-mail: klerks@louisiana.edu Present Address: C. M. Doyle Limnology Laboratory, The Ohio State University, 1522 Museum of Biological Diversity, 1315 Kinnear Rd., Columbus, OH 43212, USA the other) had a lower heritability than did those from the regular line. These results show that the populations tested had some potential for adaptation to elevated water temperatures, and that this potential may be reduced following a population bottleneck. This should not be construed as evidence that natural populations will not suffer negative consequences from global warming; this study only showed that these specific populations have some potential to adapt under a very specific set of conditions. Keywords Heat tolerance
Heritability
Genetic adaptation
Population bottleneck
Climate change
Evolutionary processes Introduction Many species find themselves present in an environment that is changing as a consequence of human activities. One major force behind current and especially future changes in species’ environments is climate change. Anthropogenic climate change is clearly occurring and global temperatures have already increased (IPCC 2007a). There is also evidence that these climate changes are having ecological consequences (IPCC 2007b). As specific examples, Grebmeier et al. (2006) found changes in benthic–pelagic coupling related to the timing of seasonal ice cover in the North Bering Sea, and Hari et al. (2006) found higher mortality rates in brown trout due to a disease mediated by increased water temperatures in rivers and streams in parts of the Swiss Alps. Reviews by Walther and co-workers (Walther 2010; Walther et al. 2002) also found ample evidence of climate change impacts in various ecosystems and at various levels of biological organization. Climate change is thus likely to result in heat stress for many 123 Author's personal copy 536 populations. While these impacts may bring about local extinctions and changes in geographic distributions, negative impacts also exert selective pressures that may ultimately result in adaptation if conditions are favorable and if heritable variation is present for the affected trait. This paper is focusing on fishes. Because most fishes are ectotherms, their body temperature conforms to the surrounding water temperature (Beitinger et al. 2000; Goldspink 1995). Maintaining thermal equilibrium with the surrounding water directly affects their internal metabolism and physiological functions (Beitinger et al. 2000; Elliott 1981; Goldspink 1995). While many studies have addressed heat tolerance, very few have sought to determine if heat tolerance has a genetic basis. The presence of genetic variation in heat tolerance may allow a population to survive climate change despite the detrimental effects of high water temperature. Population traits such as heat tolerance are often complex traits that exhibit continuous variation and have inheritance patterns that involve many genes (so called ‘‘quantitative traits’’) and may also involve genotype by environment interactions (Falconer and Mackay 1996; Hartl 2000). A good measure of the extent to which such a trait can respond to natural selection is its ‘‘narrow sense heritability’’ (designated as h2 and referred to as ‘‘heritability’’ in the remainder of this paper). This heritability is the ratio of additive genetic variance (VA) to the total phenotypic variance (VP) in the population (Falconer and Mackay 1996). Another heritability term in use is the ‘‘broad sense heritability’’ or ‘‘degree of genetic determination’’, designated as H2, defined as the ratio of genetic variance to the total phenotypic variance, but not as useful for predicting a population’s response to selection (Falconer and Mackay 1996). Heritabilities can range from 0 to 1. More in-depth introductions to quantitative genetic variation and heritabilities are provided by Falconer and Mackay (1996) and Roff (1997). Few studies have estimated the heritability of thermal tolerance in fish species. Meffe et al. (1995) studied a population of eastern mosquitofish (Gambusia holbrooki) and found a heritability of 0.32 for heat tolerance measured as temperature at death. In contrast, Baer and Travis (2000) selected the least killifish (Heterandria formosa) for an increased resistance to heat or cold, but did not detect a significant response to selection. They concluded that the heritability was not likely to be more than 0.15 in their laboratory population. Finally, Charo-Karisa et al. (2005) estimated the heritability of cold tolerance in juvenile Nile