Transposition of mobile genetic elements in interspecific hybrids of Drosophila

In situ hybridization of labeled DNA of four mobile dispersed genetic elements (mdg), isolated from D. melanogaster and C. virilis genomes, with polytene chromosomes of the larvae of several Drosophila species has been carried out. The data show that the mdg elements exhibit a high degree of species specificity. The same conclusions are derived from filter hybridization using ³²P-labeled D. melanogaster and D. virilis DNA and cloned mdg sequences immobilized on nitrocellulose filters. We attempted to induce transpositions (jumping) of mdg elements specific for D. virilis chromosomes to the chromosomes of related species (e.g. D. littoralis Meigen) originally lacking the representatives of this family of repeats. For this purpose we produced hybrid stocks with synthetic karyotoypes characterized by different combinations of D. virilis homologous chromosomes and hybrid chromosomes. In one of such stocks we did find by in situ hybridization the insertion of a D. virilis mdg element into the fifth chromosome of D. littoralis Meigen. The transposition (jumping) took place in the only region where somatic pairing between the fifth chromosomes of D. virilis and D. littoralis occurs more or less regularly in the hybrids. Since crossing-over in hybrid chromosomes of males is excluded in such synthetic stocks, gene conversion may be responsible for this transposition. The possible bearing of the phenomenon observed on the problem of hybrid dysgenesis is discussed.     

The Cytogenetic Study of Crossing over in Interspecific Hybrids between DROSOPHILA VIRILIS and D. TEXANA

Spontaneous crossing over was studied by means of combined cytological and genetic methods in F₁ Drosophila virilis x D. texana females (series I) and in D. virilis females carrying a D. texana fifth chromosome in heterozygous condition (series II). The main criterion utilized to distinguish the oogonial crossovers from the meiotic ones is the identity of cytological positions of genetic exchange in crossovers constituting a cluster. Five clusters of crossovers with identical positions of exchange were found in the first series of experiments. In the second series of experiments not a single cluster of crossovers resulting from oogonial crossing over was found.     

Evolution of the response to heat shock in genus Drosophila

Thermotolerance was studied in a wide spectrum of Drosophila species and strains originating from different climatic zones and considerably differing from one another in the ambient temperature of their habitats. The species that lived in hot climate have a higher thermotolerance. Most species of the virilis group exhibited positive correlation between the HSP70 accumulation after heat exposure and thermotolerance; however, this correlation was absent in some species and strains. For example, the D. melanogaster Oregon R strain, which had the highest sensitivity to heat shock (HS) among all strains and species studied, displayed the maximum level of HSP70 proteins after HS. The patterns of induction of various heat shock protein (HSP) families after heat exposure in a wide spectrum of Drosophila species were compared. The results obtained suggest that the HSP40 and low-molecular-weight HSPs (lmwHSPs) play a significant role in thermotolerance and adaptation to hot climate. Polymorphism in hsp70 gene clusters of Drosophila and variation in the numbers of gene copies and hsp70 isoforms in group virilis were found. The evolutionary role of the variation in the number of hsp70 gene copies observed in the strains and species of genus Drosophila is discussed.     

Qualitative and quantitative analyses of the transpositions of P element--based genetic construction into the region of Drosophila melanogaster hsp70 genes

The hsp70 genes is among the main systems underlying the adaptation of organisms to adverse environmental factors. The ever increasing amount of data in literature demonstrates an important adaptive role of mobile genetic elements in microevolution. Drosophila hsp70 genes are potential target for transpositions of various mobile elements in natural populations. We have analyzed the frequency and localization of a P element-based genetic construction, EPgy2, in the region of Drosophila melanogaster hsp70 genes. A hot spot for the transposition was discovered in the promoter regions of genes hsp70Aa and hsp70Ab. No insertions of this construction in the coding or 3'-flanking regions of hsp70 genes have been recorded. It was demonstrated that the region of 161 to 7800 bp adjacent to the original construction is in certain cases also involved in the transposition. No transpositions of any other mobile elements have been observed. The inserts were shown to change the activity of hsp70 genes and the thermotolerance of transgenic strains.     

Small heat shock proteins and adaptation to hypertermia in various Drosophila species

Expression level and kinetics of accumulation of small heat shock proteins (21-27 kDa group) have been investigated in three Drosophila species differing significantly by temperature niche and thermosensitivity. It was shown that low-latitude thermotolerant species D. virilis exceeds the high-latitude thermosensitive closely-related species D. lummei as well as distant thermosensitive species D. melanogaster in terms of small heat shock proteins expression and accumulation after temperature elevation. The data obtained enable to postulate an important role of small heat shock proteins in organism basal thermotolerance and general adaptation to adverse conditions of environment.     

Replication slippage may cause parallel evolution in the secondary structures of mitochondrial transfer RNAs

Presence of the dihydrouridine (D) stem in the mitochondrial cysteine tRNA is unusually variable among lepidosaurian reptiles. Phylogenetic and comparative analyses of cysteine tRNA gene sequences identify eight parallel losses of the D-stem, resulting in D-arm replacement loops. Sampling within the monophyletic Acrodonta provides no evidence for reversal. Slipped-strand mispairing of noncontiguous repeated sequences during replication or direct replication slippage can explain repeats observed within cysteine tRNAs that contain a D-arm replacement loop. These two mechanisms involving replication slippage can account for the loss of the cysteine tRNA D-stem in several lepidosaurian lineages, and may represent general mechanisms by which the secondary structures of mitochondrial tRNAs are altered.