Fig. 2. Model of the regulation of cell fusion and cell death in the ventral epidermis in C. elegans and P. pacificus hermaphrodites. (A) Model of cell fusion regulation in the ventral epidermis of C. elegans. lin-39 is transcriptionally regulated by EGF and WNT signaling. Several studies have suggested that LIN-39 regulates the expression of egl-18 and elt-6, which encode GATA transcription factors, which in turn regulates expression of the cell fusion effector gene eff-1. (B) Based on data described in this study, Ppa-LIN-39 acts upstream of Ppa-pax-3. (C) Model of P. pacificus cell fate specification in the ventral epidermis of the hermaphrodite. Positional information subdivides the 12 ventral epidermal cells of P. pacificus into at least five distinct areas. P(5-8).p (red box) are regulated by Ppa-LIN-39 and Ppa-PAX-3, as described in this study. P(9-11).p (yellow box) die from programmed cell death (PCD) and Ppa-pax-3 is the first gene shown to be involved in this regulatory process. P12.pa is a special cell that forms part of the rectum. In C. elegans, it is regulated by the posterior Hox gene egl-5 (green box). P(3,4).p (blue box) die from PCD and are specifically regulated by Ppa-HAIRY and Ppa-GROUCHO, which downregulate Ppa-lin-39 expression in these cells. P(1,2).p (pink box) also die from PCD, but no specific regulators have been obtained.