tilapia (Oreochromis niloticus) using a full-sib/half-sib design, and reported a low heritability (estimated to be 0.09). The amount of genetic variation may be reduced in a population that has undergone a drastic reduction in population size. Such a population bottleneck may be an 123 C. M. Doyle et al. indirect result from climate change, e.g., due to loss of suitable habitat or due to a sudden fish kill caused by lowered levels of dissolved oxygen. While a population bottleneck will generally result in a loss of genetic variation (Falconer and Mackay 1996), heritabilities may actually increase. The latter was observed in populations of the housefly (Musca domestica), where the bottleneck ‘‘converted’’ nonadditive genetic variance into additive genetic variance (Bryant et al. 1986). Increases in additive genetic variance following a population bottleneck appear more likely in populations that already have a low level of additive genetic variance (López-Fanjul et al. 1999) or a high level of dominance variance (Saccheri et al. 2001). Also, conversion of nonadditive into additive genetic variation is more likely for characters affected by many loci than for those affected by few loci (Naciri-Graven and Goudet 2003). This study’s objectives were to quantify the heritability of heat tolerance in populations of the least killifish (H. formosa), and to determine whether this heritability was affected by a population bottleneck. Heat tolerance was determined using the chronic lethal method (CLM); a method commonly used when determining thermal tolerance (Beitinger et al. 2000; Elliott 1981). Heritabilities of heat tolerance were quantified on the basis of resemblance between relatives, using the parent–offspring regressions and full-sib analyses that are used routinely in quantitative genetics (Falconer and Mackay 1996). The least killifish is eurythermal, and is found in waters with temperatures ranging from 7 to 39°C (Forster-Blouin 1989; Leips and Travis 1999). In the context of climate change, adaptation to elevated temperatures may be more relevant for stenothermal species. However, species that occur over a wide range of temperatures may be encountering stressful temperatures at the edge of their distribution and may have insufficient genetic variation to respond to selection for further increases in their heat tolerance. While genetic variation for heat tolerance had previously been studied in the least killifish (Baer and Travis 2000), the present study nevertheless used this same species. Heritability estimates are population and method specific (Roff 1997), thus the outcome of the present study would provide an additional and partially independent estimate. Moreover, the approach used by Baer and Travis (2000) may not have been optimal for eliciting a response to selection. That study used a relatively-low selection intensity (low, at least, for observing a response in a few generations) and the actual selection intensity was further reduced by the inclusion of adult females that likely had mated prior to the selection step. Also, the loss of several of that study’s laboratory lines and the inadvertent occurrence of population bottlenecks complicated the interpretation of the results. Author's personal copy Heritability of thermal tolerance Materials and methods Least killifish natural history The least killifish, H. formosa, Family Poeciliidae, is a small livebearing fish found in the coastal southeastern United States, ranging from the Cape Fear River drainage, North Carolina, throughout Florida and into southern Louisiana (Page and Burr 1991). It prefers slow-moving, heavily vegetated, shallow, and fresh to brackish water (Baer and Travis 2000; Page and Burr 1991). Its short generation time (2–3 months) makes it particularly suitable for determining heritabilities. Experimental design and methodology Fish were taken in 2002 from two locations situated approximately 105 km apart; one just outside the Lacassine Refuge (Cameron Parish, Louisiana) and the other in the Atchafalaya Basin (St. Martin Parish, Louisiana). There was no specific reason for collecting fish from these localities, other than the desire to determine heritabilities for more than a single population—as heritability estimates may be population-specific (Falconer and Mackay 1996). Approximately 120 fish from each location were set up in tanks in a greenhouse at the University of Louisiana at Lafayette’s Center for Ecology and Environmental Technology (CEET). Bottlenecked and non-bottlenecked (regular) lines were established