Worm Breeder's Gazette 7(2): 46

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Synthetic Multivulva Mutants

C. Ferguson, B. Horvitz

Figure 1

We have recently begun to analyze a series of genes with the 
property that mutations in these genes individually can result in a 
wild-type phenotype but in certain pairwise combinations produce a 
Multivulva (Muv) phenotype.  The Muv phenotypes of these strains are 
similar: hermaphrodites usually have a functional vulva and two large 
protrusions, one anterior and one posterior to the vulva; males also 
have ventral protrusions.  Our attention was drawn to this phenomenon 
by the observation that two individually silent mutations, lin-8(n111) 
II and lin-9(n112) III, were responsible for the phenotype of the Muv 
strain CB1322 (Horvitz and Sulston, Genetics, 96, 435, 1980).  A third 
such mutation, lin(n766) III, was discovered after mutagenizing the 
strain MT1312, which contains the lin-12 Vulvaless allele n676.  
Numerous Muvs were isolated during this experiment, and all proved to 
be synthetics.  All of these synthetic Muv strains contained a silent 
mutation (named 'n766') linked to n676.  Thus the original strain 
MT1312 was presumably of the genotype n766 n676.
We are pursuing the analysis of these silent Muv mutations to answer 
the following questions.  How many genes can mutate to silent Muv 
alleles?  What is the nature of these silent Muv alleles?  If these 
alleles are not null mutations, what are the null phenotypes of these 
genes?  What roles do these genes play in vulva formation?
Seventeen synthetic Muv mutations have been obtained from 
mutageneses of the three isolated silent alleles n111, n112 and n766.  
These new silent mutations have been mapped and tested for 
complementation with appropriate markers.  These 17 mutations fall 
into five complementation groups.  The table below lists the seven 
mapped genes now known to have silent alleles.  In addition we have 
isolated 35 uncharacterized synthetic Muv strains by mutagenizing lin-
15(n767) and lin(n751), two of the above seventeen alleles.
[See Figure 1]
Two genes, lin-8 II and lin-15 X, have both silent and visible Muv 
alleles.  The visible alleles but not the silent alleles behave in 
various heterozygous combinations like deficiencies, suggesting that 
the visible alleles result in the null phenotypes of these loci.  lin-
15 alleles include visibles, temperature-sensitive (ts) visibles, 
silents, ts silents and ts silents with maternal effects.  The ts 
visible allele n765 ( wild-type at 15 C, maternal effect Muv at 20 C, 
and Muv at 25 C) does not have wild-type activity at 15 C, as it acts 
as a silent allele, interacting with lin(n751) II to give a Muv 
phenotype.  This result is consistent with the hypothesis that lin-15 
silent alleles are hypomorphs, i.e.  result in a reduction but not in 
a total loss of gene activity.
Silent Muv alleles are recessive, i.e.  in all pairwise combinations 
tested, hermaphrodites of the genotype m1/m1; m2/+, where m1 and m2 
are silent alleles in different genes, are wild type.  The only 
exception so far is lin(n766) III, as certain m1/m1; n766/+ 
hermaphrodites are sometimes Muv.  However, in many cases the addition 
of a single copy of another silent allele to give the genotype m1/m1; 
m2/+; m3/+ results in a Muv phenotype ( less severe than that of 
either homozygote) .  These observations suggest that the synthetic 
Muv phenotype results from the cumulative reduction in activity of 
genes that are either partially redundant in function or that act 
sequentially in a developmental pathway.

Figure 1