Unchanged recovery of crossovers after x-irradiation of pupal Habrobracon.

Received January 16, 1961 A LTHOUGH the irradiation of Drosophila results in more crossover off spring, it has not been demonstrated that this effect has its origin in meiosis (WHITTINGHILL 1955) rather than in gonia1 exchange. Hence it is appropriate to irradiate specimens or stages which may have only oijgonia rather than the mixture of gonia and gametocytes carried by adults. Previous experiments ( CLARK 1943) showed that Habrobracon had no significant variation in recombination percentage among offspring from females of increasing age and no change following exposure of adults either to temperature extremes or to a 560r dose of X-rays. In these three respects Habrobracon differs from Drosophila. Possible conclusions are that the chromosomes of Habrobracon require stronger environmental assaults to change crossing over, that crossovers are formed and then lost, or that the wasps lack some stages of cells in the germ line which are both present and responsive in Drosophila. We wished to investigate whether heavy doses of irradiation given before meiosis began would produce crossovers in the Drosophila pattern. The four egg strings (ovarioles) of Habrobracon contain no oocytes until a late pupal stage. Irradiation of oogonial cells could be made on the “white pupa” stage on the sixth postzypotic day. Crossover sons could be distinguished from those mutation sons 17-hich might mimic some crossover phenotypes if we could use as many as four linked markers, as had been done by CLARK. The mutants black (bl) , lemon (Le). cantaloup-25 Cc) and honey (ho ) have been pictured in WHITING (1939) and offer little or no confusion in classification. Stocks and all crosses, except those to be irradiated, were made by placing three pairs of the parasitic wasps in shell vials (15 x 75 mm) with six welldeveloped prestung moth larvae (Ephestia kuerziella Zeller) . All wasp cultures were kept in an incubator at an optimum temperature of 27*1”C2 except when t.ransfers were made in the laboratory at 25°C. The parents of the wasps to be treated with X-rays were set in lucite boxes (4.7 x 4.7 x 2 cm) with five prestung caterpillars each. Seven days after making sets the lucite boxes were opened and the white pupae were irradiated in situ. The wasps were divided into four groups: a control group, which remained just outside the radiation therapy room, and three groups, which started receiving irradiation simultaneously and were removed after having received 1 OOOr, 2OOOr or 3000r. X-rays were delivered at the rate of 341r per minute with a Picker deep therapy unit (located in the Department of Radiology of the North Carolina