No abstract

Quantitative cytogenetical analysis has been used to study the synapsis of D. melanogaster neuroblast mitotic chromosomes from normal females, flies with heterozygous deletions, duplications or inversions in the heterochromatic regions of chromosome 2 and in triploid females. In all these genotypes chromocentric fusion of heterochromatic regions of heterologous chromosomes is observed. Eu-and heterochromatic regions of homologous chromosomes are intimately paired at the same time during the cell cycle. The structural rearrangements lead to reduced frequencies of chromocentric association as well as of homologous synapsis compared with the frequencies in the wild-type. The results obtained are discussed with respect to the general problem of the homologous interaction of chromosomes and the significance of heterochromatin for these processes.

The locations of the ribosomal DNA (rDNA) insertion elements type I and type II along the polytene chromosomes of three Drosophila species of the melanogaster subgroup--D. simulans, D. mauritiana and D. melanogaster--have been compared. In situ hybridization has shown that the intragenomic distribution of type I as well as of type II insertions is different for these related species. In particular, we have revealed rDNA-free autosomal sites, containing type II element sequences within the D. simulans and D. mauritiana chromosomes. This finding confirms the ability of this type of insertion to transpose, as was demonstrated earlier for Bombyx mori. The appearance of the rDNA not associated with the nucleolar organizers, evident by additional nucleoli, occurred with species-specific frequency. At the same time, for all three species the pattern of such changes (an attachment of the nucleoli to varying sites of the chromosomes and the presence of ectopic contacts between them, a composition of the rDNA repeats in the nucleolar material not integrated at the nucleolar organizer) was similar. The number of additional nucleoli in the hybrid polytene nuclei corresponded to the value of the parental species exhibiting nucleolar replicative dominance.