HARPER, F.M.*; Dalhousie University: Morphological and phylogenetic analyses of Asterias forbesi and A. rubens in sympatry and allopatry.

The seastars Asterias forbesi and A. rubens co-occur in a zone of secondary contact from Nova Scotia to Cape Hatteras. The initial vicariance event occurred about 3 Mya resulting in a North American and a European species. Asterias rubens is believed to have recolonized North America recently, possibly since the last glacial maximum. The two species hybridize in vitro with no apparent pre- or postzygotic reproductive isolation mechanisms. Cross-fertilization studies have revealed no differences between hybrid and non-hybrid crosses, and heterogametic fertilization success is high. F₁ offspring are viable and successfully backcross. If mechanisms of reproductive isolation are weak, or do not evolve in allopatry, secondary contact of taxa can result in the formation of a hybrid zone. To examine the extent of hybridization and possible introgression between A. forbesi and A. rubens in vivo, I conducted a morphological survey of ~ 800 seastars collected both inside and outside the contact zone. Analyses of character scores were used to identify phenotypic intermediates and to assess the distribution of characters across the species ranges. The mtDNA control region was sequenced from morphological hybrids and non-hybrids for phylogenetic analyses. Patterns of morphological variation inside and outside the hybrid zone can be interpreted from the phylogeny of haplotypes. For example, some individuals have morphological traits of one species and mtDNA haplotypes of the other. This pattern may be the result of introgression of mtDNA from one species into the other or the persistence in both species of mtDNA lineages that predate divergence.