Tissue-specific schedule of selective replication in Drosophila nasutoides.

Dramatic changes in the DNA composition of post-mitotic versus mitotic and germ line nuclei occur during development in different organisms. Drosophila nasutoides possesses n=4 chromosomes which were quantified with a microphotometer in females. The diploid (2 C) DNA content was 0.79 pg or 7.7×108 nucleotide pairs, calculated from brain metaphases and calibrated with hen erythrocyte nuclei. The individual elements comprised X=9%, 2=16%, 3=13%, and 4=62% of the total complement. In polytene nuclei of larval salivary glands which had undergone 11 endoreplication cycles, chromosome 4 contained only 1.55% of total Feulgen DNA. Thus, in contrast with other Drosophila genomes, where under-replicating material is dispersed to all elements, a huge quantity of non-endoreplicating DNA is restricted to a single chromosome. This permits accurate determination of the timing of under-replication in the single cell. The data presented here suggest that the schedule is tissue-specific. Larval hind gut and salivary duct nuclei begin under-replication during the first endocycle, whereas adult and larval salivary glands mainly begin during the second cycle. In Malpighian tubules the onset of selective DNA syntheses occurs during either the first or the second endocycle.