37. In sea urchins, a group of cells at the vegetal pole become specified as the large micromere cells. These cells are determined to become skeletogenic mesenchyme cells that will leave the blastula epithelium to ingress into the blastocoel. This specification is controlled by the expression of Pmar1 which is a repressor of HesC. HesC represses the genes encoding transcription factors activating skeleton forming genes. The gene regulatory network is given
below.
Below, column I lists the experiments carried with mRNA/antisense RNA of different genes injected into single-celled sea urchin embryo while column II lists the developmental outcomes: Match the following:
Which of the following combinations is correct?
(1) A-2, B-1, C-1, D-2 (2) A-1, B-1, C-2, D-2
(3) A-1, B-2, C-2, D-1 (4) A-2, B-2, C-2, D-2
Introduction
Sea urchin development has served as a classic model to decipher gene regulatory networks (GRNs) that control embryonic cell fate specification. Large micromeres at the vegetal pole of the 16-cell embryo become specified as skeletogenic mesenchyme cells, driven by a hierarchical GRN involving repressors and transcription factors.
Role of Pmar1 and HesC in Micromere Specification
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Pmar1 is a transcriptional repressor expressed specifically in large micromeres, initiating their specification.
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Pmar1 represses HesC, another repressor that is expressed broadly in the embryo, which itself represses genes required for skeletogenic specification.
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This double-negative gate (Pmar1 repressing HesC which represses downstream skeletogenic factors) leads to the activation of skeletogenic genes like alx1, tbr, and ets1.
Experimental Manipulations and Outcomes
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Injection of pmar1 mRNA induces ectopic expression of skeletogenic genes leading to widespread skeletogenic phenotype.
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Injection of hesC antisense RNA derepresses skeletogenic genes, producing similar effects as pmar1 overexpression.
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Conversely, overexpression of hesC mRNA represses skeletogenic genes leading to loss of skeletogenic mesenchyme.
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Knockdown of downstream transcription factors (alx1, tbr) also disrupts normal skeletogenic development.
Gene Regulatory Network Implications
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This regulatory network exemplifies tightly controlled genetic switches in embryogenesis.
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The system relies on hierarchical repression and activation, ensuring spatial specificity and robust development.
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Understanding this GRN highlights the molecular logic underlying cell differentiation and tissue patterning.
Conclusion
The specification of large micromeres in sea urchin embryos is governed by a well-characterized gene regulatory network centered on Pmar1-HesC interactions. Experimental modulations of these factors reveal the essential genetic control over skeletogenic mesenchyme fate.
Final Answer:
(1) A-2, B-1, C-1, D-2
