36. Given below are few statements regarding the role of Disheveled (Dsh) and β-catenin (β-cat) in the development of sea urchin.
    A. Dsh is localized in the vegetal cortex of the oocyte before fertilization and in the region of the 16-cell embryo about to become the micromeres.
    B. Dsh is localized in the cytosol of the oocyte during oogenesis and in the micromere forming
    blastomeres of a 16- cell embryo.
    C. β-cat accumulates predominantly in the micromeres and somewhat in the veg₂ tier cells.
    D. Treatment of embryos with lithium chloride does not allow the accumulation of β-cat in the nuclei of all blastula cells, and the animal cells thus become specified as endoderm and
    mesoderm.
    E. When β-cat is prevented from entering the nucleus, the embryo develops as a ciliated ectodermal ball.
    Which one of the following options represents a combination of correct statements?
    (1) B, C and E (2) A, C and D
    (3) A, C and E                                                          (4) B, D and E

The correct combination of statements about the roles of Disheveled (Dsh) and β-catenin (β-cat) in sea urchin development is:

(3) A, C and E

Where:

• A. Dsh is localized in the vegetal cortex of the oocyte before fertilization and in the region of the 16-cell embryo about to become the micromeres. This is true based on recent findings that Dsh is enriched in the vegetal cortical domain (VCD) and is crucial for localized canonical Wnt signaling activation.

• C. β-catenin accumulates predominantly in the micromeres and somewhat in the veg2 tier cells. This is consistent with the known nuclear localization pattern of β-catenin in vegetal blastomeres.

• E. When β-catenin is prevented from entering the nucleus, the embryo develops as a ciliated ectodermal ball. This aligns with experimental evidence showing that blocking β-catenin nuclear entry prevents vegetal cell specification and leads to ectodermal fate default.

Statements B and D are incorrect:

• B wrongly claims Dsh is localized in the cytosol during oogenesis and in micromeres, whereas it’s localized to the vegetal cortex.

• D states lithium chloride treatment prevents β-catenin accumulation, but it actually promotes its accumulation in all blastula nuclei.

Introduction

Early development in sea urchin embryos is orchestrated by molecular players such as Disheveled (Dsh) and β-catenin (β-cat), pivotal in the canonical Wnt signaling pathway. Their spatial localization and function determine the specification of vegetal blastomeres, setting up the embryonic body axis and germ layer fates.

Localization and Role of Disheveled

• Dsh concentrates in a specialized vegetal cortical domain (VCD) in the egg before fertilization and is inherited by vegetal blastomeres, especially at the 16-cell stage where micromeres form.

• Its enrichment in this domain facilitates localized activation of the Wnt pathway by interacting with receptors, protecting β-catenin from degradation.

β-Catenin Accumulation and Function

• β-catenin predominantly accumulates in nuclei of large micromeres and in the veg2 tier cells, activating gene transcription essential for endomesoderm development.

• Nuclear β-catenin acts as a transcriptional coactivator, promoting vegetal cell fate specification.

Consequences of Disrupted β-Catenin Nuclear Entry

• Experimental inhibition of β-catenin nuclear import blocks vegetal cell specification.

• The resulting embryo fails to form endoderm and mesoderm, developing instead as a ciliated ectodermal ball devoid of vegetal derivatives.

Conclusion

Disheveled’s vegetal cortical localization and β-catenin’s nuclear accumulation are crucial for early pattern formation in sea urchin embryos. Their proper spatial regulation ensures the correct specification of cell fates, and disruption leads to developmental arrest and fate alteration.

Final Answer:
(3) A, C and E