Fig. 1. Schematic summary of ventral epidermal cell fate specification in P. pacificus and C. elegans wild-type and mutant animals. The ventral epidermis of hermaphrodites and males derives from 12 ectoblasts, named P(1-12).p according to their anteroposterior position. The 12 cells are equally distributed between the pharynx and rectum. (A-E) Cell fate specification in the hermaphrodite. (A) C. elegans wild-type. The vulva is formed from the progeny of the 1° (blue ovals) and 2° (red ovals) vulva precursor cells. 3° cells (yellow ovals) are competent to form vulval tissue, but remain epidermal under wild-type conditions. In C. elegans, P(1,2,9-11).p fuse (`F', white circles) with the hypodermis and are not competent to form part of the vulva. P12.pa is a special cell called hyp12 and forms part of the rectum. (B) In Cel-lin-39 mutants, positional information for the formation of the vulva equivalence group is missing and P(3-8).p fuse with the hypodermis, as do their lineage counterparts in the anterior and posterior body region. (C) In P. pacificus, P(1-4,9-11).p die as a result of programmed cell death (X) and reduce the size of the vulva equivalence group further. P(5-7).p have a 2°-1°-2° pattern, as is also observed in C. elegans, and P8.p is a special epidermal cell (black oval), which is designated as a 4° cell fate. (D) In Ppa-lin-39 mutants, the vulva equivalence group is not formed and P(5-8).p die as a result of programmed cell death (`X'). (E) The Ppa-pax-3 mutants have opposite effects on the Pn.p cells in the central and the posterior body region: P(5-8).p undergo ectopic cell death, whereas P(9-11).p often survive. `E' denotes epidermal cell fate. (F-J) Cell fate specification in males. (F) In C. elegans, P(1-8).p fuse (white circles) with the hypodermis. P10.p and P11.p divide multiple times and form the hook (`H') and associated interneurons (`I'). P9.p remains epidermal, but has been shown to be able to replace P10.p after cell ablation of the latter. (G) In Cel-mab-5 mutants, P(9-11).p fuse with the hypodermis, and no hook is formed. (H) In P. pacificus, P(1-8).p undergo programmed cell death and P(9-11).p development resembles the pattern observed in C. elegans. (I) In Ppa-mab-5 mutants, P(1-11).p die as a result of programmed cell death, suggesting that mab-5 provides positional information for P(9-11).p specification in both species. (J) In Ppa-pax-3 mutants, no patterning differences are observed.