Worm Breeder's Gazette 10(3): 97

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ct45, a Mutation at a Haploinsufficient Locus?

Paul E. Mains and W.B. Wood

Figure 1

One of the mutations obtained in a screen for dominant maternal-
effect ts embryonic lethals (Mains et al., WBG Nov.  1985), ct45, may 
represent a gene that is required in two doses for normal 
embryogenesis.  The mutation, which maps between dpy-28 and unc-49 on 
LGIII, shows a strict maternal effect: at 25 C, all the self progeny 
embryos from ct45/+ hermaphrodites, including the +/+ segregants, fail 
to hatch.  When ct45/+ males are crossed to wild-type hermaphrodites 
at 25 C, no embryonic lethality is seen.  However, ct45 also shows 
associated nonmaternal phenotypes: homozygous embryos are inviable at 
15 C or 25 C, and heterozygous males hatched at the permissive 
temperature show tail defects when reared at 25 C.  In support of the 
haplo insufficiency hypothesis, the dominant maternal effect embryonic 
lethality can be almost completely rescued by a duplication of the 
region (mnDp37).  Whereas only 1 of 4450 embryos from ct45/+ 
hermaphrodites hatched at 25 C (and this larva promptly died), 86% of 
the embryos from ct45/+/+ produced viable larvae.  (If rescue were 
complete, 90% survival would be expected: based on the frequency of 
meiotic loss of mnDp37, ct45/+/+ hermaphrodites should produce 10% 
ct45/ct45 progeny, which die from the recessive zygotic lethality.) 
The haplo insufficiency hypothesis appears to be contradicted by the 
failure of an additional copy of ct45 to decrease the maternal-effect 
lethality of embryos from ct45/ct45/+ hermaphrodites.  If ct45 at 25 C 
results in complete loss of function, consistent with haplo 
insufficiency, then at 15 C it would appear to result in partial loss 
of function, since 65% of the embryos from ct45/+ hermaphrodites at 
the permissive temperature still fail to hatch (due to the maternal 
effect, based on ratios of genotypes among surviving progeny).  It 
would then be expected that ct45/ct45/+ hermaphrodites would show 
decreased maternal-effect lethality at 15 C relative to ct45/+ 
hermaphrodites.  Instead, ct45/ct45/+ hermaphrodites at 14 C or 15 C 
either were sterile or produced only dead embryos (total of 56 
hermaphrodites).  Three possible explanations of this result are: 1) 
the wild-type allele on the duplication could function less 
efficiently due to position effects; 2) there could be autosomal 
dosage compensation as observed in corn (Birchler, Genetics 97: 625,
1981) and Drosophila (Devlin et al., Genetics 118: 87,1988), so that 
the level of wildtype product in ct45/+/+ animals is reduced; or 3) 
the haplo insufficiency hypothesis is wrong, and the 4000-fold rescue 
resulting from the extra wildtype gene copy in ct45/+/+ animals 
results from competition between the wild-type and a gain-of-function 
'poison' gene product.  The latter explanation, contrary to the haplo 
insufficiency hypothesis, would predict that for a deficiency of the 
locus, Df/+ hermaphrodites should produce viable embryos, and that 
such deficiencies should be obtainable by reversion of the ct45/+ 
maternal-effect lethality.  We are attempting to isolate deficiencies 
at this locus, which have not been previously reported.  
The embryos from ct45/+ hermaphrodites at 25 C all show an 
interesting early defective phenotype (<10 lineaged so far).  In wild-
type embryos, the division of P1 begins before the 1AB2 division is 
completed.  In the defective embryos, the P1 division did not occur 
until after completion of the 1AB2 division, and the embryo rotated 
partially about the a-p axis, so that the 2AB4 divisions occurred in a 
d-v rather than the normal l-r orientation.  The division of EMS was 
delayed by a factor of about 1.5, and subsequent division rounds of 
all cells were delayed by a factor of about 2.  Embryos arrested with 
about 28 cells, prior to gastrulation.  The temperature-sensitive 
period (TSP) for lethality of embryos from ct45/+ hermaphrodites 
begins about 2 hr before and extends until about 4 hr after 
fertilization (see figure).  Curiously, however, whereas 
hermaphrodites reared at 15 C produce only 35% viable embryos, 
hermaphrodites exposed to 25 C for as little as 1.5 hr in downshift 
experiments subsequently produce 95% viable embryos.  Further 
experiments will be needed to make sense of this result.
[See Figure 

Figure 1