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.