Population genetics of the fissiparous holothurians Stichopus chloronotus and Holothuria atra (Aspidochirotida): a comparison between the Torres Strait and La Réunion

Collections of about 50 individuals from each of five populations of the fissiparous holothurian species Stichopus chloronotus and four populations of Holothuria atra were made in 1999. These populations were located in the Torres Strait (western Pacific) and La Réunion (western Indian Ocean). Allozyme electrophoretic surveys of five (S. chloronotus) and six (H. atra) loci were conducted to compare patterns of asexual reproduction and to investigate connectivity between regions separated by large geographic distances. Deviations from genotype frequencies expected under Hardy-Weinberg equilibrium, mostly heterozygote excesses, were observed in all populations of both species. The maximum contribution of sexual reproduction (calculated as the maximum number of sexually produced individuals: sample size=N*/N_i) was similar for all S. chloronotus (58-64%) and H. atra (76-92%) populations, and on the same level as previously reported for midshelf reefs of the Great Barrier Reef. The higher values in the latter species indicated greater contributions of asexual reproduction to S. chloronotus populations. Variability was strongly reduced in S. chloronotus populations at La Réunion, with only one locus being variable in that population. When the dataset was reduced to one representative per multi-locus genotype per population to reduce the effect of asexual reproduction on calculations on gene flow, F_ST values were not significantly different from zero, suggesting high gene flow between these regions. However UPGMA cluster analyses using Rogers' genetic distance, roughly clustered populations by region. In the case of H. atra, pooled populations within each region were significantly different from those of the other region. Thus, although some restrictions in gene flow and greater genetic distances between the regions may exist, those differences are distinctly less than those reported in previous studies on echinoderms over similar geographic scales. Despite the importance of asexual reproduction for the maintenance of local population size, this study also confirmed that the potential for widespread dispersal mediated by sexually produced larvae is large.