Segmental heterogeneity in replication and transcription of the X₂ chromosome of Drosophila miranda and conservativeness in the evolution of dosage compensation

Abstract

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.