Screening for x-ray-induced crossover suppressors in Drosophila melanogaster: prevalence and effectiveness of translocations.

HE sequence of meiotic events that results in synapsis is not clear. It is diffiTcult to observe chromosome behavior in early stages of meiosis even in favorable fixed material, and the early oocytes of Drosophila melanogaster are certainly not favorable cytological material. However, the many well-mapped mutant genes and the polytene chromosomes of the salivary glands of this species make it possible to screen for chromosomal aberrations, to localize their breakpoints, and to study the behavior of chromosomes through genetic analysis of living material with greater facility than is possible in other organisms. The question that prompted this investigation is: What types of chromosomal aberrations interfere with crossing over and where in the chromosome arm is this disturbance most pronounced? It was anticipated that a study of the relation of the breakpoint position to the degree of crossover suppression produced by various rearrangements would permit some deductions about the way chromosomes pair prior to crossing over. To compare the behavior of the different chromosome arms of D. melanogaster, I decided to screen for dominant X-ray-induced crossover suppressors simultaneously on all the chromosome arms that regularly cross over: XL, 2L, 2R, 3L and 3R. For this purpose, a new stock that is homozygous for 8 recessive markers located approximately 40-50 crossover units apart was formed. Each chromosome arm has a marker near the base and one near the tip. The markers are far enough apart so that they segregate independently unless there is a dominant crossover suppressor present that makes the linkage between the two markers apparent. It was expected that the type of rearrangement that would be netted most frequently by this procedure would be the inversion (a class of chromosomal aberration that has been, up to now, almost synonymous with crossover suppressor). Instead, many more translocations than inversions were recovered. It was not unexpected that translocations would have effects on crossing over; such effects have been known since the early reports of DOBZHANSKY (1931). But DOBZHANSKY'S studies were made before polytene chromosomes were discovered