Worm Breeder's Gazette 14(2): 66 (February 1, 1996)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Biology 156-29, Caltech, Pasadena CA91125, USA
In Caenorhabditis elegans, P3.p to P8.p are competent to form the vulva, and in wild type development only P5.p to P7.p, centered around the anchor cell, actually do form the vulva. The central cell, P6.p, acquires a specific fate, revealed in particular by the TTTT lineage of its 4 granddaughters. The patterning of the Pn.p fates occurs via multiple interactions: an inductive signal from the anchor cell, lateral signalling between the Pn.p cells and negative signalling, presumably from the surrounding epidermis. Induction occurs in early L3, before they divide (Kimble (1981), Dev. Biol. 87, 286-300, and M. Wang and P.W.S., unpublished). In the family Panagrolaimidae, the anchor cell is positioned between P6.p and P7.p, and the 'TTTT' inner fate is shared between their 2 central daughters, P6.pp and P7.pa. By ablating the gonad or the anchor cell in Panagrolaimus PS1732 at different times, we found two temporally distinct signals acting on the Pn.p cells: the first one originates from the gonad, in early L2 (when the gonad precursors Z1 and Z4 have not yet divided- they divide late in L2 in this species). This first signal induces the Pn.p cells to divide twice in late L3 and form vulval tissue. The second inductive signal, in late L3, originates from the anchor cell and induces the central daughters of the Pn.p cells (P6.pp and P7.pa) to divide twice more instead of only once ('TT' inner fate). In the Oscheius genus (family Rhabditidae, like C. elegans), the vulva is centered, as in C. elegans, on P6.p. The lineages of P5.p and P7.p are simple: they divide only twice, and in contrast to P4.p and P8.p, participate to the vulva invagination (Sommer and Sternberg (1995), Dev. Biol. 167, 61-74). As in Panagrolaimus, we found 2 successive inductions in Oscheius PS1131. In this case both come from the anchor cell, the first in early L3 on the P(5-7).p, and the second in late L3 on P6.pa and P6.pp. Complex patterning of the vulva thus occurs through multiple interactions acting on the Pn.p cells in Caenorhabditis, and by 2 successive localized inductions on the Pn.p cells and their daughters in Panagrolaimus and Oscheius. The second induction occurs when the anchor cell appears to contact tightly the induced cells: this signal could be membrane-bound. This is an example of evolution in the 'wiring diagram' of interactions between cells, in its connectivity and in its temporal execution. The direction of evolution is not clear: analysis of more species might infirm or confirm the hypotheses that the 2-step mechanism is ancestral to the '1-step/many-interactions' found in C. elegans and that the centering of the vulva on P6.p in Oscheius is an intermediate. We also wish to determine the molecular nature of these 2 signals.