We have previously described the properties of
ct46(I), a temperature-sensitive (ts) dominant maternal-effect lethal mutation ( Worm Meeting Abstracts, 1987). Gene dosage experiments indicate that the
ct46 mutation results in a gain-of-function 'poison' gene product that competes with the wild-type product. We presented evidence that
ct46 may interact with maternal-effect mutations of
zyg-9(II). Mutations at both loci result in similar early defects characterized by a longitudinal first cleavage with formation of an anterior cytoplast. In addition,
ct46/+;
zyg-9 double mutants show a 500 fold enhancement of maternal-effect lethality under conditions that are semi-permissive for the individual mutations. We also described the isolation of mutations that showed dominant, trans acting suppression of
ct46, as well as recessive nonconditional maternal-effect lethality. These suppressor alleles are likely to represent loss-of-function mutations based upon their frequency. After EMS mutagenesis, we found 4 alleles among 2000 animals in one screen, and at least 4 (and possibly as many as 11) additional alleles among 3500 animals in a second screen. The suppressors appear to have been induced by EMS, since we found no suppressors among 6800 non-mutagenized worms. The suppressors map <0.01 cM from
ct46 (in the
lin-10 -
lin-28 interval), but the results of gene dosage experiments are difficult (although not impossible) to reconcile with their being in the same locus. At the May 1987 Worm Meeting, Ken Kemphues told us about the recessive maternal-effect lethal mutation,
mei-1 (
b284), which has properties strikingly similar to our suppressors of
ct46 (see Springer and Kemphues, this issue). Both map to the same region and show the same early cleavage defects. We have now shown that our suppressors are alleles of
mei-1.
mei-1 (
b284) both fails to complement the recessive maternal-effect lethality of our suppressors and also acts as a dominant, trans-acting suppressor of
ct46. Since
b284 was isolated independently of
ct46, it is highly unlikely that the suppression results from a specific alteration of the
mei-1 product to allow it to compensate for the
ct46 lesion. As argued above, the dominant suppression probably results from loss of function at the mei- 1 locus. It is interesting to note that decreasing the wild-type activity of
mei-1 by one-half in
ct46+/+
mei-1 animals relative to
ct46+/++ increases the viability of eggs from 1% to 95% at 25 C. This may indicate that the stoichiometry between the
mei-1 and
ct46 gene products must be precisely controlled. Unexpectedly, deficiencies of this region (nDF23 and nDf24), which should be equivalent to
mei-1 null mutations, do not suppress
ct46. This may indicate that the suppression requires the presence of a non- functional
mei-1 product, or that there are additional genes removed by these deficiencies which interact with the
ct46 and
mei-1. One such gene may have been defined by
ct61(I), another member of our collection of ts, dominant maternal-effect lethal mutations. Embryos from
ct61/+ mothers show defects distinct from those caused by
ct46,
zyg-9, or
mei-1 mutations. The early cleavage furrows are indistinct and the spindles appear to be reduced in size. A very surprising result is that the defects resulting from
ct61 are also suppressed by
mei-1 alleles. At 25 C, only 0.3% of the eggs from
ct61/+ mothers hatch, but 95% of the eggs from +
ct61/mei-1+ mothers do so.
ct61 is linked to
ct46, but maps approximately 2 cM to the right, in the
unc-29 -
lin-11 interval.
ct46 and
ct61 were re-mapped to exclude the possibility that the stocks were mixed up. In addition, we were able to isolate ++ recombinants from
ct46+/+
ct61 animals at the expected frequency. Like
ct46,
ct61 is not suppressed by the deficiencies nDf23 and nDf24. These deficiencies remove the
ct46 and
ct61 loci in addition to
mei-1, which may account for why they do not suppress similarly to the
mei-1 mutations. Thus,
ct46,
ct61 and
mei-1 appear to represent three maternally active genes whose products may interact during the first cleavage division. We are currently testing
ct61 for enhancement of
zyg-9 mutations, and we will examine the phenotypes of double mutants of
ct61,
ct46,
mei-1 and
zyg-9 to determine epistatic interactions.