Proteomics support the threespine stickleback egg coat as a protective oocyte envelope

Area of Science:

• Evolutionary Biology
• Molecular Biology
• Reproductive Biology

Background:

• Threespine stickleback fish (Gasterosteus aculeatus) provide a model for studying adaptive radiation and speciation in freshwater environments.
• Reproductive proteins involved in gamete recognition are crucial for fertilization but remain understudied in stickleback speciation.
• Understanding these proteins can shed light on the molecular mechanisms driving reproductive isolation.

Purpose of the Study:

• To characterize the egg coat proteome of threespine stickleback fish.
• To investigate the role of reproductive proteins, specifically zona pellucida (ZP) proteins, in stickleback reproductive isolation and speciation.
• To compare stickleback ZP protein evolution and structure with those of other vertebrates, particularly mammals.

Main Methods:

• Proteomic analysis of the stickleback egg coat.
• Molecular evolutionary analyses, including tests for positive Darwinian selection.
• Comparative analysis of ZP protein structure, including glycosylation and disulfide bonding.

Main Results:

• Stickleback egg coats contain homologs of zona pellucida (ZP) proteins ZP1 and ZP3, consistent with other teleosts.
• Molecular evolutionary analyses indicate that stickleback ZP3, unlike ZP1, has experienced significant positive Darwinian selection.
• Differences in glycosylation, disulfide bonding, and synthesis sites were observed between stickleback and mammalian ZP proteins, with some selected residues aligning.

Conclusions:

• The evolution of ZP proteins, particularly ZP3, likely contributes to reproductive isolation and speciation in threespine stickleback fish.
• Stickleback ZP proteins exhibit distinct architectural features compared to mammalian counterparts, suggesting a specialized functional role.
• These findings suggest that fish egg coats may have evolved a more protective function than those of mammals.