EFFECTS OF CONTRASTING PATTERNS OF LARVAL DISPERSAL ON THE GENETIC CONNECTEDNESS OF LOCAL-POPULATIONS OF 2 INTERTIDAL STARFISH, PATIRIELLA-CALCAR AND P-EXIGUA

The closely related intertidal starfish Patiriella calcar and P exigua have markedly different dispersal capabilities. P calcar is considered to have sexually produced planktonic larvae and hence the potential for wide dispersal. If this is the case local populations should be strongly interconnected and there should be little or no accumulation of genetic variation. Conversely, P exigua has no known means of dispersal between local populations since development proceeds directly from egg masses laid on the rocky shore. Therefore, local populations of P exigua may be less genotypically diverse than P calcar, but should display marked genetic heterogeneity among local populations resulting from the accumulating effects of genetic drift and localised natural selection. To test these predictions genetic data were used to assess the genotypic structure and apparent connectedness of local populations of both species along 230 km of the southeast coast of Australia. Electrophoretic variation was surveyed for 5 loci. Single-locus genotype frequencies within all local populations of both species closely matched expectations for Hardy-Weinberg equilibria. This implies that local populations are maintained by recruitment of outcrossed sexually produced offspring, but as predicted, P exigua had fewer alleles at each locus and fewer heterozygous individuals than P calcar. Furthermore, it appears that these 2 species have vastly different levels of gene flow among local populations. The 6 local populations of P calcar were genetically homogeneous [F(ST) = 0 +/- 0.001 (SE)] and these populations were thus inferred to be strongly connected by larval dispersal. In sharp contrast, the 11 local populations of the directly developing P exigua were highly genetically heterogeneous which is reflected in consistently high levels of genetic variance (F(ST) = 0.462 +/- 0.048).