Worm Breeder's Gazette 11(5): 74
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.
Using the procedure of Howell et al (1987), the Rose laboratory has tested more than 31000 chromosomes mutagenized with EMS (dose 12-17 mM) and recovered 550 lethal mutations. Ninety lethal mutations had been mapped to the region defined by hDp19 and sDp2 by J. McDowall, K. McKim and A. M. Rose. In addition, some duplications and deletions in this region and can be used to order the lethal mutations. sem-4 was identified by M. Basson and he isolated n2088 allele of sem-4. We have found that n2088 fails to complement three other essential genes. The largest mutational target was let-385 with eighteen alleles. The visible genes dpy-14, ped to the hDp19 - hDp62 region. Mutations of these genes were complementation tested against representative alleles of the essential genes. I have found two lethal mutations, s80 and h763, which failed to complement the Unc phenotype of unc-37(e262). In contrast, one gene identified on the basis of a lethal phenotype, let-605, has been found to have an Unc phenotype in the absence of linked visible markers. A concern was that there are lethal mutations which cannot be recovered with the sDp2 system. This is an important consideration since our method of isolating and mapping essential genes relies heavily on the ability of duplications to rescue lethal mutations. To determine the frequency of these lethal mutations, a screen was done using the translocation hT1(I;V) as a balancer (Howell and Rose; Genetics 126: 583). Lethal mutations were recovered in the region between the left end of chromosome I and unc-29. Forty-four mutations from this screen were analyzed. Howell and Rose showed 22 of these mutations could be rescued by sDp2. We tested the remaining 22 mutations for rescue with sDp2 and hDp19. Nine mutations were rescued by hDp19 and five more were rescued by sDp2 but not by hDp19.Further mapping showed these eight mutations mapped outside the sDp2 region. Two duplications known to cover part (hDp70) or all of the region ( nDp4) were used to determine if the lethal mutations could be complemented by a duplication. Three were rescued by hDp70 and all eight were rescued by nDp4. Therefore, no mutations were inside the sDp2 region and lethal in let-x/let-x/Dp[let-x(+)] genotypes and thus most essential genes can be identified and mapped using duplications. More precise mapping of these eight mutations (see Figure) has shown they all map to the right of the sDp2 breakpoint and define eight new genes (let-535 to let-let-542).Although this analysis shows most essential genes can be identified and mapped by our approach, we still expect a small number to not be rescued. As an example of this, Rosenbluth et al (Genet. Res. 52: 105) proposed an X-linked duplication failed to rescue some lethal mutations because of dosage compensation. We have found two exceptions. First, two of the lethal mutations isolated with hT1, h940, an allele of let-385, and h1003, the second allele let-520, were poorly rescued by some duplications. Approximately 80-90% of the let-x/let-x/Dp strains were fertile. These effects were probably not due to second site semi-sterile mutations because when either h940 or h1003 was derived from a rare fertile let;Dp strain, the same effect was observed. In addition, these mutations were more efficiently rescued by some duplications than others. A second example is that some duplications poorly rescue most genes. hDp65 and hDp50 poorly rescue most of the genes in the regions they duplicate. This may be because these duplications are segregationally unstable or because they have a piece of the X- chromosome (unc-1 dpy-3 region) on them. [See Figure 1]