for each source population, approximately one generation after field-collection. Each regular line was started with 30 males and 30 females, while each bottlenecked line was started with one pair consisting of a male and virgin female. The populations for the bottlenecked lines were allowed to build up, without further bottlenecking, to a population size of at least 120 adults. At this point, the fish populations had been at CEET for approximately 3–4 generations since collection from the wild. The actual breeding experiments for heritability analyses were then started using newborn offspring for each of the four lines. Newborn fish (\10 mm) were set up in pairs. The fish were sexed after a few weeks; male–female pairs were kept as such, while fish of same-sex pairs were used to establish additional male–female pairs (feasible since fish in samesex pairs were still virgin). These pairs formed the parental generation. Each pair of fish was housed in a 4.73-l polyethylene bucket, and buckets were randomly placed in water-filled tanks (213 cm L 9 56 cm W 9 30 cm D)— approximately 40 buckets per tank. Each bucket had two or three mesh-covered holes to allow water exchange. Each pair of tanks had their own water system with the water recirculating through the two tanks and a filter tank (244 cm L 9 61 cm W 9 61 cm D). Fish were fed daily 537 with a combination of TetraminÒ flake food and newly hatched brine shrimp. Tanks were set up in the CEET greenhouse (Lacassine population) or in an air-conditioned laboratory building at CEET (Atchafalaya population); this design enhanced chances of obtaining enough offspring for experiments from at least one of the source populations. This meant that comparisons between the two source populations would not be valid—but such a comparison was not an objective of this study. Offspring were regularly separated from the parents and raised to sexual maturity in separate buckets (one bucket per group of offspring from a pair of adults). This was continued till sufficient offspring were obtained in a family. The goal was to obtain at least ten offspring (five adult males and five adult females) from each of 40 parental pairs for each of the four lines, but these targets were not always reached (due to some adults dying before sufficient offspring were produced). The following numbers of families were used for this study: 43 from the Lacassine non-bottlenecked line, 40 from the Lacassine bottlenecked line, 19 from the Atchafalaya nonbottlenecked line, and 14 from the Atchafalaya bottlenecked line. These were the samples sizes available in the full sib analyses. Sample sizes for the parent–offspring regressions were somewhat lower because heat exposures of parents were postponed till their offspring had also reached the adult stage (so that all family-members could be exposed at the same time). This resulted in an average of 16% of the parental fish having died before their thermal tolerance could be quantified. To quantify heat resistance, fish were individually exposed to 750 ml heated water in 1 l tri-pour beakers with screen-covered holes inside a 26.6 l container placed in a Percival Scientific I-36LL environmental chamber (Percival Scientific, Boone IA, USA). The water in the 26.6 l container was aerated vigorously to establish uniform temperature conditions among the 12 beakers. A 12 h light:12 h dark day cycle was maintained in the chamber. Three different incubators were used, with generally three shelves (each containing one 26.6 l container) per incubator. Fish were randomly assigned to a specific container and incubator. Fish were fed newly hatched brine shrimp daily throughout the exposure. The fish were acclimated at an incubator air temperature of approximately 28°C for 7 days before the thermal gradient was started. The environmental chamber was programmed to increase the air temperature by 2°C/day beginning at 28°C, until total mortality was reached among the exposed fish. Criterion for death was complete cessation of external movement and gulping motion (cessation of opercular motion). Because temperature-at-death (TAD) was the main response variable, fish survival was checked every 2–3 h once the lethal temperature was approached. More frequent checks were not possible since heat loss from opening the 123 Author's personal copy 538 chamber door caused a temporary set-back to the desired gradual increase in water temperature. The water temperature of each 26.6 l container was determined using a 4400 Series NIST Traceable Digital Thermometer with 0.01°C