Transglobal comparisons of nuclear and mitochondrial genetic structure in a marine polyploid clam (Lasaea, Lasaeidae)

Existing genetic studies have proposed that the intertidal clam, Lasaea, is one of a few animal groups with asexual lineages that has persisted for an evolutionarily significant time. This proposal is based on the exceptional mitochondrial genetic divergence between studied sexual and asexual lineages. Nevertheless, a conclusion of long-lived asexuality awaits a more comprehensive sampling of the collective global range of this taxon. We assessed the breeding system and phylogeography of geographically divergent Lasaea populations using nuclear and mtDNA genetic markers. The allozyme genetic structure of five populations (from Japan, New Zealand, South Africa, Florida and Bermuda) showed marked deviation from expected random mating patterns (within and among loci), frequent fixed heterozygosity, and reduced genotypic diversity. This pattern and the finding of multiple asymmetric allozymic heterozygotes, indicated a clonal structure consistent with allopolyploid origins for each population. Spatial analysis of mtDNA and allozyme markers revealed strong geographical structure and yielded no cosmopolitan clonal lineages. Australian sexual species formed sister taxa to a minority of the clonal lineages, but pronounced mitochondrial genetic divergence levels and developmental differences precluded their identification as convincing parental species to any of the clones. A majority of asexual lineages may have originated in areas where no sexual congeners are presently known. The observed nuclear and mtDNA genetic structure is consistent with Lasaea clonal diversity being enriched by local mutational derivatives but infrequent recombination cannot be ruled out.