Our labs are developing a genetic map of visible markers in Caenorhabditis briggsae in order to do comparative studies with the related nematode, Caenorhabditis elegans . The two nematodes, though morphologically similar, are evolutionarily separated by approximately 25-40 million years. The genome of C. briggsae is currently being sequenced by the Sanger center. We have isolated 300 visible mutations including the C. briggsae versions of dumpy, roller, uncoordinated, multivulva, egg-laying defective, small, and other phenotypes from wild-type C. briggsae . These visible phenotype mutants will be used for the construction of a genetic map. In addition to the large-scale genetic map of visible mutations, we are constructing a high-resolution genetic map of the
mip-1 r egion on chromosome D of C. briggsae . The
mip-1 region of chromosome D was chosen because the genetic organization of the region surrounding its putative homologue in C. elegans ,
unc-22 IV , has been extensively studied in terms of essential gene saturation. The
unc-22 IV region is the 2 map unit area deleted by sDf2 . In C. briggsae a large BAC/fosmid contig has been constructed which covers the
unc-22 IV region (Jacquie Schein, BC Genome Sequencing Centre, Vancouver, BC). BAC DNA plus a dominant marker gene will be injected into C. briggsae mutant worms to create transgenic lines in order to test for phenotypic rescue. This will help align the physical map with the growing genetic map of C. briggsae. Gene knockouts are being generated for all the genes in C. elegans , with human homologues, in the
unc-22 region (Don Moerman, C. elegans Knockout Centre, Vancouver, BC). This will give us a powerful tool for comparison with C. briggsae
mip-1 region. In order to assist in the maintenance of lethals, balancers were generated using low dosage X-irradiation. Balancers were isolated that suppressed recombination between the linked markers
cby-7 and
mip-1 .
mip-6 , which lies between
cby-7 and
mip-1 , was used to mark the other LGD . Our study indicates that 1500R X-irradiation on C. briggsae yields a 3.7% induction rate of mutations that balance the 6.3 map unit interval between
cby-7 and
mip-1 . A total of 36 putative balancers were isolated from this screen. A preliminary 0.025M EMS mutagenesis screen was carried out to produce lethal mutations within the balanced region of sC5. This yielded a lethal induction rate of 7.1%. We have since isolated a more well-behaved balancer ( sC6 ) that we are using to induce lethal mutations in the
mip-1 region.