Worm Breeder's Gazette 8(3): 43

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Intragenic and Extragenic Suppressors of unc-8 Mutations

E. Lander, J. Park, R. Horvitz

In an effort to understand the function of unc-8 IV, we have been 
isolating and characterizing suppressors of various unc-8 alleles.  
The gene unc-8 is defined by the recessive mutation e49.  We have 
isolated three dominant mutations (n491, n492, n773) and have showed 
them to be allelic to e49 (J.  Park and B.  Horvitz, Newsletter Vol.7 
Our attention was drawn to unc-8 for two 
(1) The mutant phenotypes suggested that the primary effect of unc-8 
mutations might be in the nervous system rather than in muscle.  
Animals homozygous for any of the dominant alleles are strong coilers, 
usually assuming such a posture for long periods.  They are capable of 
moving forward in a fairly normal, if somewhat kinky, manner, but if 
they are touched on the nose, their head and tail simultaneously curl 
inward to resume a tight coil.  Such animals are also smaller than the 
wild type.  Heterozygotes (dominant/+) move forward actively in a 
coordinated fashion, but, in response to a touch on the nose, retreat, 
suddenly stop, proceed to retreat in a slightly kinky manner and then (
sometimes) coil.  The differences between the homozygous and 
heterozygous phenotypes is somewhat subtle.  Finally, e49/e49 animals 
move forward and backward in a highly kinky manner.  (In fact, this 
phenotype is very slightly semidominant.) unc-8 mutants show no muscle 
defect visible with polarized light microscopy.
(2) While mutations in unc-8 can lead to severe effects on 
coordination, the gene itself is probably completely dispensable, 
since the phenotype resulting from apparent null mutations at the 
locus is wild-type (Newsletter, Vol.  7 No.  2).  In particular, by 
mutagenizing hermaphrodites homozygous for any of the dominant alleles,
we have obtained in the F2 animals that appear to be wild-type.  Some 
of the induced suppressor mutations map very close to the original 
n491 mutation and prove to be cis-dominant suppressors.  Trans-
heterozygotes for the suppressor-bearing chromosome and e49 display 
the kinky phenotype associated with e49 homozygotes.  These 
observations strongly suggest that the dominant mutations n491, n492 
and n773 are allelic to e49.  They also argue that the suppressor 
mutations are intragenic and possibly null.
To screen for further suppressors of n491, we mutagenized n491/n491 
hermaphrodites with EMS and examined the F2 progeny for animals not 
displaying the strong coiler phenotype.  From 23,000 haploid F1 
genomes screened, we isolated 20 independent mutations generated wild-
type, or nearly wild-type, animals in the F2.  In every case, both the 
uncoordination and the short length were simultaneously suppressed.
The suppressor mutations fall into at least two classes.  Nine 
alleles (n1142, n1143, ...  , n1150) were members of the class 
described above: intragenic, putative null mutations.  In addition, 
two other as-yet poorly characterized allelic mutations, n1160 and 
n1161, are closely linked and possibly intragenic to unc-8, but they 
appear to behave somewhat differently from the putative null alleles.  
The second class of nine fully recessive alleles (n1151, n1152, ...  , 
n1159) constitute a single complementation group defining a new gene 
on LGI.  Two-factor crosses place the gene 3.5 map units from dpy-5(
e61) (14/400 recombination events in e61+/+ n1152 heterozygotes).  
Preliminary three-factor crosses position the gene right of dpy-5.
While these extragenic suppressors were isolated only for their 
ability to suppress n491/n491, at least the reference allele n1152 has 
strong effects on all four unc-8 alleles listed above.  In particular, 
n1152; n491 animals, n1152; n492 animals, and n1152; e49 animals 
appear completely wild-type.  By contrast, n1152; n773 animals do not 
show the strong coiler phenotype but are backwards-uncoordinated.  
When tapped on the nose, they retreat in a kinky manner while partly 
coiling their head inward -- a phenotype that appears similar to, but 
distinct from, the other unc-8 phenotypes.  (We believe that this 
phenotype probably reflects incomplete suppression rather than a 
mutation linked to n773, because intragenic revertants of n773 fail to 
show this phenotype.)
The suppressors appear to cause no effect on coordination in an unc-
8(+) background.  Indeed, in the case of the reference allele n1152, 
animals of genotype n1152/n1152 move normally.  They appear, however, 
to be somewhat longer than wild-type in length.
We are currently continuing to characterize these suppressors and 
plan to isolate further suppressors by different genetic screens and 
by Tc1 mutagenesis.