Molecular evolution and phylogenetic importance of a gamete recognition gene Zan reveals a unique contribution to mammalian speciation



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Speciation genes contribute disproportionately to macroevolution, but few examples exist, especially in vertebrates. In mammals, the Zan gene encodes the sperm acrosomal protein zonadhesin that mediates species-specific adhesion to the egg’s zona pellucida. Here we identify Zan as a speciation gene in placental mammals. Zan genomic ontogeny suggested it arose by repurposing of a stem vertebrate gene that was lost in multiple lineages but retained in Eutheria on acquiring a function in egg recognition. A 112-species Zan sequence phylogeny, representing 17 of 19 placental Orders, resolved all species into monophyletic groups corresponding to known Eutherian Orders and Suborders with <5% unsupported nodes. Zan divergence by intense positive selection and by domain duplications and accelerated divergence rate in the Myomorpha Suborder of Rodentia produced dramatic species differences in the protein’s properties, and Zan ordinal divergence rates generally reflected species-richness of Eutherian Orders. We propose that species-specific egg recognition conferred by Zan divergence served as a mode of prezygotic reproductive isolation that promoted radiation and adaptive success of Eutheria.



Fertilization, Mammals, Phylogenetics, Reproductive isolation, Species-specificity, Zonadhesin