Expression and Genomic Organization of Zonadhesin-Like Genes in Three Species of Fish Give Insight into the Evolutionary History of a Mosaic Protein

Background: The mosaic sperm protein zonadhesin (ZAN) has been characterized in mammalsand is implicated in species-specific egg-sperm binding interactions. The genomic structure andtestes-specific expression of zonadhesin is known for many mammalian species. All zonadhesingenes characterized to date consist of meprin A5 antigen receptor tyrosine phosphatase mu(MAM) domains, mucin tandem repeats, and von Willebrand (VWD) adhesion domains. Here weinvestigate the genomic structure and expression of zonadhesin-like genes in three species of fish.Results: The cDNA and corresponding genomic locus of a zonadhesin-like gene (zlg) in Atlanticsalmon (Salmo salar) were sequenced. Zlg is similar in adhesion domain content to mammalianzonadhesin; however, the domain order is altered. Analysis of puffer fish (Takifugu rubripes) andzebrafish (Danio rerio) sequence data identified zonadhesin (zan) genes that share the same domainorder, content, and a conserved syntenic relationship with mammalian zonadhesin. A zonadhesinlikegene in D. rerio was also identified. Unlike mammalian zonadhesin, D. rerio zan and S. salar zlgwere expressed in the gut and not in the testes.Conclusion: We characterized likely orthologs of zonadhesin in both T. rubripes and D. rerio anduncovered zonadhesin-like genes in S. salar and D. rerio. Each of these genes contains MAM, mucin,and VWD domains. While these domains are associated with several proteins that show prominentgut expression, their combination is unique to zonadhesin and zonadhesin-like genes in vertebrates.The expression patterns of fish zonadhesin and zonadhesin-like genes suggest that the reproductiverole of zonadhesin evolved later in the mammalian lineage.