Worm Breeder's Gazette 12(4): 62 (October 1, 1992)

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.

Identification of Additional Dauer-Constitutive Mutations Using an Unbiased Screen

Elizabeth A. Malone, James H. Thomas

Department of Genetics, University of Washington, Seattle, WA 98195

In response to adverse environmental conditions worms form dauer larvae instead of entering the usual third-larval stage. A constitutively-secreted pheromone is the primary inducer of dauer formation while high temperature and limiting amounts of food also promote this process. Entrance to and exit from the dauer state is controlled by certain amphid sensory neurons (Bargmann and Horvitz, Science 251:1243). Mutations that affect dauer formation are mainly of two types. Dauer-defective (Daf-d) mutations prevent dauer formation, and dauer constitutive (Daf-c) mutations promote dauer formation even under non-dauer-inducing conditions. The Daf-c mutations identify ten genes, and nearly all were isolated as temperature-sensitive mutations that cause dauer formation at 25- and permit recovery and continued growth at 15°

Because of the method of identification, the known Daf-c genes may be strongly biased toward genes that can readily mutate to a temperature-sensitive phenotype. In addition, the Daf-c mutations are biased against those that also cause sterility, inhibit dauer recovery, or otherwise prevent propagation at 15°. We were particularly concerned about genes that regulate both dauer formation and dauer recovery because amphid neurons control both processes. In order to address these concerns, we have screened clonally for additional Daf-c mutations, a method that requires only that the homozygous mutants be viable up to the dauer stage.

After screening approximately 10,000 genomes, we have identified 22 new Daf-c alleles that result in dauer formation at 25°. Of these, 18 are recoverable as viable homozygotes at 15°. Since our screen did not demand temperature-sensitive mutations, we suspect that this temperature dependence reflects the previously described influence of temperature on the process of dauer formation itself (Golden and Riddle, PNAS 81: 819). The high frequency at which we isolated mutations with incompletely penetrant phenotypes at low temperature can be explained by a model in which two parallel pathways regulate dauer formation (Thomas and Birnby, this WBG). Complementation tests show that at least 14 of these 18 Daf-c mutations are mutations in previously identified daf-c genes. These results suggest that there is not a large class of genes in which mutations cause pleiotropic defects in addition to constitutive dauer formation.

One mutation, sa191 ,has characteristics unique among Daf-c mutations. At 25°, nearly all sa191 worms form dauers and then recover immediately. (Other Daf-c mutations result in dauers that fail to recover at 25° and take up to several days to recover at 15°) sa191 is also partially dominant; heterozygous worms become dauers about 10% of the time. We have mapped sa191 to the right arm of chromosome V. The phenotypes and the map position of sa191 suggest that it identifies a new gene.

Additional mapping and complementation testing are underway to assign the seven remaining new alleles to genes.

Literature Cited:

Bargmann and Horvitz, Science 251:1243.

Golden and Riddle, PNAS 81: 819.

Thomas and Birnby, WBG 12.4.92.