Widely differing degrees of sequence conservation of the two types of rDNA insertion within the melanogaster species sub-group of Drosophila

Abstract

We have examined the distribution of sequences homologous to the type I and type II rDNA insertions of Drosophila melanogaster in its sibling species. Each of the six species we have examined has sequences homologous to the type I insertion, which have undergone extensive divergence by the criterion of their EcoRI, BstI and HindIII restriction patterns. We have isolated cosmid clones containing type I sequences from D. simulans and D. mauritiana, the two species most closely related to D. melanogaster. Southern hybridisation analysis of these clones indicates that, as in D. melanogaster, the type I sequences can exist independently of rDNA and can also dissociate to give sub‐components homologous to the right hand segment of the D. melanogaster type I insertion. The type II sequences, on the other hand are present in five out of the six species, but their restriction endonuclease cleavage profile is highly conserved. The differences in the degree of conservation of the two types of insertion sequence are discussed.

Localization of ribosomal DNA insertion elements in polytene chromosomes of Drosophila simulans, Drosophila mauritiana and their interspecific hybrids

“…Absence of the rDNA coding sequence in the sites 75B, 80BC and the chromocenter argues that type I and type II elements outside the NOs could be arranged in a similar way in these species and in D. melanogaster (Kidd & Glover, 1980;Peacock et al, 1981;Hilliker & Appels, 1982). It seems that the D. simulans and D. mauritiana type I sequences, revealed by us in the chromocenters, correspond to the rDNA-independent type I insertion clones derived previously by Roiha et al (1983) from genomes of these species. In the same paper, altered restriction patterns of the type II insertion-containing fragments in D. simulans genomic DNA (as compared to D. melanogaster) were demonstrated.…”

“…Southern analysis of rDNA of sibling Drosophila species from the melanogaster group demonstrated the presence of type I and type II insertion elements in their genomes (Coen et al, 1982;Roiha et al, 1983). In a previous work, we studied the localization of NOs in mitotic chromosomes of two members from this group, D. simulans and D. mauritiana, and the distribution of the rDNA insertions between the NOs (Mecheva & Semionov, 1989;1991).…”

The site-specific ribosomal DNA insertion element R1Bm belongs to a class of non-long-terminal-repeat retrotransposons.

“…Several considerations suggest that the retention of Ri within the 28S genes is at least partly due to its ability to actively transpose. First, Ri insertions have been found in both lepidopteran and dipterian species (3,4,16,21,46 Unequal crossover or gene conversion mechanisms cannot explain this variation or the fact that it is limited to the 5' end. Retrotransposable elements and retroviruses are able to insert into many sites of the host chromosome.…”

Bombyx mori 28S ribosomal genes contain insertion elements similar to the Type I and II elements of Drosophila melanogaster.

“…Several other Dipteran species contain rDNA units with insertions similar to that of D. melanogaster. DNA from Type I insertions has been used as a hybridization probe to demonstrate that Type I-like inserts are present in all examined sibling species of melanogaster (Coen et al, 1982a;Roiha et al, 1983). Insertion elements have also been detected in the rDNA units of a distant Drosophila species, D. virilis (Barnett and Rae, 1979).…”