The majority of crossover suppressors in C elegans are translocations that suppress crossing over by disrupting pairing. It has been proposed that each chromosome contains a homologue recognition region; a site necessary for pairing and recombination. Crossover suppressors for regions associated with the homologue recognition sites have not been characterized. To isolate such a balancer for LG I, wild-type males were treated with 1500 rads of gamma radiation and mated to Unc-101 Unc-54 hermaphrodites. These two markers, are 14.2 m.u. apart and located on the right end of LG I. 800 Fl progeny were screened for the absence of recombinants and one crossover suppressor, hCI was recovered. The zygotic viability of hCl homozygotes and heterozygotes is not statistically different. hCI suppresses crossing over from
unc-75 to
unc-54 (17.6 m.u. apart) and is homozygous viable. An enhancement in recombination frequency was observed in the dpy-S
unc-29 region, a 3.4 m.u. interval adjacent to the boundary of hCI crossover suppression, and in the
bli-3 unc-ll region, a 14.8 m.u. interval located on the left end of LG I. The recombination frequency in 3 intervals on other chromosomes was examined and no interchromosomal effect was obseNed. Four rare recombinants were recovered from experiments measuring recombination in the crossover suppressed region. These recombinant events were associated with deletions and duplications and are being used to map the physical breakpoints of hCI. To investigate the possibility hCl is an inversion, an
unc-54 mutation has been induced on hCl. The map distance between dpy-S and
unc-54 in hCl homozygotes is 9.1 m.u. (25 m. u. in dpy-S
unc-54/ + + and 12 m.u. in dpy-S
unc-54/hCl), suggesting gene order may be different in hCl. hCI will be used to shorten sDpl, an LG I duplication that pairs and recombines with the normal homologues. An
unc-75 lethal mutation has been induced on hCl, and is being used to construct an
unc-75 unc-54 hCl/unc-75
unc-54 hCl/sDpl strain. By treating this strain with gamma and screening for viable Unc-54 progeny, indlviduals bearing shorter derivatives of sDpl will be isolated. These derivatives will be tested for their ability to recombine and in this manner the region of sDpl responsible tor this ability will be mapped. Supported by an NSERC grant to A.M.R. and a University Graduate Fellowship to M.C.Z.