17. Several marine organism release their gametes into the environment, where sperm attraction and subsequent events lead to successful fertilization. With reference to sea urchins, which one of the following statements is NOT true?
    (1) Additiion of resact into a drop of seawater containing sperms specifically attracts sperms of A. punctulata.
    (2) P₃ is formed initially at the site of sperm entry and releases sequestered Ca²⁺
    (3) Ca²⁺ prevents docking of cortical granules of the egg to the cell membrane.
    (4) Inhibitors that specifically block PLC𝛾 can be circumvented by microinjecting IP₃ into the
    egg.

The statement which is NOT true in reference to sea urchin fertilization is:

(3) Ca2+ prevents docking of cortical granules of the egg to the cell membrane.

Explanation:

• (1) Addition of resact into seawater specifically attracts sperms of Arbacia punctulata (a species of sea urchin) — True. Resact is a species-specific chemoattractant peptide released by the egg jelly that attracts sperm.

• (2) IP3 (inositol trisphosphate) is formed initially at the site of sperm entry and releases sequestered Ca2+ — True. IP3 triggers intracellular calcium release, which is critical for egg activation and cortical granule exocytosis.

• (3) Ca2+ prevents docking of cortical granules to the membrane — False. Ca2+ actually promotes the docking and fusion of cortical granules with the egg membrane, leading to exocytosis that forms the slow block to polyspermy.

• (4) Inhibitors that specifically block PLC𝛾 can be circumvented by microinjecting IP3 into the egg — True. IP3 is downstream of PLC𝛾; thus, by directly supplying IP3, one can bypass PLC𝛾 inhibition and induce calcium release.

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Sea urchin fertilization is a model system extensively used to understand molecular and cellular fertilization mechanisms. Key players include Resact peptides, IP3-mediated Ca2+ signaling, and cortical granules involved in polyspermy prevention. This article clarifies one common misconception about calcium’s role during fertilization.

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The Role of Resact in Sperm Attraction

Resact is a peptide secreted by the egg’s jelly coat. It diffuses into the surrounding seawater and acts as a chemoattractant, specifically attracting sperm of the same species, such as Arbacia punctulata. This ensures successful recruitment of sperm towards the egg for fertilization.

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Calcium Signaling Via IP3 in Egg Activation

Upon sperm binding and fusion, signaling enzymes including phospholipase C (PLC) get activated. PLC produces IP3 at the site of sperm entry, which mobilizes Ca2+ from intracellular stores within the egg. The resulting calcium wave triggers several downstream events:

• Resumption of the egg’s meiosis,

• Cortical granule exocytosis releasing contents to harden the fertilization envelope,

• Activation of the egg for embryonic development.

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Correct Role of Calcium in Cortical Granule Docking

Contrary to the incorrect statement (3), Ca2+ is not an inhibitor but an essential facilitator in the docking and fusion of cortical granules with the egg plasma membrane. This Ca2+-dependent exocytosis constitutes the slow block to polyspermy, effectively preventing multiple sperm from fertilizing the egg.

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Bypassing PLC𝛾 Inhibition by IP3 Injection

Inhibition of PLC𝛾, which catalyzes IP3 formation, can be overcome by microinjecting IP3 directly into the egg cytoplasm. Since IP3 triggers intracellular calcium release, this method rescues fertilization-related calcium signaling even when PLC𝛾 is blocked.

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Conclusion

Among the four statements about sea urchin fertilization, the incorrect one is the notion that calcium inhibits cortical granule docking. In reality, calcium promotes this critical step of the polyspermy prevention mechanism. Other statements describing Resact function, IP3-mediated calcium release, and the ability to circumvent PLC𝛾 inhibition via IP3 microinjection are accurate.

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This article dispels a common fertilization misconception and provides a clear understanding of calcium’s vital signaling functions in sea urchin egg activation.

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Final Answer: The statement that is NOT true is (3) Ca2+ prevents docking of cortical granules of the egg to the cell membrane.