Worm Breeder's Gazette 14(4): 22 (October 1, 1996)
Institute of Zoology University of Fribourg Perolles CH-1700 Fribourg Switzerland
Meiotic mutations in C. elegans have largely been isolated in screens for mutations that disrupted the segregation of the X chromosomes, resulting in an easily identifiable him phenotype. Further characterization of these mutations has suggested that they fall into two general classes: one of which specifically disrupts the segregation of the X (and is associated with an X-chromosome specific recombination-defective phenotype) and another which disrupts the segregation of all chromosomes (a recombination-defective phenotype has also been reported for him-6 and him-14). In him-3(e1256) homozygotes, crossing over is also reduced on the autosomes and has been shown to correlate with an increase in the frequency of nondisjunction (K.McKim, Ph.D Thesis). On the X chromosome, however, e1256 has little effect on the frequency of exchange (K. McKim, Ph.D Thesis), making difficult an explanation for the presence of 12% males amongst the self-fertilization progeny of mutant hermaphrodites. e1147 is a weak allele that produces 3.5% males amongst the self-progeny of homozygous, hermaphrodites yet the frequency of crossing over in e1147 homozygotes is also greatly reduced: 2.0 m.u. in dpy-11 unc-42 (V) (compared to 2.7 m.u. in controls) and 12.7 m. u. in dpy-7 unc-3 (X) (compared to 21.5 m.u. in controls). These results suggest that the him-3 mutations uncouple the strict relationship between crossing over and proper segregation and their effects on crossing over in several other intervals is now being determined. To determine the molecular basis of the him-3 phenotype, the rol-6(su1006) transformation plasmid and cosmids in the region to which him-3 has been genetically mapped on chromosome IV were co-injected into e1147 hermaphrodites. The co-injection of ZK616 and F41H10 resulted in five stable transgenic lines that partially rescued the him phenotype. The most significant rescuing activity was observed for swEx56, which segregated 1.1% males and 48% rollers. To confirm these results, the extrachromosomal array sEx125 (constructed by D.Collins), which contains the cosmid ZK1236 from chromosome III, was crossed into the e1147 background. This strain gave rise to 5.5% males amongst the self progeny, indicating that the presence of extrachromosomal DNA in general was not the explanation for the partial rescue observed with F41H10 and ZK616. To narrow down the region responsible for the rescuing activity, ZK616 and F41H10 were injected independently. A strain carrying the ZK616-containing array swEx60 segregated 41% rollers and 1.5% males, whereas four strains carrying the cosmid F41H10 (swEx61-swEx64) segregated rollers between 20-53% and males between 3.8-5.0%. This indicates that the rescuing activity maps to ZK616 and that the general presence of cosmids from this region is not responsible for the decrease in the frequency of males. Deletion derivatives are now being constructed to narrow down the region responsible and to identify potential coding elements. An unexpected result of these experiments has been the observation that the phenotype of e1147 homozygotes that bear arrays containing nonrescuing cosmids is more severe than the canonical phenotype. For example, the percentage of males rises from 3.5% in e1147 homozygotes to 5.5% in the presence of sEx125 and to 5.0% in the presence of swEx64 , suggesting that e1147 is sensitiive to the level of DNA in the cell. Thanks to Heinz Tobler for his support, Alan Coulson for the cosmids, Ann Rose and David Baillie for strains, and Jonathan Hodgkin and Phil Meneely for helpful comments on the rescue data.