| Summary: | Extensive studies of hybrid zone origin and formation have been conducted in the terrestrial environment, yet relatively few marine invertebrate hybrid zones have been well described. In the northwest Atlantic, a secondary contact zone has recently formed between two sibling species of sea stars, Asterias rubens and A. forbesi. I conducted a series of studies to determine the outcome of this contact. Gametes of A. rubens and A. forbesi were reciprocally compatible in cross-fertilization studies, however the compatibility of heterospecific crosses was highly variable. Differential compatibility of heterospecific gametes was demonstrated in sperm competition studies in which I used a nuclear DNA marker to assign paternity to larval offspring. There was evidence of conspecific fertilization preference in A. forbesi and some A. forbesi sperm were competitively superior to A. rubens in fertilizing A. rubens eggs. A morphological survey of sympatric and allopatric Asterias populations did not quantitatively support the existence of a distinct group of intermediate phenotypes that might have been hybrids. However, evidence of hybridization and introgression in the contact zone was detected in a phylogenetic survey of mtDNA variation. Rubens-like haplotypes were found in three individuals with forbesi-like phenotypes. Asymmetric introgression may reflect differential compatibility of heterospecific gametes. Asterias forbesi and A. rubens are not completely reproductively isolated in secondary contact. Asymmetric gamete compatibility appears to have led to asymmetric introgression of mtDNA from A. rubens into A. forbesi. This asymmetry suggests a prezygotic reproductive barrier not previously considered in these taxa, such as a gamete recognition protein system analogous to bindin in sea urchins. Further description of the Asterias secondary contact zone may enable future studies to test theories of speciation and hybrid zone dynamics using these closely-related species. Thesis (Ph.D.)--Dalhousie University (Canada), 2004.
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