Evolution of poecilogony and the biogeography of North American populations of the polychaete Streblospio

Invertebrate interspecific developmental patterns can be highly variable an d, taxonomically, are considered only weakly constrained. Intraspecifically , some invertebrate species possess multiple developmental modes-a conditio n known as poecilogony. Closer examination of most putative poecilogenous s pecies, however, has not supported poecilogony, but rather has uncovered hi dden or cryptic species. The polychaete Streblospio benedicti is a well-kno wn, poecilogenous species found along the coast of North America. We collec ted mitochondrial cytochrome subunit I DNA sequence data from 88 individual s taken from 11 locations along the Atlantic, Gulf, and Pacific Coasts of t he United States to provide a phylogenetic framework from which to interpre t intraspecific variation in larval life history and brooding structure mor phology in this species. Our results are consistent with a recent revision of the species into two separate species: S. benedicti, a pouched brooding form distributed along the Atlantic and Pacific Coasts, and S. gynobranchia ta, a branchiate brooding form in the Gulf of Mexico. Contrary to the redes cription, S. benedicti is paraphyletic because the pouched brooding populat ion in Vero Beach, Florida shows strong genetic affinity with Gulf of Mexic o populations (S. gynobranchiata). However, S. benedicti is a true poecilog enous species, with both lecithotrophic and planktotrophic individuals poss essing identical mitochondrial DNA haplotypes. Crossbreeding experiments fu rther support the molecular phylogeny with reproductive isolation demonstra ted between, but not within, the major phylogenetic clades consistent with the previously described species. The genetic break near Vero Beach, Florid a, corresponds to a well known phylogeographic boundary, but the estimated time of separation for the Streblospio spp., approximately 10 million years before present, predates all other known phylogeographic subdivisions in t his area. This suggests that biogeographic sundering in this region is a re current event. Divergence times within the major Streblospio spp. clades ar e recent and indicate that changes in larval life history as well as broodi ng structure morphology are highly plastic and can evolve rapidly.