Induction and characterization of premature chromosome condensation in Drosophila synkaryons and implications to dosage compensation.

Premature chromosome condensation (PCC) was induced in Drosophila melanogaster cell hybrids, with Drosophila mitotic cells and interphase cells at different phases (G1,G1-S,S,S-G2 of the cell cycle, and from male and female, using standard cell fusion technique with polyethylene glycol (PEG). A combination of Feulgen and autoradiography was used to enhance the resolution of the PCC plates. It was possible to identify the characteristics of PCC's at G1, S and G2, and the transitory intermediate phases, which are comparable with respect to the characteristics of PCC's previously described for other species. Using the combined Feulgen-autoradiography technique it was possible to critically resolve the different phases including the transitory intermediate phases in greater detail. Analysis and comparison of results obtained from M (female) x S (female/male) and M (female) x G2 (female/male) hybrids have revealed that the X chromosome from the male could be identified as a distinct acrocentric entity which showed clear allocyclic, heteropycnotic characteristics. The results thus lead us to suggest that the X chromosome in such synkaryons is indeed early replicating as is the X chromosome of male larval salivary gland of Drosophila. The X chromosome morphology is also distinctly at an advanced stage of the cell cycle. On the basis of these findings it is concluded that X chromosome is hyperactive in the normally dividing diploid cells.

Replicative activity of X-chromosome and autosomes of Drosophila melanogaster in autosomal hyperploids and the problem of dosage compensation.

Replicative behaviour of two hyperploid autosomal arms (2L and 3L) of D. melanogaster has been analysed by 3H-thymidine autoradiography. Results reveal that hyperploid autosomal arms (2L-trisomy or 3L-trisomy) replicate synchronously with other disomic non-homologous chromosome arms i.e. there is no asynchrony in the initial mid or late phase of replication patterns between the trisomic 2L or trisomic 3L and disomic arms, suggesting that the extra dose of an autosomal arm can not alter the inherent pattern of replication of that arm. Results further reveal that 2L-trisomy or 3L-trisomy does not impart any influence on X-chromosomal replication in either sex. It is suggested from these results that change in the genomic dose of autosome does not play any role in modulating the replicative organization of the autosomes, 2L and 3L. Thus, although a regulatory mechanism of autosomal dosage compensation does exist for Drosophila, the hierarchy of genetic programming of regulation for X-chromosomal and autosomal dosage compensation might be different. Neither hypertranscriptive activity nor faster replication pattern of the male X-chromosome is influenced by 2L- or 3L-trisomy.

Functional aspect of a phosphopeptide derived from calf thymus nuclei and DNA.

Phosphopeptides (PPs) isolated from highly purified calf thymus DNA (N-DNA) and extracted from calf thymus nuclei were fractionated, and the effect of one PP fraction on DNA replication has been examined. In the absence of inhibitors, the increasing PP concentration caused a linear decrease of 3H-thymidine uptake in L5178Y cells. If PP fraction was mildly hydrolysed with 1NHCl, the decrease in uptake was much steeper. The studies in which the inhibitors were used revealed that by the addition of the unhydrolysed PP fraction the inhibition of 3H-thymidine uptake by alpha-amanitin could be completely overcome, and that the inhibition by puromycin was reduced to 65-77% of the control. With puromycin, there was a gradual decrease of 3H-thymidine uptake with PP concentration above 3 mg/ml. The PPs gave an increase in incorporation of 3H-thymidine even after removal of alpha-amanitin and puromycin; thus, it is suggested that there is no direct interaction of either inhibitor with PP in the cell. Data on the utilization of 3H-cytidine for the synthesise of new DNA suggest that PP fraction might cause an acceleration of DNA replication.