Worm Breeder's Gazette 14(1): 32 (October 1, 1995)

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.

Evolution of vulva formation: Part VI: 5 vs. 12 ventral ectoblasts as a common theme

Ralf J. Sommer^1,2, Paul W. Sternberg¹

Previously, we described evolutionary alterations of the number of Pn.p-
ectoblasts in the ventral cord in Pristionchus pacificus
(Neodiplogasteridae) and Panagrolaimus sp. PS 1159 (Panagrolaimidae) (WBG
13, #3, 107). In these two species only P(5-8).p and the hypl2 cell are
present. Based on the fact that Panagrellus redivivus, another species of
the Panagrolaimidae, has twelve Pn.p cells like C. elegans, we assumed
that the five Pn.p cell state is a highly derived character.

Our original survey of vulva development within the rhabditids did not
reveal a corresponding lineage difference among species (WBG 13, #2, 29).
However, we observed a similar evolutionary alteration in Poikilolaimus
oxycerca. In this species, P(1-4).p and P(9-11).p die immediately after
birth in the L1 stage, so that only five ectoblasts survive. The vulva is
formed by P(5-7).p with a lineage typical for rhabditids. P8.p, the other
ectoblast in the central body region, does not divide. Cell ablation
experiments reveal that P8.p is a member of the vulva equivalence group:
after ablation of P7.p, P8.p generates a P7.p-like lineage. By contrast,
in Pristionchus, P8.p is not part of the vulva equivalence group and
cannot replace P7.p after ablation. Based on morphological data
Poikilolaimus is a highly evolved species of the "Eurhabditis". Thus, the
five Pn.p cell state is presumably a derived character.

Pristionchus. In our earlier report on Pristionchus we were unable to tell
what happens to the Pn.p cells in the anterior and posterior body region
(WBG 13, #3, 107). This is because the P-ectoblast migration and the
asymmetric cell division generating neuroblasts and ectoblasts take place
during embryogenesis, indicating a heterochrony in comparison to all other
analyzed nematodes. Early problems of the lineage analysis in embryos of
Pristionchus were overcome by the use of PBS instead of M9 or S-Basal. As
in Poikilolaimus, P(1-4).p and P(9-11).p die immediately after birth.
Thus, programmed cell death helps limit the vulva equivalence group in
these species.