Chromosomal replication in Drosophila virilis. III. Organization of active origins in the highly polytene salivary gland cells.

Using DNA fiber autoradiography, the rate of fork movement in D. virilis salivary glands was determined to be 0.1 micron/ml (25 degrees C). This value is 3.5 times slower than the replication rate determined in the diploid brain cells using the same experimental conditions (Steinemann, 1981). Replicon lengths in the polytene cells span from 5 to 203 micron, with a mean of approximately x = 46.7 micron and a median of approximately x = 39.5 micron. The polytene replicon length is about the same as that for diploid brain cells (31.0 micron). Dividing the haploid genome of D. virilis by the estimated number of 5,000 bands and taking the resulting 13.6 micron as average DNA length per chromomere, the average replicon length covers 3.4 chromomere units. This result does not support the concept that "one-band-plus interband" behaves as a replicating unit (Pelling, 1966). From the positive skew of the replicon length distributions, combined with the results derived from the chromosomal labelling patterns, the following schedule of chromosomal euchromatin doubling is infered: A short initiation period (discontinuous phase I), asynchronous with respect to individual origins, is followed by a phase of common replication activity at all inducible origins (continuous phase). The discontinuous II pattern is formed by few, long replicons which are still replicating while the shorter ones have already finished (weak points are not considered). Replicons, distributed in the main peak of the histogram and smaller than about 100 micron, are then responsible for the replication of the bulk of chromosomal euchromatin, the continuous phase. the interpretation is in conflict with the hypothesis assuming a clustered organization of the replicons to explain spot labelling (reviewed in Hand 1978). The diploid karyotype of D. virilis contains 45% satellite sequences, located in the alpha-heterochromatin (Gall et al., 1971). They do not replicate in the highly polytene salivary gland cells of 3rd instar larvae. Comparison of DNA fiber autoradiograph patterns from salivary glands and brains suggests that these satellite sequences replicate in short (less than 10 micron), to some extent irregularly spaced replicons.