It has been accepted that the characteristics of sperm are determined by the diploid genotype of the male producing them. However, recent and unexpected findings indicate that the phenotype of a sperm cell may also be influenced by its own genotype. This innovative project examines the degree to which the haploid genotypes of sperm cells affect their individual characteristics and the relative importance of sperm competition versus selection post-fertilization in causing biases in transmission of alleles from one generation to the next. It will provide basic information to illuminate a previously unsuspected but potentially important avenue for natural selection in evolution. The expected results have the potential to open up an important new line of research in sperm biology. They are expected to have broad implications in developmental biology, evolutionary biology, and for improvements in reporductive technologies in humans and domestic animals.
This research investigates the epistatic interactions among genes observable in hybrids between two incipient species of freshwater fish (Astyanax spp.). Previous work has shown that sperm from hybrid males pass along biased subsets of alleles and allelic combinations derived from unlinked genes. This observation suggested the hypothesis that the phenotypes of individual sperm (in this case fertilization efficiency) are determined by their individual genotypes, i.e., the probability that a package gets delivered depends upon what it contains. Consistent with this hypothesis are preliminary studies that show that sperm subpopulations - defined by differences in swimming ability or by their response to a chemical challenge - differ significantly in allelic content. This research tests whether the observed transmission biases are indeed caused by sperm competition or whether there is a contribution from zygotic selection, post fertilization. Single sperm will be genotyped from embryos formed using intracytoplasmic sperm injection. This will allow different sub-populations of sperm (e.g. slow versus fast swimmers) to be compared for allelic content. Additionally, the offspring of hybrid X non-hybrid individuals will be genotyped, allowing for the detection of transmission biases. The hypothesis predicts that biases will be restricted to families derived from hybrid males but not from hybrid females.
