Genetic monitoring of natural Drosophila populations in radiation contaminated regions of Belarus

Genetic monitoring of natural Drosophila melanogaster populations inhabiting regions of Belarus with different radiation background (Vetka and Svetilovichi villages), radonuclide-contaminated after the Chernobyl accident, compared with populations from the Berezinsky biosphere reserve (the control area) were conducted. The dominant and recessive lethal mutation levels and genetic structure of the populations were analyzed for frequencies of F- and S-alleles of Adh (alcohol dehydrogenase) of Gpdh (glycerinophosphate dehydrogenase) and Sod (superoxide dismutase) loci. Populations inhabiting the regions with high radiation background exhibited higher frequency of lethal mutations and higher heterozygosity than those from the control area. Moreover, higher frequency of polymorphous Sod locus S-allele was detected in these populations. Apparently, Sod S-alleles are more adaptively valuable under conditions of high radiation background, because as is known, superoxide dismutase is an effective radioprotector at all levels molecular, cellular and organism. Adaptation of populations to stress impacts was analyzed, since 1998. Nonspecific adaptation of natural Drosophila melanogaster populations from Vetka and Svetilovichi villages of Gomel region was reveled. They are higher adapted than the control population from the Berezinsky biosphere reserve to both ionizing radiation effect and to chemical mutagen EMS. After laboratory cultivation within 6-8 generations without irradiation adaptation to radiation in the population from radiocontaminated regions remained. The content of samples from the control natural drosophila population in the laboratory conditions is an environmental stress that led to the formation of nonspecific adaptation within 6-8 generations to unfavorable factors, including ionizing radiation. It should be taken into account that the population adaptation is formed via death of sensitive genotypes at various ontogenesis stages.     

Mutation processes in natural populations of <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> and <Emphasis Type="Italic">Hirundo rustica</Emphasis> from radiation-contaminated regions of Ukraine

Natural populations of Drosophila melanogaster and Hirundo rustica from regions of Ukraine exhibiting different levels of radiation contamination are investigated. Genetic monitoring was performed with respect to such parameters as the frequency of visible sex-linked mutations, frequency of gonad reduction in Drosophila, and rate of interphase manifestations of chromosomal instability in the erythrocytes of the birds. The results attest to a possible opposite dependence of the level of chromosomal instability among swallows and that of the rate of lethal mutations in the sex chromosome of Drosophila on the density of radiation contamination.     

Adaptation processes in natural Drosophila populations in radiation contaminated Belarus regions before and after radiation exposure removal

We have shown that natural drosophila populations from the settlement Vetka of Gomel region with increased radiation background are more adapted to mutagenic effect of radiation than drosophila populations from Berezinsky reserve (the control). After the populations were placed into laboratory thermostat adaptation of Vetka population remained within 6-8 generations without irradiation. However the control population became more resistant too. So, the keeping of natural drosophila populations under laboratory conditions was a stress and led to unspecific adaptation the same as a low level of radiocontamination did. These facts should be considered in studying dynamics of the mutation level during radionuclide removal in animals caught in radiocontaminated regions and placed in vivaria conditions.     

Selection for intragenic suppressors of lethal 23S rRNA mutations in Escherichia coli identifies residues important for ribosome assembly and function

Mutations in several functionally important regions of the 23S rRNA of E. coli increase the levels of frameshifting and readthrough of stop codons. These mutations include U2555A, U2555G, ΔA1916 and U2493C. The mutant rRNAs are lethal when expressed at high levels from a plasmid, in strains also expressing wild type rRNA from chromosomal rrn operons. The lethal phenotype can be suppressed by a range of second-site mutations in 23S rRNA. However, analysis of the functionality of the double mutant rRNAs in heterogeneous ribosome populations shows that in general, the second site mutations do not restore function. Instead, they prevent the assembly, or entry of the mutant 50S subunits into the functioning 70S ribosome and polysome pools, by affecting the competitiveness of the mutant subunits for association with 30S particles. The second-site mutations lie in regions of the 23S rRNA involved in subunit assembly, intersubunit bridge formation and interactions of the ribosome with tRNAs and factors. These second site suppressor mutations thus define functionally important rRNA nucleotides and this approach may be of general use in the functional mapping of large RNAs.     

High nucleotide substitution error frequencies in clonal pools of vesicular stomatitis virus.

