Structural organization and evolution of the marsupial zona pellucida

In this review, the biochemical composition and structural organization of the marsupial and eutherian zonae pellucidae are compared. Differences between the zonae from these two groups of mammals are observed in their response to dilute proteases and reducing agents, in their potential glycosylation patterns, and in some of their functions. However, studies on the glycoconjugates and polypeptides of the three zona pellucida genes have not explained these different responses to the proteases and reducing agents. There is high sequence similarity between the zona polypeptides of marsupials and eutherians, as well as a similarity in the oligosaccharides present, as demonstrated by lectin staining. As the marsupial and eutherian lineages diverged from a common ancestor over 100 million years ago, these observations indicate that the three-dimensional structure of these glycoproteins is highly conserved throughout all mammals, although the complexity of its molecular organization has yet to be resolved. Phylogenetic analyses indicate that there are at least four groups of paralogous zona pellucida genes in vertebrates. The marsupial ZPA and ZPB genes have been named in accordance with their orthologues but the phylogenetic relationships of the marsupial ZPC gene require further investigation.