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unpublished).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.