Nucleotide substitution error frequencies were determined for several specific guanine base positions in the genomes of cloned vesicular stomatitis virus populations. Predetermined sites were examined in coding regions for the N, M, and L proteins and at a site in the genome 5'-end regulatory region. Misincorporation frequencies were estimated to be on the order of 10(-3) to 10(-4) at all positions analyzed. Isolates taken from virus populations after disruption of equilibrium conditions displayed replicase fidelity similar to that of cloned wild-type vesicular stomatitis virus. These mutation frequencies apply to all virus genomes present, including viruses rendered nonviable by lethal mutations. At one selected site in the N gene, two of three G----N base substitutions generated lethal nonsense mutations, yet their frequency was also very high. Biological implications for rapid virus evolution are discussed.     

Mutations and their use in insect control

Traditional chemically based methods for insect control have been shown to have serious limitations, and many alternative approaches have been developed and evaluated, including those based on the use of different types of mutation. The mutagenic action of ionizing radiation was well known in the field of genetics long before it was realized by entomologists that it might be used to induce dominant lethal mutations in insects, which, when released, could sterilize wild female insects. The use of radiation to induce dominant lethal mutations in the sterile insect technique (SIT) is now a major component of many large and successful programs for pest suppression and eradication. Adult insects, and their different developmental stages, differ in their sensitivity to the induction of dominant lethal mutations, and care has to be taken to identify the appropriate dose of radiation that produces the required level of sterility without impairing the overall fitness of the released insect. Sterility can also be introduced into populations through genetic mechanisms, including translocations, hybrid incompatibility, and inherited sterility in Lepidoptera. The latter phenomenon is due to the fact that this group of insects has holokinetic chromosomes. Specific types of mutations can also be used to make improvements to the SIT, especially for the development of strains for the production of only male insects for sterilization and release. These strains utilize male translocations and a variety of selectable mutations, either conditional or visible, so that at some stage of development, the males can be separated from the females. In one major insect pest, Ceratitis capitata, these strains are used routinely in large operational programs. This review summarizes these developments, including the possible future use of transgenic technology in pest control.     

Limits to adaptation in asexual populations

In asexual populations, the rate of adaptation is basically limited by the frequency and properties of spontaneous beneficial mutations. Hence, knowledge of these mutational properties and how they are affected by particular evolutionary conditions is a precondition for understanding the process of adaptation. Here, we address how the rate of adaptation of asexual populations is limited by its population size and mutation rate, as well as by two factors affecting the fraction of mutations that confer a benefit, i.e. the initial adaptedness of the population and the variability of the environment. These factors both influence which mutations are likely to occur, as well as the probability that they will ultimately contribute to adaptation. We attempt to separate the consequences of these basic population features in terms of their effect on the rate of adaptation by using results from evolution experiments with microorganisms.     

The radiationally induced change of level of double-stranded breaks DNA in neuroblasts of larvae and frequency of lethal mutations in sex cells of males Drosophila melanogaster

The level of damage DNA in neyroblastes of larvae and frequency of recessive sex-linked lethal mutations of males from chronically irradiated populations Drosophila melanogaster, differing on mobile P-elements patterns, was estimated. Received results testify, that exposition in conditions a chronic gamma-radiation (absorbed radiation dose at one generation is compounds 10 mGy) result to increase of significance of parameters and change of sensitivity of cells to following of an acute irradiation in a dose of 3 Gy.     

Positive selection of synonymous mutations in vesicular stomatitis virus

Prevailing evolutionary forces are typically deduced from the pattern of differences in synonymous and non-synonymous mutations, under the assumption of neutrality in the absence of amino acid change. We determined the complete sequence of ten vesicular stomatitis virus populations evolving under positive selection. A significant number of the mutations occurred independently in two or more strains, a process known as parallel evolution, and a substantial fraction of the parallel mutations were silent. Parallel evolution was also identified in non-coding regions. These results indicate that silent mutations can significantly contribute to adaptation in RNA viruses, and relative frequencies of synonymous and non-synonymous substitutions may not be useful to resolve their evolutionary history.     

Biological effects of chronic radiation exposure on plant populations

The findings from long-term field studies on biological effects in plant populations inhabiting radioactively contaminated territories contrast in levels and compositions of dose-forming radionuclides are presented. Plant populations developing under radioactive impact show enhanced frequencies of gene and chromosome mutations, and their reproductive potential is inferior to reference populations. Even relatively low levels of technogenic impact are able to increase genetic diversity and destroy regularities inherent for intact populations. Chronic radiation exposure from a certain level appears to be an ecological factor changing genetic structure of wild populations. Data presented indicate the presence of adaptation processes in plant populations in territories with technogenic impact. Under ecological stress, there are selection processes for resistance improvement in plant populations. But an appearance and rate of this process can essentially differ in dependence on radioecological conditions.     

