Worm Breeder's Gazette 8(3): 37

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More About Sex-Linked Gene Expression

J. Hodgkin, E. Hedgecock

Figure 1

1.  Sex-linked amber suppressors are not dosage compensated.
An amber allele of dpy-20 IV, e2017, has been isolated, which 
provides a convenient assay for the level of amber suppression.  The 
unsuppressed phenotype is 'piggy', i.e.  an extreme dumpy phenotype 
with a rounded nose, like other strong dpy-20 alleles such as e1362 or 
e1422, which are non-amber.  The three amber suppressors sup-5 III, 
sup-7 X, and sup-21 X are roughly equal in the efficiency of 
suppression in hermaphrodites: the sup/+ phenotype is a medium dumpy, 
and the sup/sup phenotype is wildtype.  The effect of suppressor 
dosage on expression of e2017 in the two sexes has been 
[See Figure 1]
Thus, suppression in hemizygous males is markedly weaker than in 
homozygous males for both sup-7 and sup-21, indicating that these 
genes are not dosage compensated.  It follows that not all sex-linked 
genes of C.  elegans are dosage compensated.  It may be that C.  
elegans tRNA genes, such as sup-7 and probably sup-21, are in general 
not compensated, in contrast to Drosophila sex-linked tRNA genes, at 
least some of which are dosage compensated.
2.  Another chauvinistic dumpy.
Mutations in the genes dpy-21 V and dpy-26 IV are expressed much 
more strongly in XX animals than in XO animals, irrespective of sexual 
phenotype (see Mol.Gen.Genet.  192: 452).  A possible explanation is 
that these two genes are involved in controlling X chromosome dosage 
compensation.  A new mutation of this type has been identified: dpy-27(
rh18) III, mapping about 0.5% right of unc-93 III.  The rh18 mutation 
is recessive, and has properties intermediate between those of dpy-26(
n199) and dpy-21(e428).  Homozygous dpy-27(rh18) XX hermaphrodites are 
dumpy and very slow growing, while dpy-27 XO males are non-dumpy, grow 
normally, and are almost wildtype (mating efficiency 64% of wildtype). 
As with dpy-26, there is a maternal effect: dpy-27 XX daughters of 
dpy-27/+ mothers are dumpy, but grow better than dpy-27 XX daughters 
of homozygous mothers.  However, unlike dpy-26, XX strains homozygous 
for dpy-27 can be maintained, though they grow very slowly.  Also 
unlike dpy-26, odites do not have a Him 
phenotype.  The interactions of dpy-27 with sex-determining genes 
resemble those of dpy-26 and dpy-21: r-1 XO animals 
are non-dumpy hermaphrodites, while dpy-27 ls 
are dumpy males with abnormal bursae.

Figure 1