Abstract
Previous research on gametic incompatibility in marine invertebrates suggests that for highly dispersive marine invertebrate species, barriers to fertilization among closely related taxa are often incomplete and sometimes asymmetric. The nature of these barriers can dramatically affect the patterns of gene flow and genetic differentiation between species, and thus speciation. Blue mussels, in the genus Mytilus, are genetically distinct in allopatry yet hybrids are present wherever any two species within the group co-occur. The present study sampled M. edulis (L.) and M. trossulus (Gould) in May and June 2001 from the East Bay section of Cobscook Bay, Maine, USA (latitude 44°56′30″N; longitude 67°07′50″W), where the two species are sympatric. Gamete incompatibility was investigated in a series of laboratory fertilizations carried out in July 2001. The proportion of fertilized eggs typically exceeded 80% at sperm concentrations of 103–104 ml−1 among intraspecific matings (n=18), but was <30% even at sperm concentrations in excess of 105–106 ml−1 for interspecific matings (n=13). Further analysis indicated that approximately 100- to 700-fold higher sperm concentrations were required to achieve 20% fertilization in interspecific matings relative to intraspecific matings, indicating strong barriers to interspecific fertilization. The proportion of fertilized eggs did not follow this general pattern in all matings, however. The eggs from two (out of five) M. edulis females were almost as receptive to M. trossulus sperm as they were to M. edulis sperm. In contrast, the eggs from all M. trossulus females (n=3) were unreceptive to M. edulis sperm, suggesting that fertilization barriers between these species may be asymmetric. Given the experimental design employed in this study, the results are also consistent with a strong maternal or egg effect on the level of interspecific gamete compatibility in M. edulis.





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Acknowledgements
C. Slaughter participated in this research as part of the National Science Foundation Research Experience for Undergraduates program at the Darling Marine Center (OCE-99-10188). Additional support was provided by the University of Maine Sea Grant Program under contract 99-343 and Hatch funds provided by the U.S. Department of Agriculture and the Maine Agriculture and Forestry Experiment Station (project no. ME08510) to P. Rawson; P. Yund was supported by funding from the National Science Foundation (OCE-01-22031). A previous draft of this manuscript was strengthened appreciably by comments from E. Fisher. This is Maine Agricultural and Forest Experiment Station external publication no. 2636 and contribution no. 378 from the Darling Marine Center. We certify that the research described within this document complies with the current laws of our country.
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Communicated by J.P. Grassle, New Brunswick
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Rawson, P.D., Slaughter, C. & Yund, P.O. Patterns of gamete incompatibility between the blue mussels Mytilus edulis and M. trossulus . Marine Biology 143, 317–325 (2003). https://doi.org/10.1007/s00227-003-1084-x
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DOI: https://doi.org/10.1007/s00227-003-1084-x