AN X-LINKED REGION IN DROSOPHILA-MELANOGASTER THAT CONTROLS THE STRUCTURE OF THE MALE X-CHROMOSOME AND PERTURBS SEX DETERMINATION

Male third-instar larvae of an isolate of the strain In(1)BM(2) (reinverted) of Drosaphila melanogaster show abnormally decondensed polytene X chromosomes. The transcriptional activity of these abnormally diffuse X chromosomes remains at wild-type level, indicating that the phenomenon that causes the atypical decondensation does not affect the regulatory mechanisms that control male hyperactivity. We report that in this strain, (i) the morphology of the male X chromosome can be altered by heat shock; (ii) the region controlling this structural alteration of the male X chromosome is localized at segment 16A of the polytene chromosome; (iii) the abnormal decondensation is not linked to the process of X chromosome polytenization; and (iv) in females, which show normal X chromosomes, the 16A region interacts with a gain-of-function.Sex-lethal mutation, Sxl(M1), to perturb the sex-determining pathway. These results therefore suggest the existence of a regulatory pathway by which the morphology of the male polytene X chromosome may be altered without affecting the transcriptional activity of the chromosome.