Worm Breeder's Gazette 15(3): 16 (June 1, 1998)
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.
MPI for Developmental Biology, Spemannstrasse 35, 72076 Tuebingen, Germany
The comparison of cell fate specification in the vulva equivalence group between C. elegans and Pristionchus pacificus, reveals several evolutionary differences. In P. pacificus, P8.p loses its competence to adopt vulval cell fate early in the first larval stage and can only adopt an epidermal fate. To isolate mutations with defects in the specification of P(7,8).p, we carried out mutagenesis screens of a total of approximately 100 000 gametes (WBG 14, (4) page 42). Many egg-laying defective mutations were isolated, 16 of which have an ectopic vulva-like structure in the posterior body region. In 11 of these 16 mutations the specification of P8.p was affected. In most mutant animals analyzed, a posterior vulva-like structure was formed by P8.p. The specification of P7.p was also affected but in a different way. Cell lineage analysis of the mutant strains tu34, tu36, tu37, tu52 and tu74 revealed that P7.p adopted the 1! fate in approximately 5% of animals. In such animals, the main invagination was generated by the progeny of P(6,7).p and P6.p had the 2! fate. The AC in these cases was displaced posteriorly so that it was now located above P7.p. Complementation tests revealed that all 11 mutations are allelic to one another and to the previously isolated mutation Ppa-ped-4. Further genetic analysis indicated that Ppa-ped-4 is linked to the visible mutation Ppa-dpy-1 which has been shown to be closely linked to the homeotic gene Ppa-lin-39. This result suggests, that Ppa-ped-4 might be associated with, or might be a member of the Hox genes of P. pacificus. One candidate gene would be mab-5. To test if Ppa-ped-4 corresponds to P. pacificus mab-5, we cloned the Ppa-mab-5 homeodomain by PCR and isolated genomic and cDNA clones, the latter by performing RT-PCR. We analyzed the complete Ppa-mab-5 coding region in 5 of the 12 ped-4 mutant strains and found mutations in all of them indicating that Ppa-ped-4 is identical to the homeotic gene mab-5. To simplify nomenclature we renamed Ppa-ped-4 as Ppa-mab-5. When we ablated Z(1,4) in Ppa-mab-5 mutant animals, P7.p and P8.p could differentiate into vulval cells, whereas P(5,6).p remained epidermal. These results suggest that only P(7,8).p can differentiate into vulva-like structures in a gonad-independent way in Ppa-mab- 5 mutant animals, indicating that P(7,8).p are distinct from P(5,6).p. The gonad-independent differentiation properties of P(7,8).p might occur in response to an unknown signal or because of an intrinsic property of these cells to undergo vulva-like differentiation. In wild-type animals, Ppa-mab-5 functions as part of a negative signaling system that antagonizes the differentiation properties of P(7,8).p. The Ppa-mab-5-mediated process prevents the inappropriate differentiation of P8.p by making this cell incompetent to adopt vulval fate. This effect of Ppa-mab-5 is eventually restricted to P8.p, as the activity in P7.p is overruled by gonadal signaling that induces vulva formation by P(5-7).p. Comparing the loss-of-function phenotypes, the strongest difference between the Ppa-mab-5 and the Cel-mab-5 mutations is that in Cel- mab-5 no ectopic vulva differentiation occurs. Furthermore, Cel- mab-5 mutant animals have VPCs that are identical in their developmental potential (Clandinin et al., Dev. Biol. 182, 150ff). In contrast, in Ppa-mab-5 mutations P(7,8).p still differ from P(5,6).p resulting in gonad independent differentiation. This observation indicates that factors other than mab-5 distinguish P(7,8).p from P(5,6).p or vice versa. One major function of Ppa-MAB-5 is to antagonize the gonad-independent differentiation properties of P(7,8).p. Mechanistically, MAB-5 might act in different genetic circuits with different target genes in both species. For example, Ppa-mab-5 might be part of a locally restricted negative signaling system that is only present in P. pacificus P(7,8).p. However, we cannot rule out that only quantitative differences of MAB-5 activity in the regulation of identical downstream target genes led to the observed competence patterns in both species.