Worm Breeder's Gazette 2(1): 10
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.
Suppressor mutations compensating for defects in the body musculature of C. elegans have been studied in collaboration with Sydney Brenner. Suppressor mutants were found among EMS-induced revertants of the unc-54 mutants, E1258 and E1315, and the unc-15 mutant, E73. Nine independent suppressor mutations were judged to lie at a single suppressor locus, sup-3. The sup-3 locus is located 3.8% to the right of dpy-11 on LG V. Suppression is dominant, but dose- dependent, and results in improved locomotion as well as an increased ability to lay eggs. Mutations in 6 genes known to affect muscle assembly or function ( unc 15, 22, 52, 54, 66, 87) were tested for suppression by three representative suppressor alleles. Mutations in unc 15, 54, and 87 are suppressed. There is no indication of allele-specific suppression, except that dominant or semi-dominant mutants are generally not suppressed. It seems that among unc-54 alleles at least, those mutants producing no unc-54 myosin are suppressed best, while those mutants producing the full amount of protein are not suppressed. Quantitative differences were detected between independent suppressor isolates based on locomotion and on egg laying. Additionally, suppression of E73 is invariably stronger than suppression of any unc- 54 or unc-87 allele tested. These factors, coupled with the dose- dependence of suppression, gives rise to the widest possible range of suppressed phenotypes. In the case of weak suppressor alleles like e1390, heterozygous-suppressed unc-54 mutants are hardly distinguishable from the unc-54 mutant itself. At the other extreme, E73 homozygous for a strong suppressor like e1407 is almost indistinguishable from wild-type in its movement. With one notable exception, suppressor mutations are non-lethal and produce no obvious phenotype when they are placed in a wild-type genetic background, although e1407 homozygotes exhibit a marginal (approx. 20%) decrease in brood size when compared with N2. The exceptional case is e1405, a suppressor allele which is lethal when homozygous. Genetic tests revealed that e1405 is a small deletion encompassing unc-41 and unc-42, two genes adjacent to the sup- 3 locus. The e1405 mutation fails to complement unc-41 or unc-42 mutations. While unc-41 and unc-42 recombine with each other at a frequency of 0.1%, they fail to recombine with e1405 at the level of 10+E-5 . The lethality of the e1405 suppressor may be due to deletion of these or other adjacent genes. Of the 9 suppressor mutants isolated, e1405 is the only one having the characteristics of a deletion. Whether e1405 was induced by EMS or arose spontaneously in the reversion experiment is unknown. However, no revertants have arisen spontaneously in unmutagenized cultures of unc-15 or unc-54 mutants. To my knowledge, this is the first deletion characterized in C. elegans.The significance of the e1405 mutation is that it implies that a 50% reduction in the level of sup-3 gene product ( in e1405/+heterozygotes ) leads to suppression. The high frequency of suppressor mutants (approx. 10+E-4 ) arising as E73 revertants supports the idea that suppression results from a general knock-out of sup-3 gene function rather than a specific alteration in gene activity. It is known that unc-15 and unc-54 mutants exhibit thick filament defects. It is possible that sup-3 mutations compensate for these defects in some indirect way, perhaps by enhancing the function of 'residual' thick filaments present in unc-15 and unc-54 mutants. Microscopic comparison of mutant and suppressed animals in polarized light suggests that body muscles are better organized in suppressed strains.