Interchromosomal effects on crossing-over in Drosophila melanogaster. I. Effects of compound and ring X chromosomes on the third chromosome.

LTHOUGH the properties of crossing-over have been investigated extenA sively in Drosophila melanogaster, the influence of nonhomologous chromosomes on crossing-over is a problem that warrants further exploration. Since STURTEVANT’S (1919) observation that a heterozygous inversion in the third chromosome increased crossing-over in chromosome 2, many investigators such as SCHULTZ and REDFIELD in MORGAN, BRIDGES and SCHULTZ (1932, 1933), GLASS (1933)’ and STEINBERG (1936) have shown that heterozygous inversions in any given chromosome pair increase crossing-over in the other pairs. Although severa1 hypotheses have been formulated to explain these results, (MATHER 1936; STEINBERG and FRASER 1944; SCHULTZ and REDFIELD 1951 ; COOPER, ZIMMERING, and KRIVSHENKO 1955; OKSALA 1958) no one mechanism has been found to have general applicability. An impasse in the study of interchromosomal effects seemed to have been reached because of a lack of techniques for testing the schemes proposed to account for these effects. The synthesis of new compound X chromosomes (NOVITSKI 1954; SANDLER 1958) offers new tools for the investigation of interchromosomal effects. By introducing different compound and ring X chromosomes into females heterozygous for autosomal markers, it is possible to test: (1) whether the synaptic configuration of the X chromosomes in any way influences pairing patterns of the autosomes in the meiotic nucleus as reflected by altered frequencies of crossing-over; (2) whether the amount and distribution of heterochromatin along the X chromosomes changes autosomal crossover values; and (3) the role of the Y chromosome in interchromosomal effects on crossingover.