Worm Breeder's Gazette 14(2): 40 (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.
Waksman Institute, Rutgers University, Piscataway, NJ 08855
We are interested in using C. elegans genetics as a tool in identifying components of TGF-b-like signaling pathways. Our attention has focused so far on the characterization of sma-2, sma-3, and sma-4, which function with the TGF-b-like receptor DAF-4 (WM95 p. 56; Savage et al., PNAS, in press). Mutants for each of these genes have at least two phenotypes in common: small body size (Sma) and specific male tail ray and spicule defects (Mab). We have found that sma-2, sma-3, and sma-4 encode related proteins, the dwarfins, that appear to act downstream of the receptor. Although the dwarfins are novel proteins, the family includes the Mad gene product in Drosophila, which functions in the dpp TGF-b-like pathway (Sekelsky et al., Genetics 139: 1347-1358). Additional members of the family are being uncovered at an alarming rate (see abstract by Das et al., this gazette). We are therefore encouraged by the success of using the phenotypes of the sma genes as criteria for the identification of TGF-b pathway components. To identify other components of this pathway, it seemed reasonable to continue looking at Small mutants, using as a second criterion the presence of specific male tail defects. Existing alleles of the other four sma genes, sma-1, sma-5, sma-6, and sma-8, do not have the daf-4 Mab phenotype (S. Baird, personal communication). We are therefore doing an old-fashioned F2 screen to look for additional Sma mutants. In our first efforts, Sma mutants are being recovered at a rate of ~1/100 genomes scored; and we are aiming to screen at least 10,000 chromosomes. To characterize these mutations, we will place them in complementation groups, map any new genes, and determine the male tail phenotypes. Since we expect to make many new alleles of sma-2, sma-3, and sma-4, we have decided to screen through the new mutants first by doing a complementation test with a sma-4sma-3sma-2 triple mutant. This test will allow us to identify quickly those that are candidates for new genes. Six new Sma mutants have been analyzed this way: 4/6 are alleles of sma-2, sma-3, or sma-4. Of the remaining two mutants, one was X-linked, so that we were able to score for male tail defects immediately, but no defects were seen (n=20). The next step will be to make him-5 double mutants with the alleles that remain. We have already made him-5 doubles with three other new Sma mutants. Since these three mutants have not yet been tested for complementation with sma-2, sma-3, and sma-4, they may include new alleles of these genes. All three mutants had male tail defects, and one of the three showed the characteristic fusions of the Mab phenotype of TGF-b pathway components. Ultimately, we expect to clone those genes that are likely to be in this TGF-b-like pathway. The pathway is currently defined only by the type II receptor, DAF-4, and the three dwarfins, SMA-2, SMA-3, and SMA-4. In our Small mutant collection we may identify mutations in a ligand or type I receptor for this pathway. Ongoing efforts to identify such components molecularly (see abstracts by Krishna et al., this gazette) may help in fitting together the pieces of this pathway. Finally, there are likely to be other unidentified components of this signal transduction pathway, which we hope to identify using this genetic approach.