Gamete compatibility, gamete trait variation and their effect on fertilization success in northwest Atlantic blue mussel (Mytilus edulis l. and Mytilus trossulus gould) hybrid zone

Liu, Guangxu (2009) Gamete compatibility, gamete trait variation and their effect on fertilization success in northwest Atlantic blue mussel (Mytilus edulis l. and Mytilus trossulus gould) hybrid zone. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Mytilus edulis and M. trossulus coexist and form a natural hybrid zone along the Atlantic Canada coast, which provides an excellent model to study marine invertebrate evolution, speciation and especially reproductive isolation barriers that maintain species integrity. Gamete traits and their role in determining fertilization success among M. edulis, M. trossulus and their hybrids were investigated in the present study. Sperm of all three genotypes (the term genotypes was used throughout the thesis to include M. edulis, M. trossulus and their hybrids) conducted circular movement in a two dimensional plane, which may represent a trait that increases fertilization success under sperm limitation on a small spatial scale. Neither egg water nor egg presence elicited sperm chemotaxis in our experimental set-up, which suggests that sperm chemotaxis may not be necessary for successful fertilization of marine invertebrate species with dense aggregations, synchronized broadcast spawning and high gamete output. Although no significant difference in gamete output was detected among the different genotypes, sperm velocity differed among M. edulis, M. trossulus and their hybrids, F₁ hybrids producing the slowest swimming sperm. Sperm of M. edulis had faster VAP (average path velocity) and angle change rate than those of M. trossulus . Most of the variation in sperm velocity was attributed to variation within individuals, which may be caused by differences in stage of maturity among individual sperm. Both sperm velocity and sperm half life (T₅₀) decreased as temperature increased. F₁ hybrids had the shortest sperm longevity and M. trossulus was least temperature sensitive, which may be an adaptation to a longer spawning period, resulting in exposure to higher temperature fluctuations. M. edulis produced larger eggs than M. trossulus and F₁ hybrids, providing a larger target for sperm-egg interaction. Both homospecific and heterospecific fertilization success differed among parental combinations and may be due to the combined effects of gamete compatibility and gamete trait variation on fertilization success. Sperm velocity, egg size and especially the combination of these two gamete traits were positively correlated with fertilization success, supporting the hypothesis that fertilization success is partially determined by these gamete traits. The three parameters (β,β₀ and β/β₀) of the fertilization kinetics model differed significantly among the different crosses. The small β/β₀ detected for the heterospecific crosses between M. edulis and M. trossulus suggests strong reproductive isolation between these species and can explain the rarity of F₁ hybrids. In general, the poor gamete quality (small egg size, slow swimming speed and short life span of sperm) of F₁ hybrids can lead to reduced fertilization success and may help to explain the genetic composition, especially the bimodal structure, of the blue mussel hybrid zone in Atlantic Canada. The present study shows that gamete trait variation (sperm velocity, sperm longevity and egg size) and gamete compatibility are crucial factors affecting both homospecific and heterospecific fertilization success, and provides a better understanding of prezygotic reproductive isolation between and among broadcast spawning species.

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