Worm Breeder's Gazette 10(3): 97
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.
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 1]