Sperm lacking Bindin are infertile but are otherwise indistinguishable from wildtype sperm

Area of Science:

• Reproductive Biology
• Developmental Biology
• Molecular Genetics

Background:

• Cell-cell fusion is a specialized process, with sperm-egg fusion during fertilization being a key example.
• Gamete-specific proteins on sperm surfaces mediate species-specific interactions with eggs.
• Bindin is a sea urchin sperm protein essential for binding to homologous eggs.

Purpose of the Study:

• To investigate the phenotypic consequences of Bindin absence in sea urchin sperm.
• To confirm the role of Bindin exclusively in species-specific sperm-egg interactions.

Main Methods:

• Utilized Cas9 targeted gene inactivation in the sea urchin Hemicentrotus pulcherrimus to create Bindin-minus sperm.
• Analyzed sperm morphology, function (including acrosomal reaction), and binding/fertilization capacity.
• Observed the development and metamorphosis of embryos produced by Bindin-minus sperm.

Main Results:

• Bindin-minus sperm exhibited normal morphology and function, including head shape, tail motility, and acrosomal reaction.
• Sperm lacking Bindin failed to bind to or fertilize eggs, even at high concentrations.
• Bindin-minus embryos developed into normal, fertile adults, indicating no other biological roles for Bindin.

Conclusions:

• Bindin is essential and sufficient for species-specific sperm-egg recognition and fusion in sea urchins.
• The function of Bindin is strictly limited to gamete interaction, not affecting sperm viability or other developmental processes.
• Evolutionary changes in Bindin are not constrained by other cellular functions, allowing for rapid divergence.