Worm Breeder's Gazette 13(3): 107 (June 1, 1994)

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 III: Minimizing the equivalence group

Ralf J. Sommer, Paul W. Sternberg

HHMI & California Institute of Technology, Division of Biology 156-29, Pasadena, CA

Expanding our comparison of vulva formation to species of families other than the Rhabditidae revealed alterations also at the level of the P-ectoblasts, the precursors for the ventral hypodermis and the vulva. In all species of the family Rhabditidae analyzed so far 12 Pn.p cells are present and P(3-8).p or P(4-8).p form the vulva equivalence group. In Panagrolaimus of the family Panagrolaimidae and Pristionchus of the family Neodiplogasteridae just P(5-8).p and hyp12 are present in the ventral cord; P(1-4).p and P(9-11).p are absent. This character evolved independently in Pristionchus and Panagrolaimus because in Panagrellus redivivus, another species of the family Panagrolaimidae, all 12 Pn.p are present (Sternberg & Horvitz, Dev.Biol., 1982). Lineage analysis of the Pn.p cells in both species support this conclusion. In Panagrolaimus, the Pn-migration into the ventral cord and the asymmetric cell division occur during the L1 stage like in Caenorhabditis. P(1-4).p die after birth, P(5-8).p become ectoblasts, P(9-11).p move to a dorsal position in the ventral cord and are later indistinguishable from surrounding neurons. P12 .pdivides later in the L1 stage and P12 .pasurvives and forms hyp12 .P(5-8).p are involved in vulva development like in Panagrellus, thus all present central ectoblasts form part of the vulva.

In Pristionchus on the other hand, Pn-migration and the Pn-division take place during embryogenesis. At hatching 12 large precursors (presumably P1. aP12 .a),P(5-8).p and P12 .pare present in the ventral cord. A few hours after hatching the Pn.a cells divide to produce the neural progeny, P(5-8).p and P12 .penlarge during the L1 stage and adopt the typical ectoblast morphology. We do not know whether P(1-4).p and P(9-11).p die after birth during embryogenesis or if the corresponding Pn cells skip the asymmetric division. We have not seen P12 .pdividing during the Ll stage. Also, this division could have taking place during embryogenesis, so we call the surviving ectoblast hyp12 .

In contrast to Panagrolaimus, only P(5-7).p form vulval tissue in Pristionchus. P8 .pdoes not divide. Cell ablation experiments reveal that vulva development is gonad dependent and that P8 .pis not part of the equivalence group in Pristionchus. After ablation of one, two or all three VPCs, P8 .pnever migrated inward or formed vulval tissue. To get a molecular understanding of pattern formation among the P-ectoblasts we have started molecular cloning of Hom-C and " lin-3 / let-23 pathway" genes. In addition we are initiating genetic and transgenic analysis in Pristionchus.

In summary different strategies exist to remove ventral ectoblasts that are not involved in vulva development and which fuse with hyp-7 in Caenorhabditis. This process furthermore reduces the equivalence group to the number of cells which normally form the vulva.