Figure 9.—

Possible models for enhancer bypass of the Mcp and su(Hw) boundaries in trans. (A) A model depicting trans activation of the ap gene on the ap^f00451 chromosome by the ap wing enhancer on the ap^MM-Mcp chromosome (see Figure 8, B and D). The additive effects of this trans activation (solid arrow) and the activation of ap by the unblocked, ap proximal portions of the ap wing enhancer on the ap^f00451 chromosome (dashed arrow) could account for the wild-type wings observed in ap^MM-Mcp/ap^f00451 trans-heterozygotes. (B) A model depicting how homology-driven chromosome pairing between the structurally dissimilar alleles ap^MM-Mcp and ap^f00451 could cause looping out and inactivation of the boundary elements, presumably due to conformational stress. (C) A model depicting pairing between the boundaries present in the ap^MM-Mcp and ap^f00451 inserts and two hypothetical boundaries on either side of the ap regulatory region. Such pairing would function to demarcate two distinct chromosomal domains, each of which would exhibit enhancer blocking when homozygous or hemizygous, but could lead to interallelic complementation when trans-heterozygous.