GENETIC-STRUCTURE OF DICTYOCERATID SPONGE POPULATIONS ON THE WESTERN CORAL SEA REEFS

Allozyme variation at six polymorphic loci was examined in foliose dic tyoceratid sponges from isolated reefs in the western Coral Sea. Four major genetic groups corresponding to the species Phyllospongia lamell osa, P. alcicornis, Carterospongia flabellifera and Collospongia auris were examined. A further two rare morphotypes from individual reefs f ormed genetic outliers to the P. lamellosa group, and may represent fu rther taxa related to P. lamellosa. Gene frequencies in individual ree f populations were largely in Hardy-Weinberg equilibrium, suggesting t hat random mating occurred in local populations of all four common spe cies. Genetic variability was high and observed heterozygosities withi n populations ranged from 0.13 to 0.40. All four taxa showed significa nt genetic differentiation among populations (F-ST=0.05 to 0.36). Gene tic distances (Nei's D) among populations within species ranged from 0 to 0.723 and increased with increasing geographical separation. There was evidence that genetic differentiation between populations to the north and to the south of the southern limit of the South Equatorial C urrent (SEC) divergence was greater than expected on the basis of thei r geographical separation. The SEC divergence may form a partial barri er to gene flow among populations of these ecologically important spon ges on the submerged Queensland Plateau. Levels of migration among pop ulations of three of the species was less than those required to preve nt divergence of the populations through genetic drift (Nm<1). Restric ted migration among populations may provide a mechanism to explain the occurrence of highly divergent populations of dictyoceratid sponges w hose specific identity is not clear, and may allow them additionally t o develop partial reproductive isolation from other populations.