The Meiotic Effects of a Deficiency in DROSOPHILA MELANOGASTER: Identification of Two Dosage-Sensitive Sites.

Heterozygosity for a deficiency for the entire zeste-white region of the X chromosome of Drosophila melanogaster females causes both reduced recombination and increased nondisjunction. The location of the dosage-sensitive sites responsible for these two meiotic defects has been studied by use of a set of deficiencies that subdivide the region. Recombination is reduced when the zw7-zw11 region is present in one dose, while nondisjunction is increased only if the doses of both the zw8-zw10 and zw6-zw11 segments are reduced. Examination of trans heterozygotes of two deficiencies explicitly demonstrates the compound nature of the meiotic dose effect and further delimits the location of the proximal disjunctional site to the zw12-zw11 interval. In inversion/deficiency heterozygotes, reduced dose of the zw8-zw10 region alone, without reduced dose of the proximal site, yields increased nondisjunction, suggesting that the proximal element that affects disjunction is the same as that which affects recombination. Thus, the zeste-white region contains at least two dosagesensitive loci that affect meiosis: reduced dosage of one locus, in the zw7-zw11 interval, causes reduced recombination; reduced dose of another, in the zw8-zw10 region, increases the probability that nonexchange chromosomes will nondisjoin. A slight effect on the regional distribution of exchange may also be a property of the zw8-zw10 region locus, but could be an effect of yet another locus or of structural heterozygosity. The implications of these results for understanding meiotic control and the prospects for further analysis of the structure of the zeste-white interval are considered.