Genome-wide analysis of the ovodefensin gene family: Monophyletic origin, independent gene duplication and presence of different selection patterns

Ovodefensins (OvoDs) represent a group of cysteine-rich host defense peptides that are abundant in the egg white. Recent studies have found that ovodefensins are specific to birds and reptiles. However, the entire repertoire and evolutionary relationships of this gene family have not been thoroughly elucidated to date. Following our cross-species and genome-wide computational study, a total of 94 ovodefensin genes with multiple novel cysteine sequence motifs were identified from 22 phylogenetically divergent species. Phylogenetic analysis suggests that a large number of OvoDs evolved by gene duplication after species divergence. Furthermore, the OvoD genes in each species trend to be clustered densely in a syntenic region flanked by the XKR6 and MTMR9 genes, indicating that they are of monophyletic origin and appear to have emerged via independent gene duplication events in snakes, turtles, crocodiles, birds and the green lizard. Furthermore, positive selection sites are located primarily in the mature peptide region of the turtle, lizard and snake OvoD genes. Moreover, the duplicate OvoDAs in birds seem to be maintained in almost identical sequences and functions by strong purifying selection. Genome-wide identification and analyses of the OvoD gene family may greatly improve our understanding of the potential evolutionary relationship scenario of the OvoD gene family. Continued sequence mining and functional studies of OvoDs will be helpful in shedding light on the relationships between OvoDs and other defensin-related gene families.