Chromosomal sites necessary for normal levels of meiotic recombination in Drosophila melanogaster. I. Evidence for and mapping of the sites.

Meiotic exchange was measured in females heterozygous for a normal sequence X chromosome and for each of eleven T(1;4)s and each of sixteen T(1;Y)s. The results indicate that the X chromosome can be divided into five intervals, such that heterozygosity for a breakpoint in one interval strongly suppresses exchange within that interval, but has little or no effect on exchange in other intervals. The boundaries between these intervals are identified and mapped to regions 3C4-6/7, 7A-7E, 11A and proximal to 18C on the standard salivary map; each boundary is located at (or within a small region containing) a major constriction (i.e., a block of intercalary heterochromatin).--Exchange was examined in females heterozygous for translocations broken within the constriction at 11A. The results imply that a boundary occupies only a subregion of the entire constriction and is subdivisible by translocation breakpoints. Several other properties of boundaries have been elucidated. Finally, the relationship of these data to a simple model of meiotic pairing proposed by I. Sandler (1956) and to the role of intercalary heterochromatin in the meiotic process is discussed.