It should be apparent from the discussion in Chapter 8 of the textbook that the pharynx of C. elegans is generated by two sets of cells. One group of pharyngeal precursors comes from the EMS cell and is dependent on the maternal skn-1 gene. The second group of pharyngeal precursors comes from the ABa blastomere and is dependent on GLP-1 signaling from the EMS cell. In both cases, the pharyngeal precursor cells (and only these cells) are instructed to activate the pha-4 gene (Mango et al. 1994b). The pha-4 gene encodes a transcription factor that resembles the mammalian HNF3β protein. Microarray studies by Gaudet and Mango (2002) revealed that the PHA-4 transcription factor activates almost all of the pharynx-specific genes. It appears that the PHA-4 transcription factor may be the node that takes the maternal inputs and transforms them into a signal that transcribes the zygotic genes necessary for pharynx development.
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