Genomic and Structural Analysis of Gamete Recognition Proteins in a Broadcast Spawning Echinoderm Mesocentrotus franciscanus.

Gamete recognition proteins are expressed on the surfaces of sperm and eggs, where they mediate interactions between gametes. The genetic basis for gamete recognition proteins, as well as their structure and interactions, have yet to be fully resolved. Using a new high-quality de novo genome assembly for the sea urchin Mesocentrotus franciscanus, we investigated the genomic structure, expression, and protein forms of several gamete recognition proteins: sperm bindin, egg receptor for sperm (HSP110), and egg bindin receptor (EBR1), as well as the receptor for egg jelly (REJ) and its paralogs. To inform future population genetic and evolutionary studies, we resolve the genomic structure of the large EBR1 protein, identifying fewer tandem CUB-TSP1 repeats in EBR1 compared to the initial characterization of this protein. As expected for an egg receptor for sperm, EBR1 is highly expressed in female reproductive tissues (eggs and female gonad), compared to other tissues. In contrast, HSP110 shows similar levels of expression across male and female reproductive tissues, as well as across non-reproductive tissues and development stages. HSP110 might be a pleiotropic gene that in part influences fertilization. Using protein structural modeling and functional domain predictions, we propose hypotheses about potential interactions among EBR1, bindin, and HSP110 proteins that may provide insight into sperm-egg interactions in sea urchins. Resolving the genomic structure of genes encoding gamete recognition proteins, in combination with functional annotations and protein structural modeling, enables deeper investigation into the consequences of variation in gamete recognition proteins and the evolution of reproductive isolation.