Some notes on differential chromosome activity in Drosophila

Comparative study of the function of polytene chromosomes in laboratory stocks of Drosophila melanogaster and the l(3)tl mutant (lethal tumorous larvae). II. Changes of banding pattern and transcriptional activity in the salivary chromosome of l(3)tl

The salivary chromosomes of the l(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-like l(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 of l(3)tl larvae lack ecdysone-stimulated 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.

Genes controlling chromosome activity; the role of genes blocking Y-lampbrush loop propagation

Characterization of two more X-linked Y-affecting mutations in D. hydei is presented; ms(1) XL24 is less extreme than the previously described mutation, ms(1)XL2, but its general effect on the propagation of Y-lampbrush loops is similar qualitatively; ms(1)XL24 is located at the distal end of the X-chromosome and associated with a suppressor-of-white phenotype. The other X-linked sterile, ms(1)XL4, is the least extreme of the three mutations examined to date. It is recessive, located ca 2 c.o. units to the right of white, and causes different patterns of Y loop response. -- From a comparison of the effects of the three single mutations, as well as from the analysis of the various double-sterile combinations, we surmise that: 1) if specific activator genes for the Y-lampbrush loops do exist, they are not likely to be located on the X-chromosome of Drosophila; 2) every lampbrush loop is composed of several functional repeats, each of which operates when unfolded; 3) the Y-affecting mutations arrest preferentially, but not exclusively or specifically Y-lampbrush loop propagation probably indirectly through their independent metabolic effects. Since the mutations are not specific for germ line cells, and exhibit various pleiotropic effects, especially with regard to viability, they should be considered tissue conditional lethals.