Some cytological observations on the delayed effect of mustard gas.

Abstract

IN the course of a quantitative study of re-arrangements induced by mustard gas in Drosophila melanogaster 1, 757 progeny of treated males have so far been analysed by the salivary gland method. Among sixteen structural changes, one was a mosaic for two inversions involving separately the X and 3R chromosomes. Mosaic glands have also been found in the progeny of X-irradiated males2; the special features of the present case are: (1) mosaicism for two independent changes and (2) the fact that in any one nucleus both changes were either present or absent. Of thirty-eight nuclei analysed, seventeen contained both inversions, twenty-one neither. Since in the preparation of the slides only one larva was used at a time, a mixture of two sets of glands on the same slide could not occur. The possibility that nuclei in unanalysed parts of the larva may have contained only one of the inversions cannot be ruled out; but the simplest interpretation is that both changes arose simultaneously either immediately after fertilization or through delayed effect at a later mitosis. The first alternative requires that in both X and 3R only one of the broken sister chromatids formed an inversion, while the other was restituted in the old order, and that at anaphase the two inverted chromatids segregated to the same pole. The second alternative dispenses with these requirements and is therefore rather more probable. It suggests that conditions which affect the nucleus as a whole may result in the simultaneous opening up of latent breaks in several chromosomes.