Genetic structure of populations of two closely related brittle stars with contrasting sexual and asexual life histories,with observations on the genetic structure of a second asexual species

Ophiocoma pumila Lütken andOphiocomella ophiactoides (H. L. Clark) are morphologically similar brittle stars with contrasting life histories, the former obligately sexual, the latter fissiparous (capable of both sexual reproduction and asexual proliferation by binary fission). Electrophoretic analysis of five polymorphic enzymes was used to assess the genetic consequences of these differing life histories and provide a genetic perspective on the taxonomic relationship between the two species. Genotypic diversity ofOphiocoma pumila collected at Discovery Bay, Jamaica, in 1985 conformed to expectations for a sexually reproducing population. In contrast, genotypic diversity ofOphiocomella ophiactoides at this site was significantly lower than expected for a sexually reproducing population, due largely to the predominance of clonal proliferation over larval recruitment. Large variation in clonal composition over a short (50 m) distance emphasized the very localized scale of clonal mixing in this species. Allozymic data are indicative of a close sibling species relationship betweenOphiocoma pumila andOphiocomella ophiactoides which suggests that the present generic separation of the two species should be re-examined. Electrophoretic analysis was also used to examine the genetic structure of sponge- and alga-dwelling populations of a second fissiparous brittle star,Ophiactis savignyi (Müller & Troschel), which was also collected at Jamaica in 1985. Striking differences in the allelic composition of sponge- and alga-dwellingO. savignyi were observed. Genotypic diversity ofO. savignyi in sponges was very low, each sponge being dominated by a single genotype. Genotypic diversity ofO. savignyi in algae was higher, although still significantly lower than expectations for a sexually reproducing population. In the light of the highly clonal composition of fissiparous brittle-star populations, the adaptive significance of clonal growth may be related to an increase in the overall fitness of dispersed clones (genets), compared to individuals of strictly sexual counterparts, through greater genotypespecific biomass and, hence, fecundity.Contribution No. 476 of the Discovery Bay Marine Laboratory of the University of the West Indies