Chromosomal replication in Drosophila virilis

Abstract

About half of the diploid genome of D. virilis is α-heterochromatic (Heitz, 1934) and contains the satellite sequences found in isopycnic CsCl density gradients (Gall et al, 1971; Steinemann, 1976). The thymidine incorporation behavior of this material in the course of S phase was monitored by autoradiography. Labelled interphase nuclei show three types of labelling patterns, label exclusively confined to either eu- or α-heterochromatin, and simultaneous labelling of both fractions. Using the fraction of labelled mitotic index method, the duration of the DNA-synthetic period, t_s = 11.9 ± 4.3 h and G₂ period, t_{G2 + 1/2M} = 6.9 ± 3.8 h, were determined. On the assumption that the investigated brain cells belong to an exponentially growing cell population, the cell cycle is 22.9 h long and the G₁ period lasts t_G1=4.1 h. The a-heterochromatin begins to replicate later than euchromatin and continues alone after a phase of common replication of both fractions. Noteworthy is the asynchronous termination in the proximal α-heterochromatic segments of different chromosomes. Within the S phase, the first 1 h of DNA replication is exclusively confined to euchromatin, followed by 8 h of replication in both eu- and α-heterochromatin and terminated by 3 h of exclusive α-heterochromatin replication. Thus euchromatin has a doubling time of about 9 h and α-heterochromatin of about 11 h. The α-heterochromatin of D. virilis is “late” and “slow” replicating.