In Drosophila melanogaster, the polytene X chromosome of male third instar larva appears twice
as wide as an unpaired female X chromosome or an autosome. This characteristic morphology of
the male X chromosome is correlated with the increased rate of transcription of the sex-linked
genes, which ensures gene dosage compensation. In male third instar larvae of the
strain In(1)BM2
(reinverted), polytene nuclei manifest unusually puffy X chromosomes at 18±1 °C. Such ‘puffy X’
chromosomes are pompons, that is, despite the increased width of the chromosome, transcription
remains at the wild-type level. This characteristic is a caveat to the invariable correlation between
polytene chromosome puffs and transcription, and suggests that the mutant X chromosomes arise
due to perturbation of a pathway that controls the structure but not the transcription of the
polytene X chromosome. In this report we present evidence that the pompons of
In(1)BM2
(reinverted) arise due to spiralization of the male X chromosome, which results in condensing of
the chromosome. This unusual structural alteration can be induced only in male larvae of this
strain, at the third instar larval stage, through temperature shifts from 24±1 °C to 18±1 °C and
during recovery from cold shock. Furthermore, extract from male adult, pupae and third instar
larvae can induce chromosome condensation in wild-type larvae in vitro. This new evidence not
only explains the absence of correlation between chromosome width and transcription of the
pompons of In(1)BM2 (reinverted), but
also suggests that the chromosomal rearrangement perturbs a pathway that regulates the condensation of chromosomes.