9. Species-specific adhesion of sperm to the egg during sea urchin fertilization, involves the interaction of the sperm adhesive protein
   (1) Bindin (2) Avidin
   (3) Fertilin (4) Cortical granule

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Introduction to Sea Urchin Fertilization

Sea urchin fertilization has served as a classic model for understanding the molecular mechanisms of species-specific sperm-egg interactions. A key component of this specificity is the sperm protein Bindin, which mediates the precise binding of sperm to the egg surface.

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What is Bindin?

Bindin is a protein found in the acrosomal vesicle of sea urchin sperm. When the sperm undergoes the acrosome reaction, Bindin is exposed on the acrosomal process and directly interacts with receptor molecules on the egg’s vitelline layer (the protective outer layer of the egg). This binding enables the sperm to adhere tightly to the egg surface, a critical step before sperm-egg membrane fusion and fertilization.

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Species-Specific Adhesion Mechanism

• The interaction between sperm Bindin and egg receptors is highly species-specific, preventing cross-species fertilization.

• The egg’s vitelline layer contains specific receptor molecules such as EBR1 that recognize and bind only to Bindin from sperm of the same species.

• Knockout studies using CRISPR/Cas9 that eliminate the bindin gene show a complete inability of sperm to fertilize eggs despite normal sperm morphology and motility, confirming Bindin’s essential role.

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Other Candidates and Why They Are Incorrect

• Avidin: A protein involved in biotin binding in many organisms, unrelated to sperm-egg adhesion.

• Fertilin: A sperm surface protein implicated in membrane fusion in mammals but not specific to sea urchin sperm-egg adhesion.

• Cortical granule: Organelles in the egg important for preventing polyspermy, but not involved in sperm adhesion.

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The Importance of Bindin in Reproductive Isolation and Speciation

Bindin’s species specificity contributes to reproductive isolation, helping ensure that gametes from different species do not fertilize each other, thus maintaining species integrity. Variations in Bindin sequences among species are considered a driving force in sea urchin evolution and speciation.

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Conclusion

In sea urchin fertilization, the sperm adhesive protein responsible for species-specific adhesion to the egg is Bindin. It plays a critical role in ensuring that only sperm from the same species can bind and fertilize the egg, a key factor in reproductive success and species preservation.