Somatic Recombination within the White Locus of DROSOPHILA MELANOGASTER.

HE X-linked white locus of Drosophila melanogaster is a complex region of Tthe chromosome which has been subdivided into a number of sites (LEWIS 1952; MACKENDRICK and PONTECORVO 1952; GREEN 1959, 1964; JUDD 1959, 1964). Heterozygotes for two mutant alleles mapping at different sites in the white region (except for compounds with the right-most site, for white-spotted) do not show complementation and at a low frequency produce gametes containing a recombinant of the two sites. Such gametes combine either the two mutant sites on the same chromosome or the two normal sites. The former transmit a mutant chromosome, the latter a normal one. To take an example, females have white eyes if they carry a mutant allele, leading to white eyes, at a left site in one chromosome, and in the other, another mutant for white located at a right site. Among the eggs produced by these females are some which carry the left and right mutant white determinants and thus transmit whiteness and some which carry the two normal determinants and thus transmit red eye color. Meiotic recombination within the white locus is a reciprocal process presumably of the same type as meiotic crossing over in general. In Drosophila, as well as in some other organisms, recombination is not restricted to the meiotic phase, but occurs also in somatic cells. Recombination between separate loci in somatic cells has been termed “somatic crossing over” and it appears that this type of crossing over is a reciprocal process like meiotic crossing over. Whether this applies to recombination within a locus is not known for Drosophila but reciprocal intragenic recombination has been demonstrated in the fungus Aspergillus nidulans (ROPER and PRITCHARD 1955; PRITCHARD 1955). Figure 1 of this paper is basd on a reciprocal crossing-over model, but most of the text will use the neutral term recombination rather than crossing over. A priori the probability of discovering somatic recombination within a locus is low. Each eye of Drosophila, however, has approximately 750 facets any one of which might give evidence for recombination. Thus, as few as 1000 flies represent as many as 1,500,000 facets! Cases OP somatic recombination within the white region are reported herewith.