Some Developmental Effects of X-Rays in Drosophila

1. Drosophila larvae of various ages and several genotypes were subjected to heavy doses of X-rays (7000.r. units). 2. (a) In pupae aged 27 hr. after puparium formation, no morphological abnormalities were produced. (b) In late larvae (aged more than 90 hr. after laying) and young prepupae, the main effects were: roughening of the eyes, destruction of many macro- and microchaetae and of cells of the wing, arrest of morphogenesis of the wing, and faulty eversion of the legs leading to crippling. (c) In mid-larvae (aged 60-90 hr. after laying) the bristles were usually normal. The main effects were: overgrowth of parts, leading to the formation of over-sized organs and/or excrescences, e.g. on the wings; reduplication of organs, e.g. of wings, eyes, parts of the thorax, legs, etc.; changes in histogenesis, e.g. the formation of palps from presumptive eye material, of leg-like thickenings of the arista, of body-surface material from wing epithelia, etc. The eye palps may have the character of antennae, or extra ocelli may be formed. (d) In younger larvae the effects were less marked, and the youngest groups irradiated gave quite normal adults. 3. The effects on late larvae (groub b) can be simply explained as the results of the killing of individual cells, although the unduly frequent destruction of both a bristle and its accompanying socket indicate some interaction of the trichogen and tormogen cells during development. 4. The effects on mid-larvae demonstrate that the determination of the imaginal buds is by no means final at this stage, and a considerably greater degree of plasticity, both in growth and histogenesis, must be assumed than has previously been suggested. Moreover, the fact that the abnormalities affect large masses of tissue indicates that they are caused by agents (chemical substances or physical conditions) capable of spreading throughout the affected region. These agents must be producible by the affected cells themselves. This suggests that determination in normal development occurs fairly late, and through the agency of tissue interactions. 5. Attention is drawn to the occurrence of an increased quantity of growth following irradiation, but no explanation is offered for this phenomenon.