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.