An analysis of radiation-induced variation on body-weight of Habrobracon juglandis.

use of irradiation has long been proved to be effective in inducing new T z n e t i c variability in a quantitative trait. SCOSSIROLI (1954) irradiated D ~ o sophila mlanogaster populations that had ceased to respond to selection for sternopleural bristle. The irradiated populations now responded to further selection: the response to upward selection was remarkably high, while that to downward selection was not as striking. Reverse selection always brought back the selected lines to the original levels. CLAYTON and ROBERTSON (1955) irradiated two inbred lines of D. melanogaster and applied upward and downward selection pressures on abdominal bristle counts. At the same time, the lines without irradiation were also subjected to the same selection scheme as the control of the experiment. The response to selection indicated an increase in genetic variability in the irradiated selection lines. However, selection was not as effective as reported by SCOSSIROLI. In plants, GREGORY (1955) submitted dry seeds of a cultivated peanut to X rays. The yield observations made on phenotypically normal plants of the third generation after irradiation revealed an estimate of genotypic variance that was four times as large as that of the control. Upward selection on this material enabled GREGORY to produce new lines with significantly superior yield to that of the original variety. Subjecting a line of spring barley to X rays, GAUL (1961) was able to isolate 56 “earliness mutants” for which the heading time was from 10.1 to 0.2 days shorter than that of the original line over several years of observations. The same author also reported a significant induced variation with respect to 1000-kernel weight in spring and winter barley varieties. The present paper deals with a quantification of radiation-induced genetic variability in Habrabracon in terms of genetic and environmental variances in two phases. The first phase is devoted to a survey of the empirical relationship between the radiation dose and the increase in variance, and the second phase represents a more detailed analysis of induced genetic variance.