Genetic variation in genes controlling sperm-egg interactions in reproductively isolated populations of Astyanax mexicanus - In fertilization, sperm and egg must overcome reproductive barriers to locate, contact, and fuse. These barriers can include physical, biological, genetic and environmental factors. They exist not only to ensure that gametes match partners of the same species, but also that they are the most competitive and genetically fit matches. Occasionally, these pairing safeguards are breached, resulting in infertility, or at times hybridization supporting the emergence of new species. While distinct morphologies and signals have been identified between gametes, little is known about the precise mechanisms through which opposite sex gametes identify species specificity and fuse. Mammalian studies in this field are restricted by the scarcity and inaccessibility of eggs, and by limitation on in vivo studies. Natural experimental models, such as in cases of incipient speciation, provide unique insights into the genetics of pre-hybridization barriers of fertility. Taking advantage of an emerging fish system, Astyanax mexicanus, we can reveal genetic factors essential for ensuring conspecific gamete compatibility and pre-zygotic fusion. Astyanax is a proven genetic model which naturally exists as two morphs, river-dwelling and cave-adapted. This fish is remarkable for its rapid and repeated evolution within cave environments, and for hybridization capacity - even among highly diverged and reproductively isolated morphotypes. However, we have found that fertilization ability between morphs is variable suggesting on going pre-zygotic restriction and selection in these populations. With this unique and tractable experimental platform, our central hypothesis is that the strength of selection on these diverging populations will enable us to identify key genes controlling sperm-egg interaction. Given known conservation of many of the genes involved in this mechanism, these genetic studies will inform us of specific molecular regulation of fertility and gamete fusion in humans. We will exploit the ease of hybridization, the availability of genome sequencing from various morphs, and the morph-specific variation in sperm and egg morphology to conduct systematic genetic analysis of mechanisms of sperm-egg fusion in an in vivo and evolutionarily defined context. In Aim 1, we will uncover regulation of physical species-specific mediators of fertilization by disrupting the sperm-binding micropyle, thereby assessing interspecific hybridization efficiency as a consequence of changes in micropyle morphology caused by mutation of the uncharacterized vertebrate germ plasm organizer, Bucky ball. In Aim 2, we will map gamete fusion, fertilization, and fitness outcomes to allelic variation in a new, highly annotated dataset of genes implicated in reproductive regulation. In Aim 3, we will functionally test sperm:egg fusion factors, such as the ortholog of the as yet uncharacterized human SPACA4. As these genetic factors are broadly conserved, our findings hold the promise to enhance understanding of human egg and sperm biology and provide metrics of gamete quality and fertilization success predictability. Our study will uncover knowledge crucial to understanding origins of life and speciation that will translate to advances in human infertility care.
