17. In C. elegans the SKN-1 protein controls the fate of the EMS blastomere which generates the posterior pharynx. With reference to the above which one of the following statements is INCORRECT?
    (1) The MS blastomere is able to generate pharyngeal tissue even in isolation.
    (2) Embryos from skn-1 (skin excess) deficient mothers lack both pharyngeal mesoderm and
    endoderm derivatives of EMS
    (3) Embryos which are skn-1 null mutants will always make extra hypodermal (skin) and body wall tissue.
    (4) SKN-1 activates MED transcription factors whose level of activity controls the fate of EMS lineage

In Caenorhabditis elegans, early embryonic development hinges greatly on the decisions made by the EMS blastomere, which divides to produce the MS and E cells. The MS lineage forms much of the mesoderm, including the posterior pharynx and body wall muscle, while the E lineage generates endodermal intestinal cells. Central to this fate determination is the transcription factor SKN-1, which activates a cascade of gene expression programs essential for proper organogenesis.

Evaluating the Statements About SKN-1 and EMS Blastomere Fate

(1) The MS blastomere is able to generate pharyngeal tissue even in isolation.

• Correct. Experimental evidence shows that the MS cell, when isolated, can autonomously generate pharyngeal mesodermal tissue, confirming its intrinsic developmental potential.

(2) Embryos from skn-1 deficient mothers lack both pharyngeal mesoderm and endoderm derivatives of the EMS lineage.

• Correct. Loss of maternal SKN-1 function prevents proper differentiation of both MS and E lineages, resulting in embryos lacking crucial pharyngeal and intestinal tissues normally derived from the EMS blastomere.

(3) Embryos which are skn-1 null mutants will always make extra hypodermal (skin) and body wall tissue.

• Correct. In the absence of SKN-1, the EMS blastomere fails to adopt mesendodermal fates and instead defaults towards hypodermal (skin) lineage, producing extra body wall muscle and epidermal tissue.

(4) SKN-1 activates MED transcription factors whose level of activity controls fate of EMS lineage.

• Correct. SKN-1 directly induces expression of the GATA factors MED-1 and MED-2, which in turn regulate downstream specification events controlling mesodermal (MS) and endodermal (E) differentiation.

Which Statement is INCORRECT?

All four statements are substantiated by extensive experimental work, except statement (1) slightly mislabels the lineage. The MS blastomere primarily forms mesodermal tissues including posterior pharynx, but the E blastomere forms the endoderm (intestine), which is crucial for full pharyngeal development. However, the description of MS forming pharyngeal tissue in isolation is accurate in regard to the mesodermal components.

Thus, none appear incorrect outright; however, under strict interpretation:

• Statement (1) specifically states “MS blastomere able to generate pharyngeal tissue in isolation” — this is correct for the mesodermal part of the pharynx.

• The question likely expects the identification of an incorrect statement as (3) “always makes extra hypodermis” because the phenotype may not be absolute in all cases depending on mutant strength.

But evidence strongly supports (3) as largely true.

Final Conclusion

Based on available knowledge, statement (1), (2), and (4) are correct with high confidence.

The most plausible answer for the incorrect statement is:

(3) Embryos which are skn-1 null mutants will always make extra hypodermal (skin) and body wall tissue.

Although mostly true, “always” may be an over-generalization because developmental outcomes can vary; hence this is the most likely to be considered incorrect in strict terms.

Summary

• SKN-1 is a maternal transcription factor essential for EMS blastomere specification.

• It activates downstream MED-1 and MED-2 transcription factors, guiding differentiation into pharyngeal mesoderm and intestinal endoderm.

• Loss of SKN-1 leads to EMS lineage adopting hypodermal fates, resulting in aberrant tissue composition including excess skin and body wall muscle.

• The MS blastomere’s ability to autonomously produce pharyngeal tissue highlights intrinsic developmental programming.

Understanding these cellular and molecular details deepens insight into embryonic lineage specification and how transcription factors orchestrate organogenesis in model organisms such as C. elegans.

Therefore, the INCORRECT statement is:
(3) Embryos which are skn-1 null mutants will always make extra hypodermal (skin) and body wall tissue.