Worm Breeder's Gazette 2(1): 21
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.
We are interested in X-linked recessive lethal (including sterile) mutations, which we call let, induced by EMS and balanced by Dp(X;V)1. Specifically, we are looking for mutants with the following genotype: unc-3 let/unc-3 let; Dp(X;V)1/+, where Dp(X;V)1 carries unc-3+ let+. Such animals segregate inviable or sterile uncs. Dp(X;V)1 is retained in heterozygous form since homozygotes are gonadless. Our procedure is to mutagenize unc-3/unc-3; (X;V)1/+ animals and screen the offspring of individual F2 wild-type animals for inviable or sterile uncs. Wild-type sibs are kept to maintain the stock. To establish that the mutation is X-linked, the wild-type sib is crossed to N2 males. If the lethal acts early and is not sex-limited, there are no unc male offspring issuing from this cross. In any case, wild- type male offspring (putative genotype: unc-3 let/0;Dp(X;V)1) are back- crossed to the wild-type sib. If the lethal is X-linked, there will be no fertile unc hermaphrodite offspring. Complementation testing is straightforward: unc-3 let(MN1)/0; Dp(X;V)1/+ males are crossed to unc- 3 let(MN2)/unc-3 let(MN2); Dp(X;V)1/+ hermaphrodites, and the progeny are screened for fertile unc hermaphrodites. By this procedure, we have found 12 independent X-linked recessive lethals that fall into 10 complementation groups, with repeats in 2 genes. On the assumption that all genes in the region are equally mutable, we calculate, using the Poisson distribution, that there are roughly 26 essential genes in the region covered by Dp(X;V)1. This would obviously be an underestimate if some genes are much more mutable than others (by virtue of being larger, for example). The phenotypes of the lethals have so far been only crudely characterized. Three of the 10 act early: no uncs are produced. Five give uncs that hatch but are arrested at various larval stages, typically L2 or L3. One mutant, let-9, gives uncs that grow to adults but are sterile. Crossing these uncs to N2 males gives fertile wild- type hermaphrodites (showing that the oocytes are rescuable by let-9+- containing sperm) and unc males, which indicate either a sperm defect or an unusual sex-limited effect. To test this, the unc-3 odites were crossed to unc-3 /+. Unc hermaphrodites were among the progeny. This result indicates that the let-9+ gene is essential in hermaphrodites for either spermatogenesis or for permitting endogenous sperm to fertilize - although it is not necessary for the sperm to carry let-9 in order to be capable of fertilization. The remaining mutant exhibits a prolonged maternal effect: the uncs are fertile but their progeny die as larvae. Crossing these uncs with N2 males gives wild-type hermaphrodites but no mature unc males. Work on these mutants and others isolated more recently is continuing. The same procedure that gives the X-linked recessives also gives two other classes of mutants separable from these on the backcross. One type includes mutants found on LGV in the region where crossing over is suppressed by Dp(X;V)1. Complementation and characterization of these is more involved than for the X, and saturation of the region appears less easy, so little work has been done on them so far. The other mutant class includes lethals that map elsewhere but seem to be specific for unc-3 in their lethal action. There appear to be a fair number of these, but we have done nothing with them.