SLAUGHTER, C.; MCCARTNEY, M.A.; Univ. of North Carolina, Wilmington; Univ. of North Carolina, Wilmington: Variation in gametic incompatibility of the blue mussel, Mytilus edulis, within the Gulf of Maine.

Documented cases of natural hybridization among species of free-spawning marine invertebrates are few, however species within the M. edulis complex of blue mussels are found to readily hybridize in areas of sympatry. Blue mussel hybrid zones therefore provide a unique opportunity to examine the evolution of gametic incompatibility in free-spawning marine invertebrates in the absence of physical barriers to genetic exchange. Previous work demonstrated strong gametic incompatibility between most Mytilus edulis and Mytilus trossulus from sympatric populations within the Gulf of Maine, but showed that some M. edulis females are greatly compatible with heterospecific sperm. Although these highly compatible females appeared common in at least one sympatric population, little is known about the frequency of this trait throughout the range of M. edulis in the NW Atlantic and its impact on the maintenance of species integrity in the hybrid zone. Gametes were obtained from mussels at 3 sites, one within the Gulf of Maine hybrid zone, and 2 at increasing distance outside of it, and used in invitro fertilizations. Initial analysis suggests that the frequency of the compatible phenotype decreases with distance from the hybrid zone. Codominant nuclear markers and AFLP’s are being used to estimate high-resolution hybrid indices for all mussels. Nearly all hybrid individuals so identified are late-generation backcrosses, and fertilization experiments are being performed to determine whether their gametes combine assortatively with those from the pure species to which they are most genetically similar.