Comparative study of the function of polytene chromosomes in laboratory stocks ofDrosophila melanogaster and thel (3)tl mutant (lethal tumorous larvae)

Abstract

The salivary chromosomes of thel ₍₃₎tl mutant of D. melanogaster are considerably thicker and shorter than those of normal larvae. In most nuclei, chromosome shortening is associated with morphological changes of two types, a) The bands lose their distinctive pattern and become loose. The chromosome appears as a granular mass. In extreme cases pompon-like chromosomes arise. Most frequently male X-chromosomes undergo such changes and begin to shorten as early as in the middle of third larval instar. “Pompon” transformation is not associated with a change in the relative intensity of RNA synthesis: the ratio of silver grain number over the male X-chromosome to that over regions 61A-63F is the same in pompon-likel ₍₃₎tl chromosomes and in the male and female X-chromosomes of the normal lines, b) Shortened chromosomes occassionally retain distinct band organization and, in these cases, chromosome shortening is observed to be due to the condensation of the chromatin of many puffs and interbands resulting in the fusion of a large number of bands into “new” chromatin blocks. In regions of fused bands, transcriptional activity is decreased as compared with regions where this union does not occur. The chromosomes ofl ₍₃₎tl larvae lack ecdysonestimulated puffs and other prominent puffs. In 144–192 hour larvae, puffs can be induced by ecdysone and until 384 hours by temperature shock. The capacity of puff induction decreases with larval age.