Publication Date


Document Type

Doctoral Dissertation

Academic Program

Interdisciplinary Graduate Program


RNA Therapeutics Institute

First Thesis Advisor

Craig C. Mello, PhD


Blastomeres, Caenorhabditis elegans, Carrier Proteins, Cell Differentiation, DNA-Binding Proteins, Ectoderm, Endoderm, Mesoderm, Transcription Factors


Dissertations, UMMS; Blastomeres; Caenorhabditis elegans; Carrier Proteins; Cell Differentiation; DNA-Binding Proteins; Ectoderm; Endoderm; Mesoderm; Transcription Factors


How do embryos develop with such poise from a single zygote to multiple cells with different identities, and yet survive? At the four-cell stage of the C. elegans embryo, only the blastomere EMS adopts the endo-mesoderm identity. This fate requires SKN-1, the master regulator of endoderm and mesoderm differentiation. However, in the absence of the RNA binding protein POS-1, EMS fails to fulfill its fate despite the presence of SKN-1. pos-1(-) embryos die gutless. Conversely, the RNA binding protein MEX-5 prevents ectoderm blastomeres from adopting the endo-mesoderm identity by repressing SKN-1. mex-5(-) embryos die with excess muscle at the expense of skin and neurons.

Through forward and reverse genetics, I found that genes gld-3/Bicaudal C, cytoplasmic adenylase gld-2, cye-1/Cyclin E, glp-1/Notch and the novel gene neg-1 are suppressors that restore gut development despite the absence of pos-1. Both POS-1 and MEX-5 bind the 3’UTR of neg-1 mRNA and its poly(A) tail requires GLD-3/2 for elongation. Moreover, neg-1 requires MEX-5 for its expression in anterior ectoderm blastomeres and is repressed in EMS by POS-1. Most neg-1(-) embryos die with defects in anterior ectoderm development where the mesoderm transcription factor pha-4 becomes ectopically expressed. This lethality is reduced by the concomitant loss of med- 1, a key mesoderm-promoting transcription factor.

Thus the endo-mesoderm identity of EMS is determined by the presence of SKN- 1 and the POS-1 repression of neg-1, whose expression is promoted by MEX-5. Together they promote the anterior ectoderm identity by repressing mesoderm differentiation. Such checks and balances ensure the vital plurality of cellular identity without the lethal tyranny of a single fate.



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