Domain expansion and functional diversification in vertebrate reproductive proteins

The rapid evolution of fertilization proteins can result in remarkable diversity in their structure and function. Many of the proteins in vertebrate egg coats contain copies (1-6) of the ZP-N domain. These ZP-N domains can facilitate multiple reproduction functions, including species-specific sperm recognition. We integrated phylogenetics and machine learning to investigate how ZP-N domains diversified in structure and function. The most C-terminal ZP-N domain of each paralog is associated with another domain type (ZP-C) which together form a "ZP module." All modular ZP-N domains were phylogenetically distinct from non-modular or free ZP-N domains. Machine learning-based classification identified 8 residues that form a stabilizing network in modular ZP-N domains that is absent in free domains. Positive selection was identified in some free ZP-N domains. Our findings suggest that purifying selection has conserved an essential structural core in modular ZP-N domains, while free N-terminal domains have been able to experience functionally diversify.