10. In echinoderms sperm direction is provided by
    (1) Calcineuric (2) Bindin
    (3) Resact (4) EBRI

Introduction to Sperm Chemotaxis in Echinoderms

In echinoderms like sea urchins, fertilization happens externally, posing the challenge of how sperm locate eggs in the vast aquatic environment. To overcome this, eggs release chemical signals that guide sperm movement—a phenomenon known as sperm chemotaxis. The well-studied chemoattractant responsible for sperm directional guidance in sea urchins is Resact.

What is Resact?

Resact is a species-specific, small peptide molecule secreted by the egg jelly coat surrounding the egg. It binds to receptors on the sperm’s surface, triggering a signal transduction pathway involving cyclic GMP (cGMP) and changes in intracellular calcium levels. These signals modulate the sperm’s flagellar beating pattern, causing changes in swimming behavior that steer the sperm toward higher concentrations of Resact, and thus toward the egg.

The Mechanism of Resact-Mediated Sperm Direction

• Resact binds to guanylyl cyclase receptors on the sperm membrane.

• This results in production of cGMP inside the sperm.

• cGMP signaling induces calcium influx and alterations in flagellar beating.

• Sperm swim more rapidly and steer in the gradient of Resact concentration toward the egg.

This sophisticated chemotactic response ensures sperm efficiently navigate through the external environment to reach the egg surface.

Why Other Options Are Incorrect

• Calcineuric: No known role in echinoderm sperm chemotaxis or fertilization.

• Bindin: Involved in sperm-egg binding after contact, but not in guiding sperm direction.

• EBR1: Egg receptor protein involved in sperm binding, not in providing directional cues.

Importance of Resact in External Fertilization

Resact’s role in sperm navigation is critical for ensuring species-specific fertilization success amidst oceanic dilution, maximizing reproductive efficiency in echinoderms. Its discovery has provided fundamental insights into molecular mechanisms of chemotaxis, signal transduction, and cellular navigation.

Conclusion

Resact is the primary molecule responsible for providing directional cues to echinoderm sperm, enabling them to swim toward the egg by chemotaxis. Its interaction with sperm receptors modulates motility and steering, playing a crucial role in fertilization.

Thus, the correct option is:

(3) Resact