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

Abstract

Because of their definite order in the ovariole, Habrobracon eggs may be treated at specific meiotic stages, and then allowed to develop parthenogenetically. Those treated at metaphase I give hatchability percentages which show a linear decline with dosage from 50 to l,400r, but with no changes following aging after treatment, time-intensity differences, or fractionation of dose. The eggs treated in prophase I give hatchability percentages equal to or higher than controls for doses from 50 to 750r. At medium doses, a linear relationship is apparent between dosage and hatchability but this relationship becomes obscured at the higher doses. The lethal dose for prophase I is about 44,800r. For prophase I, hatchability increases with fractionation of dose but does not change following time-intensity changes. Eggs treated at either stage with lethal doses will continue meiosis and early cleavage at normal rates. Those treated in metaphase I lack bridges in division I, but otherwise eggs treated at both stages show fragments and bridges at both division I and II and also in cleavage. Fertilization with untreated spermatozoa does not change the hatchability percentages of eggs treated at either stage. The correlation of cytological aberrations with hatchability percentages seems to prove that death is caused by single hit chromosome breaks. The high sensitivity of the metaphase I eggs indicates that ionization effects may depend upon chromosome tension. Other explanations for differential sensitivity at various stages are discussed and found not to apply.