November 2, 2007

Comparative phylogeography and introgressive hybridization in two Caribbean sea slugs with non-planktonic development

Albert Rodriguez and Dr. Patrick J. Krug

For benthic invertebrates, dispersal depends on planktonic larvae or rafting by adults. Elysia pratensis and E. subornata, sister species of Caribbean sea slugs, lack a dispersing larval stage; thus, populations from different islands are predicted to diverge genetically. To compare phylogeographic patterns, E. pratensis and E. subornata were sampled from Florida, five Bahamas islands, Bermuda and Jamaica. Portions of the mitochondrial cytochrome c oxidase I (COI) and nuclear large ribosomal subunit (28S) genes were sequenced from all specimens. Bayesian phylogenetic analysis of COI haplotypes revealed four distinct clades up to 8% divergent in E. pratensis; analysis of molecular variance revealed highly significant differences among populations (Φst=0.935), with most sites exhibiting reciprocal monophyly. Two E. pratensis clades from the northern Bahamas were more closely related to E. subornata, suggesting hybridization and historical introgression of the mitochondrial genome from E. subornata into E. pratensis. A fixed difference in the nuclear 28S gene, morphology and host use distinguished the two species, supporting the hypothesis of introgression and “mitochondrial capture.” Less population structure (Φst=0.389) and no phylogeographic breaks were evident in E. subornata, comparable to results for Elysia spp. with short-lived larvae; adult E. subornata may raft between sites on their buoyant host alga, Caulerpa racemosa. These data offer insight into Caribbean population connectivity, critical to conservation and management of threatened reef ecosystems. Introduction of E. subornata into the Mediterranean has been suggested for biological control of invasive Caulepa spp., but its potential for dispersal and hybridization with native species urges a cautious approach.