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

Collections of about 50 individuals from each of five populations of the fi ssiparous holothurian species Stichopus chloronotas and four populations of Holothuria atra were made in 1999. These populations were located in the T orres Strait (western Pacific) and La Reunion (western Indian Ocean). Alloz yme electrophoretic surveys of five (S. chloronotus) and six (H. atra) loci were conducted to compare patterns of asexual reproduction and to investig ate connectivity between regions separated by large geographic distances. D eviations from genotype frequencies expected under Hardy-Weinberg equilibri um, 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 Grea t Barrier Reef. The higher values in the latter species indicated greater c ontributions of asexual reproduction to S. chloronotus populations. Variabi lity was strongly reduced in S. chloronotus populations at La Reunion, with only one locus being variable in that population. When the dataset was red uced to one representative per multilocus genotype per population to reduce the effect of asexual reproduction on calculations on gene flow, F-ST valu es were not significantly different from zero, suggesting high gene flow be tween these regions. However UPGMA cluster analyses using Rogers' genetic d istance, roughly clustered populations by region. In the case of H. atra, p ooled populations within each region were significantly different from thos e of the other region. Thus, although some restrictions in gene flow and gr eater genetic distances between the regions may exist, those differences ar e distinctly less than those reported in previous Studies on echinoderms ov er similar geographic scales. Despite the importance of asexual reproductio n for the maintenance of local population size, this study also confirmed t hat the potential for widespread dispersal mediated by sexually produced la rvae is large.