Worm Breeder's Gazette 13(1): 68 (October 1, 1993)

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.

How to make a posterior vulva

Ralf J. Sommer, Paul W. Sternberg

California Institute of Technology, Division of Biology 156-29, Pasadena, CA 91125

We have started an evolutionary analysis of vulva formation in different nematode species. One obvious morphological difference is between species with a vulva in the central versus posterior body region. In the family Rhabditidae species with a posterior vulva are known in the genera Mesorhabditis, Teratorhabditis and Cruznema, which are thought to belong to three different evolutionary lines. In the strains Rh. (Mesorhabditis) spec., (PS 1179, Sternberg Lab-collection) and Rh. (Cruznema) tripartita the vulva forms at 80% body length, whereas in Rh. (Teratorhabditis) palmarum the vulva forms at 95% body length, just anterior to the anus. Which P-ectoblasts form the vulva? Taking Panagrellus with P(5-8).p making the vulva as an example, it seemed most likely that the more posterior P-cells generate vulva tissues in this species. But cell lineage analysis revealed that in all three species the vulva is formed by the central Pn.p- cells (P4.p-P8.p and P3 .p-P8.prespectively) after migration to the posterior body region. In addition to the posterior vulva, all these species have a monodelphic gonad. The gonad primordium is located in the central position like in C. elegans, and grows towards the posterior during the L2 and L3 .There is thus no contact between the VPCs and the gonad in the early L3 ,when vulva induction occurs in C. elegans. How does vulva induction takes place in these species? In Cruznema the VPCs don't divide until the AC contacts P6 .pin the late L3 .Indeed, ablation of Z(1,4) in the L1 or the AC in the early L3 prevents vulva formation and give rise to 3° lineages in all the VPCs. We therefore conclude that vulva formation is gonad dependent in Cruznema and that the signaling event takes place in the late L3 .This difference in the timing of vulva induction with respect to C. elegans is a clear case of heterochrony. In Mesorhabditis and Teratorhabditis the VPCs undergo at least two rounds of cell divisions before the gonad contacts the forming vulva. Ablations of Z(1,4) in the L1 results in normal vulva formation. Thus vulva development is gonad-independent in these species. To understand VPC pattern formation in this species we did various 1, 2 or 3 - VPC isolation experiments. It is very striking that a cell of the pair P 5,6 L/R is always 1° over any other cell, independent of whether it is in the anterior or posterior position. Furthermore, in the "1-cell isolation" only P5 .pand P6 .pare able to express the correct 1° fate. In contrast, an isolated P7 .por P8 .pshow a hybrid lineage with an incorrect AC contact and abnormal invagination in later stages. P4 .pexpressed only the 3° fate as an isolated cell. The result of different "3 cell isolations" also suggests that the VPCs are not equivalent to each other; P5 .pwas always 1° over P7 .p.This was also seen after ablation of just P6 .p: P5 .pwas 1° in all 21 animals, an experiment that gives different results in C. elegans. Our current working model contains several steps in the vulva-formation process: 1.) An yet unidentified inductive signal, probably from the posterior body region and probably redundant. 2.) A prebias of the VPCs making P5 .pand P6 .pmore likely to adopt the 1° cell fate. In the intact situation P6 .padopts the 1° fate, probably because it gets the inductive signal earlier or in a higher dose. 3.) Intercellular signaling events specify the final cell fate pattern in the equivalence group, like in C. elegans.