Worm Breeder's Gazette 8(1): 6

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.

Genetics of X-Chromosome Expression

P. Meneely, W.B. Wood

Genes whose mutant phenotype depends on the number of X-chromosomes 
may be involved in regulating or interpreting X-chromosome expression. 
Four genes, the autosomal dpy-26 and dpy-21 and the X-linked dpy-22 
and dpy-23 have such X-dependent mutant phenotypes (Hodgkin, MGG, in 
press).  The autosomal mutants are non-Dpy when the X:A ratio is below 
about 0.67, Dpy when the X:A ratio is around 1, and inviable when the 
X:A ratio is above about 1.3 ( Meneely & Wood, Genetics, in press).  
The parallel between these latter two phenotypes and the phenotypes of 
X hyperploids suggests that in dpy-21 and dpy-26 mutants the level of 
X expression may be inappropriately high.  In contrast, the phenotypes 
of the X-linked dpy-22 and dpy-23 mutants are that 2X animals are sick 
Dpys and 1X animals are inviable except for occasional dpy-22 XO 
escapers which are very Dpy.  We postulate that the Dpy phenotype in 
2X animals results from inappropriately low X-expression, and that 1X 
animals are inviable for the same reason.
Although a definitive test of these hypotheses will require 
molecular assays of X-chromosome transcription, we have obtained 
genetic evidence that supports them, by examining the phenotypes of X-
linked hypomorphic mutations in the presence of dpy-21 and dpy-22 
mutations.  By definition, a hypomorph is a mutation whose defective 
phenotype becomes less defective with increasing dose of the mutant 
allele; that is, if m is a hypomorphic mutation, the severity of 
phenotype will vary in the manner m/DF > m/m > m/m/m.  We have used 
the three X-linked hypomorphs unc-3(e54), 14), and 
lin-15(n767).In 2X animals homozygous for dpy-21(e428) and any one of 
these mutations, the hypomorph is suppressed; that is, its phenotype 
becomes less severe.  In 1X dpy-21(e428) animals carrying any one of 
the hypomorphs, the hypomorph is also suppressed.  Other dpy-21 
alleles as well as dpy-26 alleles are being tested.  Since the 
hypomorphic mutations themselves do not appear to have a common 
physiological basis, this finding suggests that the dpy-21 mutation 
suppresses them by increasing overall expression of the X in both 1X 
and 2X animals.  Consistent with this view is that dpy-21(e428) does 
not suppress an apparent null (amorphic) allele of unc-3, nor does dpy-
21(e428) suppress any of eight autosomal hypomorphs so far tested.
Conversely, dpy-22(e652, the only known mutant allele) increases the 
severity of all three X-linked hypomorphs in 2X animals, suggesting 
that it decreases overall expression of the X.  Furthermore, dpy-22 
67) XO escapers have the phenotype of strong lin-15 
alleles in XO animals: they are Dpy males with multiple vulvae.  The 
dpy-22 mutation does not affect autosomal hypomorphs so far tested.
To examine the interaction of dpy-21 and dpy-22, we have constructed 
the double mutant dpy-21(e428);dpy22(e652).  Its phenotype is similar 
to that of the single mutant dpy-22(e652): XX animals are sick Dpy 
hermaphrodites, and XO animals are inviable.
Taken together, our results suggest that dpy-21+ is a negative 
regulator and dpy-22+ is a positive regulator of X-expression in both 
XO and XX animals.  The finding that dpy-22(e652) is apparently 
epistatic to dpy-21(e428) in the double mutant suggests further that 
dpy-21+ may act by negatively regulating dpy-22.  We have preliminary 
evidence that the one known dpy-23 mutation does not affect the three 
X-linked hypomorphs.  We would like to have a larger collection of 
both X-linked and autosomal hypomorphs, and will welcome any