The correct answer to the question about what happens when GSK-3 is blocked in the developing sea urchin embryo is:

(3) β-catenin will accumulate in the nuclei of all blastula cells leading to animal cells getting specified as endoderm and mesoderm.

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Introduction

In sea urchin embryogenesis, β-catenin plays a pivotal role in specifying vegetal cell fates such as endoderm and mesoderm. Under normal circumstances, β-catenin accumulates predominantly in nuclei of micromeres at the vegetal pole. Glycogen synthase kinase-3 (GSK-3) regulates β-catenin levels by tagging it for degradation, thus spatially restricting β-catenin accumulation and cell fate specification.

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Role of GSK-3 in Regulating β-Catenin

• GSK-3 phosphorylates β-catenin, marking it for proteolytic degradation in cells where β-catenin should not accumulate, mainly the animal pole cells.

• By inhibiting GSK-3 using chemical blockers (e.g., lithium chloride), β-catenin degradation is prevented.

• This leads to β-catenin accumulation in the nuclei of all blastula cells, not limited to the vegetal pole.

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Effects of GSK-3 Inhibition on Embryonic Cell Fate

• The ubiquitous nuclear presence of β-catenin reprograms animal pole cells, which normally give rise to ectoderm, to adopt endodermal and mesodermal fates.

• This transformation causes vegetalization of the entire embryo, leading to an abnormal loss of ectoderm and an embryo that is mostly endomesodermal.

• The experiment highlights the critical role of spatial regulation of β-catenin in embryo patterning.

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Experimental Evidence

• Studies show that when GSK-3 activity is blocked in sea urchin embryos, nuclear β-catenin localization expands from micromeres to all cells.

• Resulting embryos fail to form normal ectoderm, causing them to develop into radialized veg- talized embryos enriched for endodermal and mesodermal tissues.

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Conclusion

GSK-3 inhibition disrupts the normal restriction of β-catenin, causing it to accumulate across all blastula cells and fate conversion of animal pole cells into vegetal cell types. This experiment underscores the importance of β-catenin spatial regulation by GSK-3 during sea urchin development.

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Final Answer:
(3) β-catenin will accumulate in the nuclei of all blastula cells leading to animal cells getting specified as endoderm and mesoderm.