Abstract
Cryptic female choice (CFC), a form of sexual selection post mating, describes processes that lead to a bias of fertilization outcomes towards certain males. In Chinook salmon (Oncorhynchus tshawytscha), an external fertilizer, the ovarian fluid surrounding the ova differently enhances the sperm velocity of males. Sperm velocity is a key ejaculate trait that determines fertilization success in externally fertilizing fishes, thus its differential enhancement might bias male fertilization outcomes and represent a mechanism of CFC.
Once sperm reach the oocyte, CFC could potentially be further facilitated by sperm-egg interactions, which are well understood in externally fertilizing marine invertebrates.
In this thesis, I explored the potential genetic basis of both possible mechanisms by examining whether i) the genotypic combinations of females and males at the Major Histocompatibility Complex (MHC) class I and II and/or ii) the genome-wide relatedness of mates, assessed with a 6000 Chinook salmon SNP chip, explain the variation in sperm velocity and/or male fertilization success.
A series of paired-male competitive fertilization trials were performed in which competing males exhibited different sperm velocities in the focal female's ovarian fluid. Male fertilization success was evaluated using microsatellite-based paternity assignment.
I found that relative sperm velocity was positively correlated with fertilization success, confirming that the differential enhancement of sperm velocity of different males may be a mechanism of CFC in Chinook salmon. The variation in sperm velocity was MHC-independent and random with respect to genome-wide relatedness, but explained by relatedness between mates on two of a total of 29 examined SNP Linkage Groups (LG). After correcting for multiple testing, relatedness on these two LGs was not significantly correlated with sperm velocity and thus, they might be false discoveries. Besides relative sperm velocity, similarity of mates at the MHC class II, but not at the MHC class I, is correlated with fertilization success, indicating that this locus might influence sperm-egg interactions. Further, fertilization success was positively correlated with genome-wide relatedness but independent of relatedness on any LG.
The data in this thesis suggests that the MHC plays an overall minor role in CFC in Chinook salmon. This thesis provides an indication for an assortative form of CFC in the study species acting on the MHC class II and on genome-wide relatedness. Given relatedness on none of the LGs appears to be significantly correlated with the variation in sperm velocity and fertilization success, this thesis can not provide candidate genomic regions other than the MHC class II locus that might hold genes with a possible role in these post-mating processes.