A MODEL FOR SYNAPSIS IN FEMALES HETEROZYGOUS FOR THE CHERRY SUB-LOCUS DUPLICATION IN DROSOPHILA.

Abstract

IN Drosophila the white-eye locus consists of five subloci in the fourth of which JUDD (1964) has reported a duplication (herein indicated as 4 4). The allele cherry ( wCh) is at this sublocus. The duplication ordinarily carries no mutant allele and by itself causes no change in eye color. It is detectible by virtue of the fact that it enhances zeste (z, an eye color mutation at the left end of the X). The identification and location of the duplication involved the employment of W“ (apricot at sublocus 3) and wrh (cherry, at sublocus 4) as well as zeste and outside markers close to the white locus. A testcross of a female heterozygous for the duplication in one X (red in expression) and w‘ wch in the other (a combination which is white) yields F, females which are predominantly either red or white. However, as a result of crossing over between wa and wCh, JUDD occasionally detected F2 females that were either apricot or cherry. These are referred to as “exceptions.” Some of the apricot exceptions had the duplication (five in a total of about 71,000 females) and some did not (21 in the total); none of the cherry exceptions (six in the total) had it. If we show all the elements at the white locus (1 to 5), then the above F2 exceptions would be as listed in Table 1 (only the X derived from the female parent is shown, the X from the male is wa wCh). In order to account for the above results, JUDD assumes that pairing in the heterozygous female may be of two kinds, as shown in Figure 1 (in which the arrow indicates the crossover point). Crossing over between Wa and wCh in type I pairing would give apricot with the duplication ( wa 4 4), and in type 11, apricot with no duplication ( wa 4). The reciprocal crossover products would be formed in each case. In type I pairing they would be 3 wch, the cherry exceptions. In type 11, they would be 3 4 wCh. This