GENETIC AND COLOR INTERACTIONS AT A CONTACT ZONE OF ACANTHOCHROMIS-POLYACANTHUS - A MARINE FISH LACKING PELAGIC LARVAE

Acanthochromis polyacanthus is an unusual tropical marine damselfish t hat uniquely lacks pelagic larvae and has lost the capacity for broad- scale dispersal among coral reefs. Different color morphs exist in dif ferent regions of the Great Barrier Reef, and morphs from northern and southern regions are genetically distinct. In the Hydrographers Passa ge area, which is a large break through the reef matrix in the central Great Barrier Reef that may have acted as a bottleneck on the migrati on of these animals during sea level rise, three morphs recognized fro m other regions were found on neighboring reefs. The transition betwee n them is abrupt with three loci (AAT-2, GPI-1* and PGM*) showing all elic frequency patterns close to fixation between opposite alleles wit hin a few kilometers. On two reefs (Hyde, Bebe), a pair of morphs was found to coexist and exhibited a habitat partitioning pattern with eac h morph restricted to one side on the reef and steep transitions in be tween. Outside these transition zones, phenotypes and genotypes matche d those on surrounding reefs without coexistence and were Little chang ed from reefs several hundred kilometers away. An electrophoretic surv ey across one transition zone on Hyde Reef showed steep genetic gradie nts along one kilometer of reef slope. Significant linkage disequilibr ia in samples collected in Hyde Reef as a result of dispersal of paren tal combinations of alleles into the center or because parental combin ations of alleles confer greater fitness, allowed us to estimate the d ispersal rate (189 m/generation) and the selection pressure on the mar ker loci (0.411). Finally, we investigated models that could lead to s uch a steep transition in genotypic and phenotypic combinations. Both contact zones on each side of Hyde Reef were associated with geomorpho logical discontinuities in the reef structure. We suggest that assorta tive mating may be a proximal mechanism for maintaining isolated each color morph, which could be reinforced by selective predation against hybrids outside the zone of their formation (i.e., the frequency depen dent selection model of Mallet and Barton (1989). Acanthochromis is a midwater planktivore and, when in coexistence, the two morphs forage i n different habitats amid multispecific Becks of other damselfishes of matching colors.