Gene conversion and crossing over along the 405-kb left arm of Saccharomyces cerevisiae chromosome VII

Abstract

Gene conversions and crossing over were analyzed along 10 intervals in a 405-kb region comprising nearly all of the left arm of chromosome VII in Saccharomyces cerevisiae. Crossover interference was detected in all intervals as measured by a reduced number of nonparental ditypes. We have evaluated interference between crossovers in adjacent intervals by methods that retain the information contained in tetrads as opposed to single segregants. Interference was seen between intervals when the distance in the region adjacent to a crossover was < approximately 35 cM (90 kb). At the met13 locus, which exhibits approximately 9% gene conversions, those gene conversions accompanied by crossing over exerted interference in exchanges in an adjacent interval, whereas met13 gene conversions without an accompanying exchange did not show interference. The pattern of exchanges along this chromosome arm can be represented by a counting model in which there are three nonexchange events between adjacent exchanges; however, maximum-likelihood analysis suggests that approximately 8-12% of the crossovers on chromosome VII arise by a separate, noninterfering mechanism.

Figure 1.—

(A) Genetic and physical map of markers along the left arm of chromosome VII. In diploid MG100 (set II), the diploid did not contain the URA3 marker. Diploid YFS0407 (set I) contains both the URA3 marker and a BLE marker (not shown) that subdivides the cyh2-crl3 interval but was not used in the subsequent genetic analysis. (B) Expected vs. observed numbers of crossovers along the left arm of chromosome VII. The expected numbers of exchanges for the tetrads in set I were calculated from a Poisson distribution for each possible tetrad type, based on the average number of crossovers per tetrad (open bars), as described in materials and methods. The observed distribution is shown with solid bars. Also shown is the predicted distribution assuming the two-pathway model in which m = 3 and 8% of crossovers are noninterfering (shaded bars).