Functional organization of polytene X-chromosome in two X-chromosome inversion carrying larvae of Drosophila melanogaster reared at 24°C or 10°C

Abstract

Larvae of D. melanogasler carrying either the In(I)B^M1 or In(I)B^M2 inversion have been reared at 24° or at 10°C to study the morphology, transcription and replication of the X-chromosome in the salivary gland polytene nuclei. These two inversions share a similar left-hand breakpoint in euchromatin (16A 2-5 region in polytene X-chromosome) but have different right-hand breakpoints in the proximal heterochromatin. In 10°C reared B^M1 male larvae, the polytene X appears somewhat more diffused than in 24°C reared larvae. On an average, in 36% of the polytene nuclei of 10°C reared B^M2 male larvae the single X-chromosome appears considerably shortened in length, enlarged in width and has very indistinct band, while the other nucIei show "normal-looking" X as in B^M1 larvae. This highly disorganized form of X-chromosome, referred to as "pompon-Iike" X, is never seen in cold-reared female or in warm-reared female as well as male B^M2 or B^M1 larvae. Hoechst 33258 fluorescence reveals that the "pompon-like" morphology of the X is due to loose packing of chromatin in different band regions. It is suggested that the "pompon-like" morphology is due to position effect variegation associated with the particular heterochromatin breakpoint in the B^M2 inversion, ³H-uridine and ³H-thymidine labelling and autoradiography of polytene nuclei from cold-reared male and femal larvae of the two genotypes shows that the "pompon-like" or the "normal-looking" X in male nuclei continues its hyperactive transcription and faster replication as in 24°C reared wild-type larvae. It appears that the hyperactive organization of the hemizygous X in larval male polytene nuclei predisposes its pattern of chromatin condensation to be specifically affected by a variety of genetic, chemical and physical factors. However, the type of chromatin dispersion seen in "pompon-like" X in cold-reared B^M2 male larvae does not seem to affect the basic functional (hyperactive) organization of the hemizygous X in male polytene nuclei.