X-ray induced visible mutations in Habrobracon oocytes.

HE technique for classifying oocytes of irradiated females of the parasitic Twasp, Habrobracon, into those exposed in first meiotic prophase (prophase I ) and those exposed in first meiotic metaphase (metaphase I) was described by the author in 1945 (1945a). Methods for greater accuracy in estimating lethal rates were described by ATWOOD, VON BORSTEL and WHITING 1956. The striking difference in sensitivity to X rays between the two stages was first noted in 1940 (WHITING). LD,, for prophase I is about 12,000 r, for metaphase I about 400r. There is no evidence for recovery if oocytes are retained in females for varying periods after exposure and, under conditions of the experiments, there is no mortality due to degeneration of stored oocytes within females. Cytological observations (WHITING 194510) demonstrated that irradiation of prophase I may induce bridges in division I, division I1 or both. A bridge in division I is permanent, the dicentric chromatid being withheld from the terminal nuclei, one of which is the functional ootid nucleus. This may be a factor in the somewhat lower dominant lethal rate in relation to recessive mutations induced in prophase I. Fragments appear in telophase I after irradiation of metaphase I but no bridges have been found in this stage in more than 1500 eggs examined. Bridges in division I1 are frequent and are present at least as often between nuclei 3 and 4 (the inner pair) as between 1 and 2 (the outer pair) after completion of meiosis. When bridges are formed between the inner pair of nuclei, they are stretched to a comparatively great length, remaining attached to the egg pronucleus and pulling it into a “tear-drop’’ form, a phenomenon lethal in the unfertilized egg and with a dominant lethal’ effect when such a nucleus reaches the sperm pronucleus in the fertilized egg. Bridges in cleavage follow irradiation of either stage. The study of visible mutation rates was postponed, as it so often has been in experiments of this kind, because of the possibility of subjective error in their detection.