Worm Breeder's Gazette 4(1): 24

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.

Vulva Development in C. elegans

C. Ferguson, B. Horvitz

Figure 1

The 22-cell vulva of C.  elegans is formed during the late L3 and L4 
stages by the divisions of three ventral hypodermal cells (P5.p, P6.p, 
and P7.p) and the subsequent differentiation of their progeny.  A 
variety of observations have suggested that vulval development is 
controlled by the developing gonad (e.g., Sulston and Horvitz, Devel.  
Biol.  56, 110, 1977).  The most compelling evidence derives from 
laser ablation experiments performed by John White: killing the gonad 
primordium in the L1 prevents the last two rounds of vulval cell 
division.  Thus, the gonad induces vulva formation.
We have identified 16 cell lineage mutants that affect vulva 
development.  In 'vulvaless' (Vul) mutants, ventral hypodermal cell 
divisions fail and no vulva is formed.  Because no eggs are laid by 
Vul mutants, progeny hatch inside of the body of the hermaphrodite, 
which turns into a 'bag of worms' (from which the progeny eventually 
escape).  Vul mutants so far define 5 genes: lin-?a(e1438, e1439) I; 
lin-4(e912) II; lin-?b(e1413) II; lin-3(e1417) IV; and lin-2(e1309, 
e1424, e1437, e1453) X.  In 'multivulva' (Muv) mutants, more ventral 
hypodermal cells divide to produce up to 5 pseudo-vulvae, which appear 
as tumor-like protrusions along the ventral side of the adult 
hermaphrodite.  A proper vulva may or may not be formed, depending 
upon the mutation.  One multivulva mutant, E1322, actually contains 
two separate mutations, n111 on chromosome II and n112 on chromosome 
III; both n111 and n112 are required for the expression of the Muv 
phenotype.  The other Muv mutants so far define at least 2 genes: lin-?
c(n137) III; lin-l(e1206, e1275, n176) IV; and lin-?d(n177).  n137 and 
n177 are dominant.
We are attempting to identify the site of action of the genes 
defined by these mutations.  If only the gonad and hypodermis are 
involved in vulva formation, each mutant must be defective either in 
gonadal cell(s) (i.e., in the production of the inductive signal or in 
hypodermal cells (in the reception of this signal).  Four classes of 
mutants might be expected: 1) gonadal Vul, 2) hypodermal Vul, 3) 
gonadal Muv, and 4) hypodermal Muv.  The site affected in the Muv 
mutants can be determined directly using the laser to ablate the gonad 
in the L1.  Pseudovulvae development by definition is dependent on the 
presence of a gonad in gonadal Muv mutants.  Hence, a gonad-ablated 
Muv should not develop pseudo-vulvae.  John White has performed this 
experiment on two Muv strains, E1275 and E1322.  In both, pseudo-
vulvae still form, implying that both are hypodermal Muv mutants.
These hypodermal Muv mutants can be used to identify hypodermal Vul 
mutants.  Muv Vul doubles are constructed using the hypodermal Muv 
mutants.  The possible classes of animals and phenotypes 
[See Figure 1]
Only if it is a hypodermal Vul can the Vul phenotype be epistatic to 
the Muv phenotype.  The laser experiments indicate that even in the 
complete absence of a gonad, hypodermal Muv mutants are still Muv; 
hence, in the presence of a ' vulvaless gonad', the hypodermal Muv 
would also be Muv.  Thus, a Vul mutant that affects pseudo-vulvae 
formation of a hypodermal Muv strain must be a hypodermal Vul.  If the 
double mutant is always Mw, the site of action of the Vul gene cannot 
be determined, but is consistent with action within the gonad.  Of the 
5 genes defined by Vul mutations, 4 (lin-?a, lin-?
b, n this basis to be of the hypodermal Vul 
class; the other, lin-3, does not affect pseudo-vulvae formation of 
the hypodermal Muv strains and thus could be a gonadal Vul.
Two Muv strains (N176 and N177) have never been observed to lay eggs,
and one Vul strain (E912) lays eggs very rarely.  These mutants can 
be reverted by selecting for eggs.  One spontaneous revertant of E912 
has been identified and found to contain an extragenic X-linked 
suppressor.  The isolated suppressor mutant displays no obvious 
phenotype in the hermaphrodite but leads to the formation of an 
abnormal tail in the male; the suppressor may itself be a cell lineage 
mutant.  No EMS-induced revertants of E912 have been obtained; it 
should be noted that E912 was isolated by Babu after p32 mutagenesis.  
Eight independent EMS-induced revertants of N176 have been obtained, 
all of which are temperature-sensitive; at 25 C all individuals are 
Vul, whereas at 15 C some are Muv and some are Vul.  These revertants 
do not appear to resegregate the original Muv mutation after 
backcrossing with N2, suggesting that both Muv and Vul phenotypes may 
be defined by mutations in a single gene.  One linked and one unlinked 
suppressor of n177 have been obtained.  The isolated extragenic 
suppressor has a Vul phenotype.
Because of the relative ease of isolating and characterizing vulval 
cell lineage mutants, we hope that this system will prove amenable to 
saturation genetics.  By answering a variety of questions regarding 
vulva cell divisions --e.g., how many genes are involved in each cell 
division?  In how many and what other cell divisions is each such gene 
involved?  What factors are common to those cell divisions controlled 
by a given gene? -- we hope to learn more about the nature of the 
partitioning of the genetic program for C.  elegans development.

Figure 1