Worm Breeder's Gazette 7(2): 49

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Intragenic Revertants of some Dominant Mutations

J. Park, B. Horvitz

Figure 1

In the last Newsletter we described the phenotypes and map positions 
of some new dominant mutations.  To explore the nature of genes that 
can mutate to dominant alleles, we have begun to attempt to determine 
the null phenotypes of a set of such genes.  Since dominance often 
reflects the synthesis of a gene product altered in function or 
increased in level (e.g., see Greenwald and Horvitz, Genetics, 96, 147,
1980; also, 110, in press), the introduction of loss of function (
'null') mutations should generate alleles that are recessive.  Thus, 
the intragenic reversion of dominant alleles offers one approach to 
the isolation of null mutations.
We have attempted to obtain intragenic revertants of mutants in 
seven genes with dominant alleles.  In general, F1 and F2 progeny of 
EMS-mutagenized homozygous hermaphrodites were screened for revertants 
('homozygous reversion').  If this protocol failed to produce tightly-
linked (presumably intragenic) revertants, as it must for certain 
genes (e.g., those with null phenotypes that are either lethal or 
indistinguishable from the dominant phenotypes), N2 males were mated 
with EMS-mutagenized homozygous hermaphrodites and wild-type F1 
progeny sought ('heterozygous reversion').  Only tightly-linked 
revertants (less than 0.1% from the original mutation), which we have 
tentatively assumed to be intragenic, are described below.
The following table summarizes the number of revertants and the 
homozygous phenotypes of the putative intragenic revertants obtained 
from these protocols.
[See Figure 1]
Mutants in unc-105,  to 
homozygous viable phenotypically wild-type animals.  Mutants in unc(
n498) and unc(n494) 'reverted' from one Unc phenotype to another: n498 
animals are almost paralyzed, where as n498 revertants are only mildly 
uncoordinated; n494 animals are very sluggish and assume coiled 
postures, whereas n494 revertants are kinky and relatively active.  
unc(n493) reverted only as a heterozygote, and the revertant 
heterozygotes segregated larvae that failed to develop.  All of these 
revertants are recessive, and all appeared at the frequency expected 
after 4 EMS-mutagenesis for null mutations in a typical C.  elegans 
gene (10+E-3 to 10+E-4); in addition, spontaneous intragenic 
revertants, which are likely to reveal null phenotypes (Greenwald and 
Horvitz, 1980), have been obtained for unc-105, 
n498).  Thus, it is likely that these 
revertants display the null phenotypes of these genes (although for 
unc(n498) and unc(n494) it remains possible that the null phenotype is 
wild-type, and the original strains contain tightly-linked recessive 
unc mutations).  That rol-6 and unc-8 have wild-type null phenotypes 
is further supported by the observation that wild-type intragenic 
revertants failed to complement visible recessive alleles (e187 and 
e49, respectively) of these loci.  We have been unable to isolate 
revertants of n501 by either protocol, suggesting that n501 either 
itself reduces gene activity or defines a locus that is relatively 
difficult to mutate with EMS.

Figure 1