Worm Breeder's Gazette 9(3): 88
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.
C. velopment is dependent on maternally expressed genes which can be identified by maternal-effect lethal mutations (mels). Homozygous progeny of heterozygotes for mels are viable and fertile, but produce self-fertilized eggs which fail to hatch. Strict mels are those for which progeny of homozygous mothers die even when a wild-type allele has been provided by mating to males. Partial mels are rescued by the wild-type allele. We know there must be two classes of maternally expressed genes required for embryonic development: those that are expressed at many or at all stages of the life cycle, and those that are expressed exclusively by the maternal genome (pure maternal-effect genes). We wished to know how these two classes of genes would be represented in collections of mels, and whether we could establish critera for identifying pure maternal- effect genes, so that we could estimate the number of such genes in C. lts suggest that there are between 25 and 60 such genes. The data base for our analysis is a set of mels on chromosome 11 (WBG 8 (2), 5). We have now screened 13,900 chromosomes to identify 54 chromosome 11 mels which fall into 29 complementation groups, uniformly distributed on the chromosome. Of the 26 loci for which tests are completed, 15 mutated only to strict mels. The frequency distribution of these mutations is shown below. Typically, estimating the size of the gene pool being sampled by mutation is done using calculations based upon the Poisson distribution. However, in order for these calculations to be valid it is necessary that the events being sampled occur at equal frequency, in this case that the genes be equally mutable to maternal-effect lethality. Chi square analysis to test our data for a fit with the Poisson reveals a significant deviation from expected frequencies for genes with higher than three mutations. The basis for the deviation from expected could be either that the loci with high numbers of mutations are mutational hot spots or that the other mutations are mutational cold spots. {Figure 1} The average mutation rate for loci in our frequent class is 3.6 x 10- 4, reasonably close to knockout rate. Because we would expect null mutations in pure maternal effect genes to cause strict maternal- effect lethality, it seems likely that the loci in which we isolated multiple alleles are pure maternal-effect genes. In fact, two of the loci in the high frequency class, zyg-11 and zyg-9, had previously been identified as pure maternal-effect genes (Kemphues et al. Dev. Biol. 113: 449). We hypothesized that the low frequency mutations resulted from rare mutations in essential genes. Null mutations in these genes should result in phenotypes other than maternal-effect lethality (for example, larval lethality or defective gonadogenesis) and would not be isolated in our screens. However, rare mutations that reduce the activity, alter the function, or affect an embryo-specific domain of the gene product could result in maternal-effect lethality and appear in our screens. To test this hypothesis, we have carried out complementation tests of nine loci that map under chromosome 11 deletions with lethal mutations mapping within the same deletions. Two of the nine mel loci tested were identical with lethal loci (lethal mutations failed to compliment the maternal-effect lethal phenotype). These results support our hypothesis that there are two classes of genes that can mutate to maternal-effect lethality: rare mutations in essential genes, and pure maternal-effect genes. Results of rescue tests are consistent with this interpretation. Alleles at 26 of the 29 loci have been tested for male rescue. All loci with mutations in the high frequency class mutate only to strict mel while high proportions of the loci in the low frequency class mutate to partial mel. If all genes in the high frequency class are pure maternal-effect genes, then we have reached saturation for such genes on chromosome 11 with a total of four. If we then make allowances for statistical fluctuation and the possibility that some pure maternal-effect genes are hypomutable, we can estimate that there are probably fewer than ten pure maternal-effect genes on chromosome 11. Extrapolating to the whole genome, there are as few as 24, and probably no more than 60, pure maternal effect genes in the entire C. We are currently studying mutations in the two uncharacterized genes in the high frequency class to determine if any of the mutations are amber suppressible and hence likely null alleles, and also to determine their embryonic phenotypes. Our preliminary observations are: 1) mutations in one of these genes (mel- 5) results in severe defects in cellular organization in the one-cell embryo leading to abnormal early cleavage, 2) all mutations in the other gene (mel-4) are incompletely expressed and 1/4 to 1/2 of the surviving progeny are males. This latter phenotype suggests that the gene plays a role either in chromosome segregation or in sex determination.