Data from: Izumo1 and Juno: the evolutionary origins and coevolution of essential sperm-egg binding partners

Reproductive proteins are among the most rapidly evolving classes of
proteins. For a subset of these, rapid evolution is driven by positive
Darwinian selection despite vital, well-conserved, reproductive functions.
Izumo1 is the only essential sperm–egg fusion protein currently known on
mammalian sperm, and its egg receptor (Juno; formerly Folr4) was recently
discovered. Male knockout mice for Izumo1 and female knockout mice for
Juno are both healthy but sterile. Here, both sperm–egg binding proteins
are shown to be evolving under positive selection. Within mammals,
coevolution of Izumo1 and Juno is also uncovered, suggesting that similar
forces have shaped the evolutionary histories of these binding partners
within Mammalia. Additionally, genomic analyses reveal an ancient origin
for the Izumo gene family, initially reported as conserved exclusively in
mammals. Newly identified Izumo1 orthologues could serve reproductive
functions in birds, fish and reptiles. Surprisingly, these same analyses
support Juno's presence in mammals alone, suggesting a recent
mammalian-specific duplication and neofunctionalization of the ancestral
folate receptor. Despite the indispensability of their reproductive
interaction, and their apparent coevolution within Mammalia, this binding
pair arose through strikingly different evolutionary forces.