Genetic effects of cosmic radiation in Drosophila melanogaster]

To examine possible effects of space radiation on living organism, we have analyzedtwo types of mutations, sex-linked recessive lethal mutations and somatic mutations, in fruit fly of the species Drosophila melanogaster. Drosophila strains used were wild type strains and a radiation-sensitive strain mei-41. Two different developmental stages of samples were sent into space; young adult males to analyze sex-linked recessive lethal mutations and about 30hr-old larvae to detect somatic mutations in wing epidermal cells. For wild type and mei-41 strains each, about 200 adult male flies and about 6,000 larvae were loaded on space shuttle Endeavour. The male flies returned from space were mated to virgin female flies of a tester strain, and the presence of the lethal mutations was analyzed at F2 generation. The frequencies of sex-linked recessive lethal mutations in flight groups were 2 and 3 times higher for wild type Canton-S and mei-4 1, respectively, than those in ground control groups. Most larvae sent to space emerged as adult flies within about 10 days after the landing. The presence of wing-hair somatic mutations, which give morphological change in hairs growing on the surface of wing epidermal cells, was analyzed under microscope. In wild type strain Muller-5, the frequency of wing hair mutant spots in flight group was about 1.5-fold higher than that in ground control, and in Canton-S-derived wild type strain the frequencies were similar between the two groups. By contrast, for mei-41 strain the mutation frequency was lower in flight group than in control group. The observed higher frequency of lethal mutations in the flight group might be due to a possibility that radiation effects on reproductive cells could be greatly enhanced under micro gravity. However, if this would be the case, we do not have appropriate explanation for the apparent absence of such synergistic effects on somatic wing-hair mutation system.     

Population genomic footprints of selection and associations with climate in natural populations of Arabidopsis halleri from the Alps

Natural genetic variation is essential for the adaptation of organisms to their local environment and to changing environmental conditions. Here, we examine genomewide patterns of nucleotide variation in natural populations of the outcrossing herb Arabidopsis halleri and associations with climatic variation among populations in the Alps. Using a pooled population sequencing (Pool‐Seq) approach, we discovered more than two million SNPs in five natural populations and identified highly differentiated genomic regions and SNPs using F_ST‐based analyses. We tested only the most strongly differentiated SNPs for associations with a nonredundant set of environmental factors using partial Mantel tests to identify topo‐climatic factors that may underlie the observed footprints of selection. Possible functions of genes showing signatures of selection were identified by Gene Ontology analysis. We found 175 genes to be highly associated with one or more of the five tested topo‐climatic factors. Of these, 23.4% had unknown functions. Genetic variation in four candidate genes was strongly associated with site water balance and solar radiation, and functional annotations were congruent with these environmental factors. Our results provide a genomewide perspective on the distribution of adaptive genetic variation in natural plant populations from a highly diverse and heterogeneous alpine environment.     

Accumulation of Deleterious Genes in a Cage Population of DROSOPHILA MELANOGASTER

Lethal and sterility mutations were accumulated in a cage population which was initiated with lethal- and sterility-free second chromosomes of D. melanogaster. It took about 2,000 days for the frequencies of these genes to reach equilibrium levels, i.e., 18% lethal and 9% male-sterile chromosomes. Two other cage populations which were initiated with random chromosomes sampled from natural populations and kept for more than eleven years in the laboratory showed 19-20% lethal content. The elimination rates of lethals by homozygosis in these populations were smaller than the mutation rate. By using NEI's formulae, the deleterious effect of a lethal gene in heterozygous condition (h) was estimated to be 0.035. The effective population number in the cage populations was estimated to be 1,000-2,900, while the actual population number was 3,500-7,800.     

Experimental evolution of human influenza virus H3 hemagglutinin in the mouse lung identifies adaptive regions in HA1 and HA2

The genetic basis for virulence and host switching in influenza A viruses (FLUAV) is largely unknown. Because the hemagglutinin (HA) protein is a determinant of these properties, HA evolution was mapped in an experimental model of mouse lung adaptation. Variants of prototype A/Hong Kong/1/68 (H3N2) (wild-type [wt] HK) human virus were selected in both longitudinal and parallel studies of lung adaptation. Mapping of HA mutations found in 11 independently derived mouse-adapted populations of wt HK identified 27 mutations that clustered within two distinct regions in or near the globular frameworks of the HA1 and HA2 subunits. The adaptive mutations demonstrated multiple instances of convergent evolution involving four amino acid positions (162, 210, and 218 in HA1 and 154 in HA2). By use of reverse genetics, convergent HA mutations were shown to affect cell tropism by enhancing infection and replication in primary mouse tracheal epithelial cells in vitro and mouse lung tissue in vivo. Adaptive HA mutations were multifunctional, affecting both median pH of fusion and receptor specificity. Specific mutations within both adaptive regions were shown to increase virulence in a mouse lung model. The occurrence of mutations in the HA1 and HA2 adaptive regions of natural FLUAV host range and virulent variants of avian and mammalian viruses is discussed. This study has identified adaptive sites and regions within the HA1 and HA2 subunits that may guide future studies of viral adaptation and evolution in nature.     

