Age dependence of the locations of x-ray induced somatic crossing over in Drosophila.

OMATIC crossing over was first demonstrated in Drosophila melanogaster by STERN (1936). Many factors are known to influence the frequency of somatic exchanges, including temperature ( BROSSEAU 1957; KAPLAN 1953; STERN and RENTSCHLER 1936), X-irradiation (BECKER 1957; LEFEVRE 1949), the presence of Minute genes (STERN 1936), the polygenic background (WEAVER 1960), and the amount of heterochromatin (WALEN 1964). In addition to these factors the frequency of somatic crossing over has been found to vary from one body region to another (ABBADESSA and BURDICK 1963; STERN 1936), with the presence of heterologous inversions ( RONEN 1964), and with maternal age, although the effect of this last factor is subject to controversy (BAKER and SWATEK 1965; BROWN and WELSHONS 1955; SCHWARTZ 1954). The frequency of somatic crossing-over events induced by temperature shock ( BROSSEAU 195 7) and X-irradiation (BECKER 1956) has been found to vary, depending upon the time during ontogeny when the treatment is given. The experiments by BROSSEAU suggested that the position on the chromosome where the somatic exchange event occurs can be influenced by temperature treatments given at certain periods during ontogeny. The experiments to be reported here concern the effect of Xirradiation delivered at different times during development on the location in the chromosome of somatic crossing-over events. While the junior author was investigating the developmental action of the dominant gene Tufted (Tft) located on the left arm of the second chromosome, he discovered that more of the somatic exchange events induced by X rays in young larvae occurred not between Tft and the kinetochore (i.e., to the right of T f t ) , but distal to the Tft locus (to the left of Tft) (ARNHEIM 1966). This was somewhat unexpected, since it is often assumed that somatic crossing over occurs primarily in the region most proximal to the kinetochore. To investigate this phenomenon further, two experiments were planned. It was known that the scute' inversion chromosome suppresses the recovery of X-ray induced somatic crossovers in the X chromosome (LEFEVRE 1949); therefore in our first experiment an inversion-carrying chromosome was used to reduce, if possible, the recovery of somatic crossovers to the left of Tft. The results of this experiment