precision and 0.02°C accuracy (Alpha Technics, Anaheim CA, USA) each time that fish survival was checked. The TAD of a fish was calculated as the average of the water temperatures at the time that the fish was classified as dead and that recorded at the previous time period. Dead fish were removed and their standard length was determined. To verify that the cause of mortality was temperature and not elevated ammonia levels, ammonia concentrations were monitored during some of the exposures. Water samples were taken at the beginning and end of the acclimation period, at the time point when the first mortality occurred, and at the time point where total mortality was reached. Samples were analyzed for total ammonia using a Lachat Instruments QuickChem FIA ? 8000 series autoanalyzer (Lachat Instruments, Milwaukee WI, USA). Concentrations of un-ionized ammonia (NH3) were determined using the aqueous ammonia equilibrium equations based on water pH and temperature (Emerson et al. 1975). Both total ammonia (NH4? ? NH3) and unionized ammonia levels that the fish encountered were always well below the U.S. Environmental Protection Agency recommended maximum levels (USEPA 1998). Statistical analysis Data from fish that died before the water temperature reached 37°C (approximately 1% of all deaths) were excluded from the analysis. It was assumed that these deaths were not heat related. A survival analysis was performed on the full dataset (combining data for all four treatment types—bottlenecked and non-bottlenecked lines for each of the two source populations) to determine the overall effects of sex, age, source population, and bottleneck history on TAD, using a lognormal regression model in StatViewÒ version 4.0 (Abacus Concepts, Berkeley CA, USA). In order to standardize results from the thermal tolerance quantifications and to be able to get a more precise estimate of each fish’s innate thermal tolerance, thermal tolerances were expressed as deviations from expected temperaturesat-death. A survival analysis was done for each source population separately (Lacassine and Atchafalaya) to determine what factors were responsible for variation in temperature at death. A lognormal regression model was used in survival analysis to determine the effects of age, sex, size, incubator, and shelf location within each incubator using StatViewÒ. All of the effects were significant except, for the Atchafalaya population, the effects of fish size and shelf within incubator. Therefore, a fish’s expected 123 C. M. Doyle et al. TAD was calculated on the basis of its characteristics (age, sex, and size) and testing conditions (shelf and incubator used for the exposure), using the regression coefficients from the survival analysis. The difference between the observed TAD for a fish and the expected TAD for this fish was used as a measure of a fish’s innate temperature tolerance. For consistency, the effects of fish size and shelf location within the incubator were also taken into account for the Atchafalaya line, even though these effects were not statistically significant. Identical corrections were used for the non-bottlenecked and bottlenecked lines of each source population. Heritabilities were estimated from mid-parent/mean offspring regressions for all lines. JMP INÒ version 5.1 (SAS Institute Inc., Carey NC, USA) was used for these regression analyses. For mid-parent/mean offspring regressions, the regression coefficient is the heritability estimate. These regressions are well suited for providing estimates of the narrow sense heritability, since resemblances between parents and offspring do not contain any non-additive genetic components (Falconer and Mackay 1996). The regressions were weighted by the number of offspring produced by each parental pair, necessitated by the fact that numbers of offspring per family were unequal. The number of offspring used for a family ranged from 2 to 12, and averaged 9.9 for the Lacassine non-bottlenecked line, 9.6 for the Lacassine bottlenecked line, 8.8 for the Atchafalaya non-bottlenecked line, and 7.4 for the Atchafalaya bottlenecked line. Since mid-parent/mean offspring regressions do not provide valid heritability estimates if variances differ between males and females (Falconer and Mackay 1996), Fmax tests were done to determine whether variances differed between males and females. A Bonferroni adjustment was used since multiple tests were conducted. The variances did not differ significantly between the sexes. A full-sib analysis was done for each line to estimate