GENETIC-RELATIONSHIPS OF THE COLOR MORPHS OF ACANTHOCHRONIS-POLYACANTHUS (POMACENTRIDAE) ON THE NORTHERN GREAT-BARRIER-REEF

Acanthochromis polyacanthus is an unusual tropical damselfish because it lacks pelagic larvae and has lost the capacity for broad-scale disp ersal among coral reefs. On the modern Great Barrier Reef, this fish h as multiple colour morphs and we have previously described steep genet ic dines in the central region between three colour morphs. A similar transition of colour morphs has been found in the far northern region (Latitude 11 degrees S), where at least five colour morphs and intergr ading hybrids were found during a survey of 14 locations on nine adjac ent reefs. On two of these reefs, monochromatic white and bicoloured m orphs coexist, partitioning the reef slope vertically when sympatric, but occupying the full depth range elsewhere. In areas of overlap, the y interbreed, producing hybrids of intermediate colour. Although the d iscriminatory power of our genetic analysis was weakened by the low le vel of variability in these populations (only four polymorphic loci), allozyme electrophoresis revealed significant genetic differentiation within and among reefs. There was neither simple linkage between colou r and protein variation, nor correlation between genetic similarity an d proximity of sampled populations. We suggest that the observed chaot ic structure is the result of differences among the founding populatio ns, although the two colour-based stocks have been homogenized through a long history of introgression during multiple episodes of isolation and secondary contact. Despite genetic mixing, phenotypic diversity ( i.e. colour morphs) seems to have been preserved in these populations by sexual selection, perhaps reinforced by natural selection. The long -term stability of this outcome, however, is suggested by the presence of unique morphs on two reefs that are detached from the continental shelf and surrounded by very deep water. The strong identity of these populations can be attributed to long occupation of these isolated sit es compared with those on the continental shelf which have only been e stablished in the last 15 000 yr and still retain differences arising from the initial founder events.