Worm Breeder's Gazette 13(2): 29 (February 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.
Several evolutionary lines of the family Rhabditidae contain species with a vulva in the central body region. Cell lineage analysis and cell ablation experiments in Dolichorhabditis carpathica, Oscheius tipulae, Rhabditella axei, Rhabditoides regina, Pelodera strongyloides, and Protorhabditis spec. revealed that in all these species the vulva is formed by the progeny of P(5-7).p, like in species with posterior vulva formation (WBG13, 68). Nonetheless evolutionary alterations are present at three levels:
1.) Number of cells contributing to the equivalence group:
In most species only P(4-8).p are part of the vulva equivalence group. Ablation experiments revealed that the cell-fate pattern is totally symmetric (3°-2°-1°-2°3°). Because just one cell of the pair P3 /4L,R is part of the equivalence group we tested, if the migration of the two cells of this pair to the ventral cord is still random. Ablation of P3 /4Land P3 /4Rin Dolichorhabditis give both rise to the failure of P3 .por P4 .p,suggesting that the migration of these cells is random, and thus the equivalence and boundary is set at the Pn or Pn.p level, but not at the level of the L/R pairs. Only in Caenorhabditis and Protorhabditis also P3 .pis part of the equivalence group. Because of the proposed position of Protorhabditis at the base of the phylogenetic tree, we cannot distinguish if the 5 or 6 cell equivalence group is ancestral.
2.) The final cell lineage pattern of all the VPCs:
P6 .p,which assumes the 1° cell fate, produces 8 progeny. On the other hand, the 2° cell fate of P5 .pand P7 .pshows very different cell lineages in the different species. The least complex lineage is in Dolichorhabditis, in which P(5,7).p generate only 4 progeny. In other species all intermediates from 5 to the 7 progeny occur. The 3° lineage of P(4,8).p varies between 2 progeny and 6 progeny. In Rhabditoides regina and Pelodera strongyloides, the two inner cells of the 4 cell stage undergo a third round of cell division, shown in Fig.1. Thus an asymmetric 3° lineage exist in these
two species. How is the mirror image difference between the P4 .pand the P8 .plineage regulated? After ablation of the gonad all VPCs express the 3° lineage of P4 .p,suggesting this to be the ground-state. Thus, vulva induction by the gonad is directly, or more likely asymmetric lineage of P8 .p.This is similar to what Wendy Katz has found in C. elegans for the 2° lineage.
3.) Variability of cell lineages: Most striking is the finding of variability in Pelodera strongyloides. In this species P3 .por P9 .pdivide in nearly 50% of the females and express a partial or complete 3° lineage. I ncreased temperature changes the frequency of this variability. To test if one or both cells can become part of the equivalence group, we performed cell ablation experiments. So far we have not seen P3 .por P9 .pform part of the vulva, even after ablation of P(5-8).p. Variability in cell lineages might play an important role in the transition between invariant cell lineages.