Worm Breeder's Gazette 13(5): 45 (February 1, 1995)

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.

An Alarming Number of Synthetic Daf-c Mutations.

James H. Thomas, Kouichi Iwasaki, Elizabeth Malone, Michael Ailion

Dept. of Genetics, Univ. of Washington, Seattle, WA

A small number of genes can mutate to produce a moderately
strong dauer constitutive (Daf-c) phenotype. When combined,
mutations in certain groups of these genes produce a strongly
enhanced Daf-c phenotype, suggesting these groups act
in parallel to regulate dauer formation. Over the past
several years, there have been scattered reports of a Daf-c
phenotype in double mutants comprised of single mutations
with little or no effect on dauer formation. These genes
include aex-3, egl-4, flr-4 unc-3, unc-31, and several
of the cilium-structure genes including che-3, osm-l,
and osm-6. We have recently added dec-S (defecation cycle
abnormal) and unc-64 to this list Not all combinations
of double mutants among these genes have been analyzed
but it appears that the synthetic pattems will be complex.
While this list of genes is not yet alarmingly long, two
observations suggest that it probably will become so.
First, nearly all of these synthetic interactions seem
to have been found by chance in the process of mapping or
other routine genetic manipulations. For example, we
found that unc-64; dec-5 is Daf-c in the process of using
unc-64 as a marker in a dec-5 construction. Such unsystematic
analysis surely has missed many interactions among known
genes. Second, in a 5,000 genome screen by Katsura, Suzuki,
and Ishihara (WBG 13-2), more than forty mutations that
cause a synthetic Daf-c phenotype with unc-31 were found.
The naive calculation would suggest about sixteen genes
can readily mutate to this phenotype (with unc-31 alone!).
If anyone out there has been surprised to notice some dauers
unexpectedly forming in an unstarved stock we'd like to
know about them.
Why so much redundancy and so much complexity? We offer
some observations that suggest why and where much of this
complexity occurs. Dauer formation is a complex process
that requires the inteagration of many developmental
and sensory cues. Among the sensory cues are dauer pheromone,
temperature, and food (itself almost certain to be complex).
Less is known about developmental cues, but at least heterochronic
genes are involved in determining the stage of dauer-formation
competence. Integration of complex cues requires convergence
of parallel pathways of information, and this implies
functional redundancy. Such neuronal integration in
larger organisms occurs when information from diverse
sensory neurons converges on interneurons (often gradually,
over more than one level of cells), whose task is to extract
relevant features of the sensory input and orchestrate
an appropriate response. Given our poor state of knowledge
of the functions of interneurons in C elegans, how can we
begin to address whether any of these synthetic Daf-c processes
occur in interneurons? We have two simple thoughts at the
moment. The first is a consideration of the pleiotropies
caused by single mutations in each gene. At least some aspects
of the Unc, Aex, Dec, and Flr phenotypes conferred by mutations
in the various genes seem unlikely to be caused by sensory
neuron defects (in contrast for example, all aspects of
the phenotypes of the cilium-structure mutations are
easily explicable as sensoly defects). In addition, the
synthetic Daf-c phenotype seems unlikely to be caused
by motor neuron defects. Thus, for several of these genes
it is plausible to suppose that they affect interneurons
(though of course they may affect other cells as well).
Our second thought is to test epistasis with daf-3 and daf-5.
Daf-d mutations in these two genes block the Daf-c phenotype
caused by killing amphid sensory neurons, suggesting
that daf-3 and daf-5 function downstream of the sensory
neurons. Any neuronal gene that functions downstream
of daf-3 and daf-5 is thus a candidate for affecting dauer-controlling
interneurons. We have just begun to analyze these relations,
and have thus far found that in all cases the synthetic Daf-c
double mutant is epistatic to daf-3 and daf-5 but not daf-12,
consistent with a dauer-related function of both genes
in interneurons. We would like to think that these synthetic
Daf-c interactions provide a long-awaited avenue into
interneuronal function.