Worm Breeder's Gazette 10(1): 102

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.

mei-1 Mutations Suppress Dominant Maternal-effect Embryonic Lethality Resulting from Mutations at Two Loci

P. Mains, I. Sulston and W.B. Wood

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.