ENZYME ELECTROPHORETIC EVIDENCE FOR THE PREVALENCE OF OUTCROSSING IN THE HERMAPHRODITIC BROODING ASCIDIAN DENDRODOA-GROSSULARIA (CHORDATA, UROCHORDATA)

The genetic structures of three relatively dense populations of the sessile solitary tunicate Dendrodoa grossularia (van Beneden) were examined using enzyme electrophoresis to study a single polymorphic locus (Pgi). Little or no genetic differentiation was apparent on a scale of tens of metres, and only moderate differentiation was observed between localities 25-1 10 km apart. Levels of heterozygosity yielded estimates of the outcrossing rate ranging from 0.84 to 1.18 at the three localities; values of the fixation index F(IS) were; not significantly different from zero. The minimum outcrossing rate giving no statistically significant deviation from observed genotype frequencies (alpha = 0.05) ranged from 0.62 to 0.80 at the three localities. The genotypes of reared young of known brood parents confirmed the occurrence of extensive outcrossing during internal fertilization, and yielded an estimate of the outcrossing rate, based on 16 broods, in excess of 1.0. These results demonstrate unequivocally that exogenous sperm are able to reach the site of internal fertilization in a solitary brooding ascidian.