V. G. Mitrofanov

Variation of the mitochondrial genome in the evolution of Drosophila

The evidence on mitochondrial genome variation and its role in evolution of the genus Drosophila are reviewed. The mitochondrial genome is represented by a circular double-stranded DNA molecule 16 to 19 kb in length. Mitochondrial genes lack introns and recombination. The entire mitochondrial genome can be arbitrarily divided into three parts: (1) protein-coding genes; (2) genes encoding rRNA and tRNA; and (3) the noncoding regulatory region (A + T region). The selective importance of mutations within different mtDNA regions is therefore unequal. In Drosophila, the content of the A + T pairs in mtDNA is extremely high and a pattern of nucleotide substitution is characterized by a low transition/transversion ratio (and a low threshold of mutation saturation). The deletions and duplications are of common occurrence in the mitochondrial genome. However, this genome lacks such characteristic for the nuclear genome aberrations as inversions and transpositions. The phenomena of introgression and heteroplasmy provide an opportunity to study the adaptive role of the mitochondrial genome and its role in speciation. Analysis of evidence concerning mtDNA variation in different species of the genus Drosophilamade it possible to ascertain data on phylogenetic relationships among species obtained by studying nuclear genome variation. In some species, mtDNA variation may serve as a reliable marker for population differentiation within a species, although evidence on the population dynamics of the mtDNA variation is very scarce.

The relationships among the species of the Drosophila virilis group inferred from the gene Ras1 sequences

Comparative analysis of a group of closely related Drosophila species (D. virilis, D. lummei, D. novamexicana, D. americana texana, D. flavomontana, D. montana, D. borealis, D. lacicola, D. littoralis, D. kanekoi, and D. ezoana) was conducted based on an incomplete sequence of gene Ras1. The pattern of the relationships among the species corresponded to that expected from analysis of morphological and cytogenetic characters. Statistical data favoring neutrality of the substitutions examined in the Ras1 gene are presented. This character of the gene Ras1 evolution confers more reliability to reconstruction of phylogenetic relationships among closely related species. The resultant tree for main phylads of the group is as follows: D. virilis (D. lummei, D. montana, D. ezoana).

[Variation of 3'-terminal fragment of 16S rRNA gene in closely related species of Drosophila virilis group].

Primary sequence of 3′-terminal fragment of the mitochondrial 16S rRNA gene has been determined in 12 Drosophila species of the virilis group. The functionally important elements in secondary structure of the RNA product were defined. The region corresponding to the peptidyltransferase center has been localized. Variation of the 3′-terminal region of 16S rRNA gene has been described in 12 species of the virilis group. Phylogeny of the Drosophila virilis species group is discussed.

Morphometric Analysis of Male Genitalia in Sibling Species of Drosophila virilis Sturt.

The structure of male genital organs in sibling species of the virilis group of Drosophila was examined using methods of multivariate statistics. The differences among these species were estimated using 33 indices and 2 angle parameters. The intraspecific and interspecific correlation structure of the examined characters and the order of their divergence were established. The key characters with respect to forming interspecific differences in the virilis species group were identified. Based on these results, the relative systematic positions of the sibling species are discussed as well as similarities and differences of the pattern of relationships among the species from that generally accepted for the virilis group.

Dynamics of mitochondrial polymorphism in a natural population of Drosophila littoralis

During seven years, we observed stable mtDNA polymorphism in a local population of Drosophila littoralis. Using RFLP, a number of mitochondrial haplotypes were revealed, two of which were the core and in condition of stable equilibrium. To explain the absence of fixation of one haplotype, we checked a hypothesis that the D. littoralis population had a complex structure, being subdivided into several partially isolated races existing on the same territory. Analysis of highly hypervariable nuclear sequence of retrotransposons Tv1 showed positive correlation of the mitochondrial haplotype with a particular allelic form of Tv1. This supports the proposal that the D. littoralis natural population forms the population system consisting of genetically differentiated races.

Mitochondrial DNA Polymorphism in Natural Populations of the Drosophila virilis Species Group

Restriction fragment length polymorphism (RFLP) analysis has been used to evaluate mitochondrial DNA (mtDNA) variation in 12 sibling species forming the Drosophila virilis species group. The variation thresholds corresponding to the interspecific and interstrain levels have been determined. The results indicate that interspecific hybridization has significantly contributed to the evolutionary history of the virilis species group.

A RAPD fingerprinting of sibling species of the Drosophila virilis group

A comparative analysis of the sibling species of Drosophila virilis was performed by RAPD-PCR technique using a set of random primers. The degree of relatedness was studied by cluster analysis (UPGMA) and multidimensional scaling. The resulting pattern of species relationships contradicts the classical taxonomy. The main result of the cluster analysis is that D. virilis does not cluster with the remaining three species of its phylad, while according to multidimensional scaling, D. virilis is equidistant from all the species of its group, from both the species of its phylad and the species of the montana phylad. The montana phylad is extremely heterogeneous; moreover, the species D. littoralis, D. ezoana, and D. kanekoi appear to be closer to the virilis phylad than to the other species of the montana phylad, wherein these species are traditionally included. The phylogenetic relationships between the studied species discovered using RAPD fingerprinting comply with the results obtained using protein markers and quantitative traits.

The effect of population density on the elimination dynamics of a recessive lethal mutation l(2)M167DTS from experimental populations of Drosophila melanogaster

Dynamics of the elimination of the temperature-sensitive lethal mutation l(2)M167DTS from experimental populations of Drosophila melanogaster under permissive conditions (25°C) was studied. We have shown a rapid elimination of the mutation from the populations, selection for fitness of heterozygous individuals, association of the selection with high larval density and with the direction of the cross that had produced the founder males of the l(2)M167DTS/+ population, and the effect of relative competitive ability of l(2)M167DTS/+ males on the efficiency of the mutation introduction. Modification systems were shown be involved in fitness selection under conditions of high larval density.

[The effect of male mating competitiveness, developmental rate, and viability of larvae and pupae in Drosophila melanogaster heterozygous for the temperature-sensitive lethal mutation l(2)M167DTS on the dynamics of the mutation elimination from the population].

The dependence of selection on an introduced mutation l(2)M167DTS on male mating competitiveness, viability, and developmental rate of larvae and pupae of Drosophila melanogaster, heterozygous for this mutation, was examined in population experiments with preset conditions. The limitations of fitness parameters of individuals l(2)M167DTS/+ relative to individuals +/+ were estimated according to the conditions of the experiment and phenotypic characteristics of the mutation studied. Under conditions of limited food supply and dependence on emergence time in each generation, the sequence of female mating was shown to be of less importance than the order of medium utilization by the progeny of a certain genotype related to the male success in the first mating. The limiting factors acting on the l(2)M167DTS mutation were viability and developmental rate.

Determination of fitness components of flies bearing the recessive lethal l(2)M167(DTS) mutation with dominant heat sensitivity in artificial Drosophila melanogaster populations

Elimination of the heat-sensitive l(2)M167DTS mutation from artificial Drosophila melanogaster populations at constant temperature 25°C and various frequencies of the mutation in the parental generation was studied. Components of fitness of the l(2)M167DTS mutation were estimated in the artificial populations by means of the recurrent model of the dependence of the frequency of this mutation in a given generation on its frequency in the previous generation. The model was solved by a numerical method with limitations on the values of some fitness components obtained in test experiments. According to the limitations and frequencies of the l(2)M167DTS mutation, the leading role and limits of the variation in egg-to-adult viability and female fertility were determined. The previously suggested effect of the positive selection for viability of individuals heterozygous for l(2)M167DTS was confirmed.