Recombination and the Function of Chromosome Pairing Sites. John R. Merriam. Dept Molec/Cell/Dev Biol, Univ California, Los Angeles, CA.

   That chromosome rearrangement heterozygosity reduces crossing over in the vicinity of the breakpoints is no surprise. Rearrangements at some breakpoint locations, however, are notable for reducing or eliminating crossing over for interval(s) distal to a break. In(1)delta-49 is perhaps the best known example. This feature has been used to map specific pairing sites that serve as gateways to eliminating crossing over distally (Hawley 1980, Szauter 1984 and Sherizen et al 2005). The mechanism of such sites is not clear, however, since normal chromosome synapsis, seen as the formation of the synaptonemal complex along the entire chromosome, is not affected even with multiply rearranged chromosomes (Gong et al 2005). - New data will be presented that map such a site on the left arm of chromosome 3 by comparing several pericentric inversion heterozygotes. Located between 65D and 69F, the ca. 13% crossovers observed distal to the 69F break are completely eliminated by the inversion with the break at 65D. - A model will be presented that proposes the function of such sites is for them to act as probabilistic boundaries in determining whether an exchange that commits the bivalent to segregation has been established. Normally this occurs by a crossover in the middle third of the chromosome arm, as most crossovers are found there. Crossing over in the distal and proximal thirds is much less frequent than would be expected on physical distance alone, increasingly so towards either pole. Assurance is the term that describes both recognition of the requirement for a committing crossover as well as activation of a mechanism that promotes crossing over more in keeping with physical distance over the remaining distal and proximal intervals in the absence of a committing crossover. Heterozygosity for a rearrangement breakpoint seems to activate the assurance program, a phenomenon recognized as the interchromosomal effect (Schultz and Redfield 1951) but blocks further crossovers within its boundary interval, perhaps by interference with breakpoints mimicking bona fide crossovers.