Transcription in X-chromosomal segmental aneuploids of Drosophila melanogaster and regulation of dosage compensation

Transcription of X chromosomal DNA has been examined autoradio-graphically in various 1X2A and 2X2A normal larvae and 1X2A (+ X fr) and 2X2A (+ X fr) segmental aneuploid larvae of species Drosophila melanogaster. The segmental aneuploids contained duplications for the segment 9A–11A and 15D–ISA of the X chromosome. Results show that in the aneuploid male containing 9A–11A duplicaton both the homologous segments involved in the aneuploidy are autonomously hyperactive; their combined activity, measured by X/A grain ratio, is found to be nearly 70% more than the activity in normal male and about 100% more than that in diplo-X female. In the aneuploid female, containing the aneuploid segment 15D–18A and having three doses of the segment of the X chromosome, the activity was over 100% more than the diplo-X activity. The per gene dose activity for the two segments in the aneuploid male and female, respectively, is also significantly higher than their male and female counterparts. The possible role of lack of contiguity of the genetic segments and an intra-nuclear variation has been ruled out by appropriate analysis. We, therefore, interpret these findings to be due to an autonomous expression of the X linked compensatory genes, resulting from a primary modulation in the organization of the entire X chromosome. The autosomal signal then renders the individual genetic locus hyperactive

Segmental heterogeneity in replication and transcription of the X<SUB>2</SUB> chromosome of Drosophila miranda and conservativeness in the evolution of dosage compensation

A detailed analysis of the replication and transcription patterns of the X chromosomes of Drosophila miranda has been made using 3H-thymidine and 3H-uridine autoradiography. The purpose of this investigation has been to examine the relative duration of replication and hyperactivity of the different regions of the X2 element of this species. Results reveal that there is a considerable amount of heterogeneity among different segments of the X2 but not among the segments of X1 chromosome. Secondly, there is a clear site wise relationship between early replication and hyperactivity in X1 as well as X2. Thirdly, the X2 element appears to replicate relatively earlier than the X1 element in the male. - Our data do not support a spreading effect or polarization in the distribution of hyperactive segments. Furthermore, the X2 element appears relatively thinner than X1 in almost all nuclei from male larval glands. The significance of these results has been discussed in the light of the conservativeness of dosage compensation mechanism in a particular evolutionary line