Worm Breeder's Gazette 14(2): 55 (February 1, 1996)
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.
MRC-LMB, Cambridge CB2 2QH, England
Extensive genetic experiments have indicated that seven genes
involved in sex determination in C. elegans (her-1, tra-2 & tra-3,
fem-1,-2,-3, tra-1) are organized in a regulatory hierarchy. In all
previous tests examining somatic sexual phenotype, mutations of tra-2
and tra-3 were found to be epistatic to mutations of her-1, mutations
of the fem genes were found to epistatic to her-1, tra-2 and tra-3, and
mutations of tra-1 were epistatic to mutations in any of the other six
genes. Hence the basic model:
her-1 --| tra-2,-3 --| fem-1, -2, -3 --|
tra-1
At the 1995 International C. elegans Meeting, a surprising
result was described that contrasts with this consistent pattern. Betsy
Goodwin reported that the strongest hypermorphic (feminizing) mutation
of tra-2, e2020, is epistatic to loss-of-function (masculinizing)
mutations of tra-1. In view of this report, which could modify our
current understanding of the sex determination pathway, I have
re-examined the epistatic relationship between tra-2(e2020) and
tra-1(lf) mutations. I was unable, however, to repeat Betsy's
finding. The following results indicate that tra-1(lf) mutations
appear to be fully epistatic to tra-2(e2020), in all aspects of somatic
phenotype. The germ line phenotype also exhibits partial epistasis.
First, tra-2(e2020); tra-1(lf) XX animals were generated by
carrrying out single pair matings between individual tra-2(e2020)
unc-4/++; tra-1/+ XX females and tra-2(e2020) unc-4/++; tra-1 XX
males. Initial crosses were carried out using a standard strong lf
mutation of tra-1, e1099. Parental genotypes were verified by progeny
testing. A total of 241 Unc progeny were scored, of which half
(129/241) were clearly male in phenotype. These must have been e2020;
e1099 XX homozygotes, apart from a few rare e2020/+; e1099
recombinants. 51 of these were examined by Nomarski microscopy: all
were unambiguously and perfectly male with respect to tail structure,
absence of yolk, body size and so on. Somatic gonad phenotypes were
variable, like tra-1(e1099) alone, but never feminized. Germ line
phenotypes exhibited more oogenesis than e1099 alone (about half the
animals contained oocyte-like cells), but also sperm were seen in some
individuals, in contrast to e2020 XX females, which never make sperm.
Similar crosses were carried out using tra-1(e1834), which is a deletion
of most of the 5' half of tra-1, with essentially identical results to
those obtained with tra-1(e1099).
Second, in order to test for possible maternal effects, and to
examine the male fertility of the tra-2(e2020); tra-1(lf) XX
homozygotes, a male/female strain was constructed using e2020 and
e1099. The strain CB5172 consists of tra-2(e2020); tra-1(e1099)/+ XX
females and tra-2(e2020); tra-1(e1099) XX males. This strain was
propagated for many generations, segregating 50% females and 50% males
at each generation. No animals of ambiguous sexual phenotype were
observed, even after starvation, which enhances the feminizing
properties of tra-2(e2020).
Survival of this strain necessarily depends on the fertility of
the e2020; e1099 XX males. All exhibit mating behavior, but only
about 1 in 9 is able to sire progeny (as compared to about 1 in 5 for
normal e1099 XX males). This is nevertheless enough to permit
propagation. When crossed with marked hermaphrodites, the males sire
only XX female progeny, confirming homozygosity for e2020 and e1099.
As expected, the XX strain is unstable: when populations expand above a
few hundred individuals, rare e2020; e1099/+ XO animals arise. Mating
by these high fertility males and their e2020 XO sons soon leads to a
pure e2020 strain. This has the same distinctive feminizing properties
as the original e2020, indicating that no modifiers have accumulated.
A stable version of CB5172 was also constructed (CB5190), by including
xol-1.
These results, and the strain CB5172, were communicated to Betsy
Goodwin last fall, in the hope of clarifying the source of the
discrepancy between our observations.