Cytological analysis of partial and total X-chromosome loss induced by X-rays in oocytes of Drosophila melanogaster

With the aid of a cytological technique (analysis of metaphase chromosomes of larval cerebral ganglia) it was shown that, in experiments on X-chromosome loss induced by X-rays in oocytes of Drosophila melanogaster, one has to distinguish between partial and total chromosome loss. For this purpose a scheme has been devised allowing the detection of aberrant F₁ individuals already at the larval stage. After treatment of mature oocytes, X-chromosomal fragments of various sizes were found. On the other hand, most of the X-chromosomal fragments observed after irradiation of immature oocytes had the same size as chromosome IV (“points”). Possibly this finding is, partly at least, simulated by the combined induction of complete X loss and nondisjunction of chromosomes IV. Otherwise preferential breakage close to the X-chromosomal centromere after irradiation of immature oocytes would have to be assumed to account for the observation of “points”.

About 39% (13/33) of the losses induced in mature oocytes by 400 R were shown to be partial ones. Depending on the classification of the “points” observed after treatment of immature oocytes with 3500 R, between 7% (3/43) and 23% (10/43) of the losses were partial ones. No indication was obtained either after irradiation of mature or of immature oocytes that the loss frequencies observed for imagoes and larvae differed from each other, e.g. because of selection.

The two-track component of the dose-effect curve of X-ray-induced (total plus partial) X-chromosome loss seems to be based—completely in the mature, partly in the immature oocyte experiments—on the induction of partial losses requiring two independently produced breaks.