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.