Phylogenetic Analysis of Molecular Lineages in a Species-Rich Subgenus of Sea Stars (Leptasterias Subgenus Hexasterias)

Among diverse marine invertebrate taxa, the evolution of a non-planktotrophic larva is often associated with increased rates of cladogenesis, compared to related taxa that retain the ancestral planktotrophic larval form. Molecular phylogenetic analyses of non-planktotrophic (and, presumably species-rich) clades of marine invertebrates are rare. Here we analyze 1542 base pairs of mitochondrial DNA sequence comprising two gene regions, the cytochrome oxidase I gene and the putative control region and flanking sequences, for 23 molecular lineages in the obligately brood-protecting asteroid genus Leptasterias. Using maximum likelihood, minimum evolution, and maximum parsimony methods, five major clades were identified that corresponded to five taxa (species or species complexes) in the subgenus Hexasterias, section camtschatica (following the taxonomy of Walter K. Fisher). Two clades (L. aequalis> complex and L. aleutica/L. camtschatica complex) were composed of numerous molecular lineages (7–8 lineages/clade), and several clades had multiple shallow nodes, suggestive of recent radiations. Two of the clades (L. aleutica/L. camtschatica complex and L. hexactis complex), with geographic ranges restricted to latitudes higher than 48°N, were lacking deep phylogenetic nodes. This pattern is consistent with the hypothesis that high-latitude taxa have high rates of extinction due to repeated climatic crises. A log-likelihood ratio test performed on the camtschatica section, including a member (Leptasterias polaris) of the polaris section and using a representative (L. mülleri) of Leptasterias subgenus Leptasterias as an outgroup, demonstrated that the camtschatica section is monophyletic.