The influence of cellular physiology on the initiation of mutational pathways in Escherichia coli populations

The factors affecting the direction of evolutionary pathways and the reproducibility of adaptive responses were investigated under closely related but non-identical conditions. Replicate chemostat cultures of Escherichia coli were compared when adapting to partial or severe glucose limitation. Four independent populations used a reproducible sequence of early mutational changes under both conditions, with rpoS mutations always occurring first before mgl. However, there were interesting differences in the timing of mutational sweeps: rpoS mutations appeared in a clock-like fashion under both partial and severe glucose limitation, while mgl sweeps arose under both conditions but at different times. Interestingly, malT and mlc mutations appeared only under severe limitation. Even though the ancestors were genotypically identical, the semi-differentiated properties of bacteria growing with mild or severe glucose limitation sent the populations in characteristic directions. Mutation supply and the fitness contribution of mutations were estimated and demonstrated to be potential influences in the choice of particular adaptation pathways under severe and mild glucose limitation. Predicting all the mutations fixed in adapting populations is beyond our current understanding of evolutionary processes, but the interplay between ancestor physiology and the initiation of adaptation pathways is demonstrated and definable in bacterial populations.     

Generation of seal influenza virus variants pathogenic for chickens, because of hemagglutinin cleavage site changes.

Influenza virus A/seal/Mass/1/80 (H7N7) was adapted to grow in MDCK cells and chicken embryo cells (CEC) in the absence of exogenous protease. The biological properties of the virus variants obtained coincided with intracellular activation of the hemagglutinin (HA) by posttranslational proteolytic cleavage and depended on the cell type used for adaptation. MDCK cell-adapted variants contained point mutations in regions of the HA more distant from the cleavage site. It is proposed that these mutations are probably responsible, through an unknown mechanism, for enhanced cleavability of HA in MDCK cells. Such virus variants were apathogenic in chickens. CEC-adapted variants, on the other hand, contained an insertion of basic amino acids at the HA cleavage site, in addition to scattered point mutations. The insertions converted the cleavage sites in the variant virus HAs so that they came to resemble the cleavage site found in highly pathogenic avian influenza viruses. CEC variants with such cleavage site modifications were highly pathogenic for chickens. The lethal outcome of the infection in chickens demonstrated for the first time that an influenza virus derived from a mammalian species can be modified during adaptation to a new cell type to such an extent that the resulting virus variant becomes pathogenic for an avian species.     

Tracking the origin of the American colonization by Drosophila subobscura: genetic comparison between Eastern and Western Mediterranean populations

Several pieces of evidence indicate a Mediterranean origin of the colonization of America by Drosophila subobscura . To ascertain whether the origin was from the Eastern or the Western Mediterranean region, samples from Barcelona (Spain) and Mt Parnes (Greece) were collected and O chromosomal inversion polymorphism and lethal genes were analysed. The frequencies of lethal chromosomes were 0.244 ± 0.039 in Barcelona and 0.336 ± 0.043 in Mt Parnes, consistent with the expectations for large populations located in the central area of the species distribution. Lethal genes seem to be distributed at random along the O chromosome in both populations. The intra-populational allelism frequencies of Barcelona and Mt Parnes were 0.016 ± 0.007 and 0.012 ± 0.005 respectively. Thus, the estimates of the effective population size were high in both populations (between 6964 and 13 004 in Barcelona and 11 874 to 26 828 in Mt Parnes). The cases of allelism in Mt Parnes were observed only between individual lethal genes, but in Barcelona some concatenated clusters of allelism were detected. This pattern of allelism can be explained by synthetic lethality, hybrid dysgenesis, the induction of recurrent lethal mutations by different factors or an effect of microdifferentiation in subpopulations. In both populations, a reduction in fitness in the heterozygotes for lethal genes has been detected. Furthermore, the estimates of the migration coefficient (between 0.0085 and 0.0120 in Barcelona, and 0.0057 and 0.0087 in Mt Parnes) confirm the existence of gene flow between Palearctic populations of D. subobscura . Our lethal genes and chromosomal inversion results are consistent with a Mediterranean origin of the colonization of America by D. subobscura , but are inconclusive with regard to the identification of the population from which these colonizers came.