Effects of X-Rays on Hatchability and on Chromosomes of Habrobracon Eggs Treated in First Meiotic Prophase and Metaphase

A. Unlaid Habrobracon eggs x-rayed in late metaphase. I and allowed to develop parthenogenetically (1) Show a linear decline in hatchability from 50 r to 1,400 r, approximately the lethal dose. (2) Show no change in hatchability percentages with aging after treatment, time-intensity differences or fractionation of dose. (3) Continue meiosis and early cleavage after doses several times lethal, at rates not noticeably different from controls. (4) Show fragments but no bridges in division I, bridges, fragments or both in division II, fragments and bridges in cleavage. (5) Give a curve expressing dose-percentages of eggs without fragments which at high doses converges toward the dose-hatchability curve. (6) Show a number of fragments per treated egg which varies as the 0.94 power of dose between 500 r and 1,000 r, 0.53 power between 1,000 r and 1,500 r, and 0.58 power between 1,500 r and 2,000 r. B. Unlaid eggs x-rayed in prophase I and allowed to develop parthenogenetically (1) Give hatchability percentages equal to or higher than controls from 50 r to 750 r. (2) Show a decline in hatchability from 1,000 r to 44,800 r, approximately the lethal dose. (3) Give a hatchability curve which appears to be linear at low doses and to become "mixed" at high doses. (4) Show no significant change in hatchability in preliminary test of time-intensity differences but an increase with fractionation of dose. (5) Continue meiosis and early cleavage at lethal dose at rates not noticeably different from controls. (6) Show fragments and bridges in division I, either or both in division II, fragments and bridges in cleavage. C. Unlaid eggs treated in either stage (1) Show no change in hatchability when fertilized by untreated spermatozoa. (2) Develop as far as blastoderm in about twenty per cent. at approximately lethal doses. D. It can be concluded that (1) Consistent and correlated cytological and hatchability patterns of eggs treated in late metaphase I appear to prove conclusively that death is due to chromosome breaks arising from single hits. (2) Consistent differences in cytological effects and in nature of dose-hatchability curves correlated with consistent differences in stages of chromosomes at time of treatment argue strongly for the dependence of ionization effects upon the physical (or chemical) condition of the chromosomes. (3) High sensitivity of metaphase I, the stage in which tensions of chromosomes have been considered very high, argues strongly for the dependence of ionization effects upon the tension of chromosomes. Other suggestions made to explain differential sensitivity of chromosomes in different stages do not apply to material used in this study.