Action des uv sur la recombinaison génétique dans les ovogonies de drosophiles

Abstract

The action of UV on genetic recombination in Drosophila melanogaster oogonia In this investigation, results concerning the action of UV radiation on the genetic recombination in females of Drosophila melanogaster are presented. Utilizing the technique of transplantation introduced by Ephrussi and Beadle ovaries which had been irradiated in vitro were injected into the body cavity of larvae. This technique was used since it allowed good penetration of UV rays through the gonad. The Latarjet' dosimeter was used to measure the amount of exposure. Four experiments were performed and seven different exposure levels were used. Recombination was studied between (a) genes b pr located near the centromere on chromosome II (Expt. I, exposure: 820 ergs/mm 2); (b) genes al, dp, c, px also on chromosome II (Expt. II, exposure: 1500 ergs/mm 2); (c) genes y, cv, f on the X chromosome, (Expt. III, exposures: 1000, 1500, 2000, and 3000 ergs/mm 2; (d) closely linked genes th, st, and cp located adjacent to the centromere on the third chromosome (Expt. IV, exposures: 500, 1500, and 2500 ergs/mm 2). The data show that: (1) UV increased the meiotic recombination frequency; (2) UV irradiation produced some anomalies in the segregation of the genes under study. Investigation of these anomalies indicated that disjunction of each pair of alleles in the irradiated sample was not different from the control. However, comparisons between the individual progenies were very heterogenous. The extent of this heterogeneity varied with the position of the gene along the chromosome. The regions where the heterogeneity was maximum were called ‘hot regions’. When the segregation of two genes was investigated, it appeared that the recombination frequency was increased. This increase was particularly important for closely linked genes located near the centromere. Evidence from this study indicated that somatic recombinations were induced in oogonia, however, these recombinations were not as symmetrical as crossing over. They resembled a process similar to conversion observed in fungi. In fact, an absence of correlation was observed between the frequencies of reciprocal cross-over classes and no systematic association existed between a cross-over and a non cross-over class. The maximum dose of effectiveness was located around 1500 ergs/mm 2. The nature of the delayed mechanism through which meiotic recombination is affected by UV remains a point of speculation. The finding that some somatic recombinations resemble conversions suggests that (a) UV induces lesions in the DNA and (b) the cell presumably contains an efficient mechanism for repairing these lesions. During this process, the non reciprocal recombinations are thought to result from an error in the rejoining of DNA fragments.