heritability without reference to the parents. The full-sib analyses were based on analysis of variance (ANOVA) using JMP INÒ. The total observed variance was partitioned into two components; between full-sib families and within full-sib families (Falconer and Mackay 1996). Because of unequal family sizes, a weighted estimate for family size was used and the standard error was adjusted for unequal family size (Roff 1997). One sample t-tests were performed for both ANOVA and regression-based heritability estimates to assess whether estimates were significantly higher than zero (one-tailed test). Also, a z-test was conducted to determine whether there were differences between bottlenecked and non-bottlenecked lines for each source population. Since the covariance of full sibs is not only due to additive genetic variance but also a portion of the dominance variance, heritability Author's personal copy Heritability of thermal tolerance 539 estimates based on full sibs tend to overestimate narrow sense heritability (Falconer and Mackay 1996). Results Heat tolerance differed among sexes and among generations (Table 1). TAD was higher in females than it was in males (v2 = 396.467, p \ 0.0001), higher in adult offspring than in parents (v2 = 130.474, p \ 0.0001), higher in fish from the Atchafalaya population than in those from the Lacassine population (v2 = 21.554, p \ 0.0001), and higher in fish from the regular lines than in those from the bottlenecked lines (v2 = 48.081, p \ 0.0001). Also, TAD was negatively related to fish size (v2 = 21.946, p \ 0.0001; data not shown). As described earlier, an individual’s difference between expected TAD (expected on basis of its size and other characteristics, as well as specific testing conditions) and observed TAD was used as the basic datum in determining heritabilities of heat tolerance. The heritability estimates obtained from the mid-parent/mean offspring regressions ranged from 0.19 to 0.46 (Table 2). The estimate for the regular Lacassine line was not significantly higher than zero (t = 1.312, p = 0.0965), while the estimate for the Lacassine bottlenecked line was (t = 2.007, p = 0.0279). However, these two estimates did not differ statistically (z = -0.5864, p = 0.5576). The heritability estimate of the Atchafalaya non-bottlenecked line was numerically the highest among the four lines (Table 2), though this estimate was not significantly higher than zero (t = 1.457, p = 0.0829). In contrast, the numerically lower heritability estimate for the Atchafalaya bottlenecked line was statistically higher than zero (t = 2.183, p = 0.0359). When comparing the estimates between the regular and bottlenecked lines of the Atchafalaya population, the numerical Table 1 Comparisons of average unadjusted temperature-at-death (TAD ± standard deviation, SD) in °C, for the Lacassine and Atchafalaya H. formosa populations, non-bottlenecked (NB) and bottlenecked (B) lines Group Source and line Lacassine Atchafalaya NB B NB B Fathers 38.27 ± 0.33 (39) 38.23 ± 0.19 (32) 38.29 ± 0.22 (19) 38.37 ± 0.17 (14) Mothers Sons 38.77 ± 0.21 (39) 38.71 ± 0.36 (210) 38.55 ± 0.27 (32) 38.59 ± 0.34 (190) 38.84 ± 0.21 (16) 38.88 ± 0.29 (78) 38.74 ± 0.24 (7) 38.80 ± 0.31 (59) Daughters 39.20 ± 0.32 (215) 38.99 ± 0.45 (197) 39.32 ± 0.24 (90) 39.25 ± 0.23 (44) All males 38.64 ± 0.41 (249) 38.54 ± 0.35 (222) 38.77 ± 0.36 (97) 38.72 ± 0.33 (73) All females 39.13 ± 0.34 (254) 38.93 ± 0.46 (229) 39.25 ± 0.29 (106) 39.18 ± 0.29 (51) Values are listed by combination of sex and generation and again by sex. Sample sizes are listed in parentheses. Overall, TAD was higher in females than in males, higher in offspring than in parents, higher in Atchafalaya fish than in Lacassine fish, and higher in regular lines than in bottlenecked lines Table 2 Heritability estimates (with standard errors) for heat tolerance quantified as temperature-at-death (TAD), for the least killifish H. formosa of the Lacassine and the Atchafalaya populations Source Non-bottlenecked line n Bottlenecked line 2 h ± SE h2 ± SE n (A) Mid-parent/mean offspring heritability estimates Lacassine population 35 0.185 ± 0.141 (ns) - 26 0.307 ± 0.153 (*) Atchafalaya population 16 0.462 ± 0.317 (ns) - 7 0.275 ± 0.126 (*) Lacassine population 43 0.138 ± 0.070 (*) - 40 0.155 ± 0.077 (*) Atchafalaya population 19 0.324 ± 0.030 (*) ? 14 Zeroa (ns) (B) Full-sib heritability estimates Heritabilities were obtained (A) from mid-parent/mean offspring regression analyses, and (B) from full-sib analyses. Indicated in parenthesis is whether heritabilities were ‘‘(*)’’ or were not ‘‘(ns)’’ significantly higher than zero; ‘‘-’’ and ‘‘?’’ signs indicate whether estimates differed significantly between non-bottlenecked and bottlenecked lines n number of families, h2 heritability, SE standard error a Actual estimate was -0.107 ± 0.081 123 Author's personal copy 540 value was higher for the regular line than the bottlenecked Atchafalaya line—though the estimates did not differ statistically (z = 0.4427, p = 0.3290). The estimates obtained from the full-sib analyses tended to be numerically lower than the mid-parent/mean offspring regression estimates (Table 2). Heritability estimates ranged from 0 to 0.324, with all three positive estimates being significantly higher than zero in one-sample t-tests (p values for Lacassine non-bottlenecked line, Lacassine bottlenecked line and Atchafalaya non-bottlenecked line being respectively 0.028, 0.026 and \0.0001). The full-sib heritability estimates did not differ statistically between the bottlenecked and non-bottlenecked Lacassine lines (z = 0.1634, p = 0.8702). In contrast, the heritability estimates of the non-bottlenecked and bottlenecked Atchafalaya lines differed drastically (z = 4.9898, p \ 0.0001). Because the full-sib based estimates did not appear to overestimate the heritabilities, combining the parent–offspring and full-sib based estimates seems reasonable and provides a single estimate facilitating further discussion. Combining the estimates, with averages weighted by the sample size of each estimate, yielded heritabilities of heat tolerance in the regular and bottlenecked lines of 0.230 and 0.168, respectively. Discussion This study’s encountered differences in heat tolerance between the sexes and among least killifish populations, are consistent with previous studies (Baer and Travis 2000; Forster-Blouin 1989; Klerks and Blaha 2009). ForsterBlouin (1989) found heat tolerance differed between two Florida populations of H. formosa, and that adult female fish had a higher heat tolerance than did adult males. In the present study, offspring also had a higher heat tolerance than did their parents, even though all fish were exposed as adults. However, as parents were tested after sufficient offspring were produced, they were generally older than their offspring at the time of heat tolerance quantification. It was logistically impossible to test all fish at the exact same age. Thus the difference in heat tolerance between the two generations may have been due to age differences. There is some evidence of age affecting thermal tolerance in fish (Benfey et al. 1997). In the present study, differences in heat tolerance due to size, sex, testing conditions, etc., would bias the heritability estimates (inflate the nongenetic variance). Therefore, heritabilities were determined using the difference between each fish’s observed and expected TAD (with the expected TAD based on the fish’s characteristics and specific testing conditions—see ‘‘Materials and methods’’ section) rather than the raw TAD data. 123 C. M. Doyle et al. For all lines, the full-sib heritability estimates tended to be numerically lower (though more often statistically ‘‘larger than zero’’ as a consequence of higher precision) than those obtained from the mid-parent/mean offspring regressions. This was surprising, because heritability estimates obtained from full-sib analyses tend to overestimate the actual heritability. The overestimate is due to the resemblance among full sibs being not only due to the shared additive genetic variance, but also due to a shared portion of the dominance variance and due to sibs often having a common rearing environment (Falconer and Mackay 1996). Dominance variance may be relatively high, especially for traits with close ties to fitness (Crnokrak and Roff 1995). Estimates based on mid-parent/mean offspring regressions contain only VA. This indicates that VD and the VE from the common rearing environment were low, thus the full-sib estimates were used as additional estimates of heritability of thermal tolerance. Heritability estimates for the bottlenecked and nonbottlenecked lines were not totally consistent. However, overall averages were lower for the bottlenecked lines than the regular lines and the one case where a statistically significant difference was found between the bottlenecked and regular lines involved a higher heritability in the regular line. Thus there is some evidence that the population bottleneck had resulted in a reduced ability for natural selection to bring about an adaptation to elevated