Interchromosomal Effects on Crossing over in Drosophila Melanogaster. Ii.a Reexamination of X Chromosome Inversion Effects.

NTEREST in interchromosomal effects on crossing over in Drosophila melanoIgaster has been renewed recently (COOPER, ZIMMERING and KRIVSHENKO 1955; OKSALA 1958; HART and SANDLER 1961; RAMEL 1962; SUZUKI 1962a; ROBERTS 1962). As a result of the new approaches taken in these studies, two general types of mechanisms, one physiological in nature ( STEINBERG and FRASER 1944; RAMEL 1962; ROBERTS 1962), the other mechanical ( SCHULTZ and REDFIELD 1951; OKSALA 1958; SUZUKI 1962a, b) have been suggested to explain these effects. Although, as SCHULTZ and REDFIELD (1951) indicate, a clear-cut distinction between these hypotheses is difficult, for the present paper the physiological mechanism has been interpreted as an alteration of gene action (e.g., position-effect) , and the mechanical effect as altered chromosome behavior (e.g., loop formation, asynapsis). While the mechanical hypotheses have been more readily tested, the number of conditions known to cause interchromosomal effects is extensive, and no mechanical hypothesis yet proposed adequately accounts for all of the observations. On the other hand, while physiological explanations are sufficiently general to encompass most known situations, no critical experiments have been carried out testing their validity. ~ By interpreting the interchromosomal effects on crossing over on a mechanical basis, it is possible to separate chromosomes having such effects into two groups: those that are postulated to synapse as loops or large rings (SUZUKI 1962a) and those postulated to have a definite frequency of asynapsis (SUZUKI 196213). It is possible to minimize such influences of loop configurations and abnormal crossover values by testing autosomal exchange in X chromosome inversion homozygotes. Such tests might indicate whether the effects of inversion heterozygotes are due to loop formation, to heterozygosity per se, or to the intrinsic properties of the rearrangement. It was also hoped that inversion-homozygote tests would