In
lin-17 animals a variety of normally asymmetric cell divisions are instead symmetric or display reversed symmetry. That many diverse cell lineages are affected in a similar way suggests that the function of
lin-17 is to establish the asymmetry of certain cell divisions. We are studying
lin-17 both molecularly and genetically. Our molecular studies have focused on cloning
lin-17. We first identified Tcl elements linked to the
lin17 locus by backcrossing the Bergerac strain 10x to Bristol. We cloned two Tcl restriction fragment length polymorphisms (RFLPs) linked to the
lin-17 locus. One, an EcoRI- HindIII restriction fragment that genetically maps 0.5 map units from
lin-17, has unique sequence flanking the Tcl element that we used to isolate lambda clones from a C. elegans genomic library and to identify one clone, Y71F9, in a YAC (yeast artificial chromosome) library. These clones enabled us to localize the RFLP to a small region on a set of cosmids and YACs that have been physically linked and that span the
lin-17 locus. One of these cosmids, W09G8, identified allele-specific RFLPs in DNA from two of ten
lin17 alleles. We have achieved rescue of the
lin-17 phenotypes by microinjecting cosmid W09G8 in concert with YAC Y71F9. In addition, a terminal 5 kb subclone from W09G8 maps the allele-specific polymorphisms to a small region of overlap between W09G8 and Y71F9. This same 5 kb fragment hybridizes to a single RNA species, approximately 2 kb in size, when used to probe Northern blots of poly A+ RNA from either mixed-stage animals or embryos. We are currently trying to determine if this message is encoded by
lin-17. We are now attempting to localize the
lin-17 locus by gene rescue from a single lambda or cosmid clone. Our genetic studies have focused on screening for suppressors of
lin-17 and analysis of the various
lin-17 alleles in trans to deficiencies. One phenotype with nearly 100% penetrance and for which one could readily screen for suppression is the male-mating defect. Most other phenotypes have a low penetrance. Thus far, we isolated two suppressors of
lin-17 by screening for male-mating proficiency. The two suppressors are recessive, extragenic, fail to complement each other, and moderately decrease the penetrance of all
lin-17 mutant phenotypes. Other genetic studies have suggested that the most highly penetrant
lin-17 alleles might be null alleles.