Worm Breeder's Gazette 10(2): 128
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.
The question arises if epistatic interactions exist between genes that affect the circuitry and structure of the nematode nervous system in similar fashion as the pathway involved in sex determination. One wonders if such inhibitory interactions are present between neural genes as represented by the unc mutations which have non-muscle lesions. To address this query, ten neural unc mutations (uncs 1,3,4, 5,6,7,13,24,30,55) were selected either because they are easily identified or have been properly reconstructed, and the matrix of double unc mutants was constructed. The double mutants were in each case picked up by phenotype but were then tested by backcrossing to both original parent strains (37 out of a possible 45 positives have so far been examined). The canonical alleles were used in the crosses and for behavioral tests. Both single and double mutants were analyzed behaviorally to quantify resting position, forward and backward movement, movement in liquid, head and tail response, and progress of movement over time. Each different movement was scored for form, speed and distance covered. The test was performed on both L1 worms with their embryonic set of motoneurones and adults with the complete set of motoneurones. Details and results of the test are available from the author. Comparison of the results of each double unc mutant against those from their parental unc strains have lead to these conclusions: 1. No double unc mutant expresses a behavioral phenotype more wild type than either parent. Each double mutant usually expresses the more severe phenotype of its two parent strains. Example: All double mutants containing the unc-13 mutation are paralyzed, but the form of the worm is influenced by the other mutation. unc-3,13 double mutants are paralyzed in coils and unc-13,24 double mutants are paralyzed in kinks. 2. The behavioral phenotype of all double mutants seem to represent the sum of their parental behavioral phenotypes. Example: unc-3 worms tend to coil with their ventral side inward especially during backward movement. unc-30 worms shrink upon either head or tail tap. A head tap on a unc-3,30 double mutant will cause the animal to shrink and then to back into a ventral inside coil. 3. The behavioral phenotype of a double unc mutant consisting of parent mutations that have been reconstructed may be interpreted as expressing the neural lesions of both parents. Example: In unc-4 worms, most VA motoneurones have gap junctions with AVB interneurones instead of the AVA interneurones, therefore adult unc-4 worms have difficulty with backing and usually end up as dorsal inside coils. In unc-7 worms, the DB and VB motoneurones have aberrant gap junctions with the AVA interneurones as well as normal gap junctions with the AVB interneurones, therefore unc-7 worms have difficulty with forward movement and prefer to back. The movement of the unc-4,7 double mutant is limited to less than a half body length in either the forward or backward direction. Although this double mutant has not been reconstructed, one may expect that all the forward movement motoneurones (DBs and VBs) being improperly wired, while half of the backward movement motoneurones (VAs) are connected as forward movement motoneurones (VBs) and thus are also improperly connected to both AVA and AVB interneurones. The behavioral phenotypes of this set of double unc mutants suggest that these neural genes act independently in the construction of the nematode nervous system. One is lead to the opinion that the worm nervous system is pieced together gene by gene, with each gene being selected against mutation since its wild type function bestows the worm with a natural wriggle having clear selective advantages.