Localization and characterization of rDNA in Drosophila tumiditarsus

Evolution of the chromosomal location of rDNA genes in two Drosophila species subgroups: ananassae and melanogaster

The evolution of the chromosomal location of ribosomal RNA gene clusters and the organization of heterochromatin in the Drosophila melanogaster group were investigated using fluorescence in situ hybridization and DAPI staining to mitotic chromosomes. The investigation of 18 species (11 of which were being examined for the first time) belonging to the melanogaster and ananassae subgroups suggests that the ancestral configuration consists of one nucleolus organizer (NOR) on each sex chromosome. This pattern, which is conserved throughout the melanogaster subgroup, except in D. simulans and D. sechellia, was observed only in the ercepeae complex within the ananassae subgroup. Both sex-linked NORs must have been lost in the lineage leading to D. varians and in the ananassae and bipectinata complexes, whereas new sites, characterized by intra-species variation in hybridization signal size, appeared on the fourth chromosome related to heterochromatic rearrangements. Nucleolar material is thought to be required for sex chromosome pairing and disjunction in a variety of organisms including Drosophila. Thus, either remnant sequences, possibly intergenic spacer repeats, are still present in the sex chromosomes which have lost their NORs (as observed in D. simulans and D. sechellia), or an alternative mechanism has evolved.

Ribosomal RNA genes in mosquitoes: localization by fluorescence in situ hybridization (FISH)

Fluorescence in situ hybridization (FISH) was used to localize the 18S-28S ribosomal RNA gene clusters on the chromosomes of 15 mosquito species belonging to the Anophelinae and Culicinae subfamilies. In the genus Anopheles the rRNA genes are localized on the heterochromatic arm of both sex chromosomes. The association between rRNA genes and sex determining chromosomes also applies to the homomorphic karyotype of Culicinae mosquitoes, at least in those cases in which localization of the sex locus/loci has been determined. In these species ribosomal genes are often found within or adjacent to heterochromatic regions (C bands). Differences in the location of rRNA genes among and within genera suggest the occurrence of several chromosomal rearrangements during the evolution of mosquitoes.

Allocyclic behaviour and underreplication of the nucleolus chromosome in Pseudodiamesa (Chironomidae)

The chironomid midge Pseudodiamesa branickii shows three polytene, banded elements in salivary gland and Malpighian tubule cells, none of which carry a nucleolar organizer. The present investigation revealed four chromosome pairs in mitotic metaphases, the nucleolus was localized in the tiny chromosome 4. Microphotometric determinations of the DNA contents in mitotic and salivary gland nuclei of Ps. branickii yielded a 2C value of 0.23 pg DNA and a maximum level of 12 endoreplications. In both salivary glands and Malpighian tubules, chromosome 4 appeared as a nonbanded network of chromatin in Feulgen preparations. Indirect immunofluorescent staining with antibodies specific for RNA X DNA hybrids indicated that this structure was transcriptionally active. Its reaction to heavy metal staining suggested that it included the nucleolar organizer, and this was conclusively demonstrated by in situ hybridization with 125I-rRNA. Chromosome pair 4 comprises about 11.4% of the total DNA in metaphase, but only 3.7% in the highly polytene salivary gland complement. According to a numerical simulation, the degree of underrepresentation suggests that about 70% of chromosome 4 is precluded from polytenization. The allocyclic aspect is probably due to this underreplication, with the nucleolar activity causing a structural loosening of the rest of the chromosome. The possible nature of the underreplicated sequences is discussed in the light of recent work on molecular aspects of rDNA structure in various Diptera.