38. The specification of sea urchin micromeres involves the activation of a repressor protein Pmar1, which represses the expression of hesC, which also encodes a repressor protein. One of the genes controlled by HesC is Delta, whose expression is used as a marker for micromere lineage. The image below represents a sea urchin embryo on which whole mount in situ
hybridization (WMISH) was performed using delta probe, indicated by the area ‘A’. The rest of the embryo is labeled ‘B’.
The table below summarizes a set of experiments (column A) and the area in which hybridization is observed (column B)
Which one of the following options is a correct match between columns A and B?
(1) A-ii, B-iii, C-l (2) A-ii, B-iii, C-iv
(3) A-i, B-ii, C-iv (4) A-i, B-ii, C-iii
Sea urchin micromere specification involves tightly regulated gene interactions. Key players include Pmar1, a repressor of HesC, which itself represses downstream genes like delta, a micromere lineage marker. Exploring how different experimental manipulations affect delta expression enhances understanding of embryonic patterning.
The Gene Regulatory Network: Pmar1 and HesC
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Pmar1 is expressed in large micromeres and represses HesC, a global repressor of micromere-specific genes including delta.
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This repression creates a double-negative gate resulting in activation of delta exclusively in micromeres.
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When Pmar1 is ectopically expressed or HesC function blocked, delta expression can expand abnormally throughout the embryo.
Experimental Observations of Delta Expression
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Injection of pmar1 mRNA (A-i) leads to delta expression in broader areas of the embryo (B-ii), reflecting derepression due to HesC inhibition.
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Injection of hesC antisense RNA (B-ii) similarly causes delta expression outside micromere regions (B-ii).
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Injection of hesC mRNA (C-iv) represses delta expression, restricting it to the normal micromere domain.
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Control or other treatments maintain delta expression in the micromeres (B-iii/B-i depending on experiment).
Conclusion
The tightly controlled regulatory interaction between Pmar1 and HesC governs delta expression and micromere specification in sea urchins. Experimentally manipulating these genes alters delta spatial patterns, revealing the intricacies of the underlying gene regulatory network.
Final Answer:
(3) A-i, B-ii, C-iv
