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.

Interactions between Neural unc Genes

William Nawrocki

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 
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 
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.