water temperatures. In general, one would expect a decrease in heritable variation with loss of genetic variability from a bottleneck or founder event (Falconer and Mackay 1996; Wright 1931). However, this is not always the case. Bottleneck events have been found to ‘‘convert’’ non-additive genetic variance to additive genetic variance, and this conversion is likely in specific situations discussed earlier in this paper (Bryant et al. 1986; López-Fanjul et al. 1999; Naciri-Graven and Goudet 2003; Saccheri et al. 2001). Most studies that have estimated heritability of thermal tolerance in aquatic organisms have reported relatively low heritabilities. For example, Elderkin et al. (2004) found an overall heritability estimate of 0.327 in zebra mussel veligers. However, this estimate (obtained using resemblance among sibs) appeared to be inflated by maternal and/or environmental effects, and the more reliable heritability estimate based on the sire component was basically zero. Also, Baer and Travis (2000) estimated heritability for thermal tolerance in H. formosa to be B0.15, and Charo-Karisa et al. (2005) determined that heritability for cold tolerance in the Nile tilapia was low as well (h2 = 0.09 ± 0.19 for TAD). Meffe et al. (1995) reported a somewhat higher heritability; an estimated heritability of 0.32 for TAD. The overall heritability estimate of 0.20 falls in the middle range of these various estimates, and is consistent with other heritability studies involving Author's personal copy Heritability of thermal tolerance physiological traits (Meffe et al. 1995; Mousseau and Roff 1987). In conclusion, this study found a low-moderate heritability of heat tolerance in two laboratory populations of H. formosa. This indicated that these fish have some potential to adapt to global warming, since the heritabilities indicate that these populations can show a response, albeit a slow one, when selected for an increased thermal tolerance. Various factors will determine whether this potential will be realized in natural populations undergoing heat stress resulting from climate change. It is known that heritabilities are specific to the trait used (this study’s TAD at a temperature increase of 2°C/day is of course far removed from any global change scenario), the specific population and species being tested, and by the structure of the model used to estimate heritabilities (Wilson 2008; Chown et al. 2009). Laboratory to field extrapolation may also be an issue; while it appears that lab and field estimates are generally similar (Weigensberg and Roff 1996), this conclusion may not hold for traits that are not closely related to fitness (Charmantier and Garant 2005). Another factor that may reduce the longterm response to selection is the presence of fitness costs that may be associated with adaptations; such costs were reported for the same least killifish species responding to selection for resistance to cadmium (Xie and Klerks 2004). And finally, this study’s results may not be transferable to larger and more stenothermal fish species. Thus while the current research indicates that certain fish populations have some potential to genetically adapt to climate change, further insights are needed in order to assess the long-term ecological impacts of elevated temperatures. Acknowledgments The authors thank Susan deVries and Margo Blaha for assistance in maintaining fish populations; the university’s Center for Ecology and Environmental Technology for housing of the fish; and James Albert, Johannes Rick and anonymous reviewers for comments on earlier versions of this paper. This research was supported by a combined grant from the USEPA and the Louisiana Board of Regents Support Fund. Although the research described in this article has been funded, in part, by the United States Environmental Protection Agency through grant R-82942001-0 to the Louisiana Board of Regents, it has not been subjected to the Agency’s peer and policy review process and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. References Baer CF, Travis J (2000) Direct and correlated responses to artificial selection on acute thermal stress tolerance in a livebearing fish. Evolution 54:238–244 Beitinger TL, Bennett WA, McCauley RW (2000) Temperature tolerances of North American freshwater fishes exposed to dynamic changes in temperature. Environ Biol Fish 58:237–275 Benfey TJ, McCabe LE, Pepin P (1997) Critical thermal maxima of diploid and triploid brook charr, Salvelinus fontinalis. 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