Evolution of natural populations in the Drosophila melanogaster sigma virus system I. Languedoc (Southern France)

In natural populations of Drosophila melanogaster, sigma virus is usually present in a minority of individuals. The virus is transmitted transovarially but is not contagious from fly to fly. Two viral Types (I and II) are found in populations. One of them (Type II) is better adapted to an allele for resistance to the virus, present as a polymorphism in fly populations. Previous observations have led to the hypothesis that a viral Type II originating in central France might be invading populations. The study of Languedoc populations was undertaken to examine this hypothesis. Two striking phenomena were observed. The strong increase in Type II clones frequency (from 0.53 to 0.91) confirmed that there was invasion in this region. The frequency of infected flies also increased dramatically, at levels never observed elsewhere yet, which indicates that Languedoc should present some unusual characteristics. The epidemiological consequences of such a burst, in the case of a pathogenic virus would have to be taken into consideration. Significant changes in other viral characteristics, from 1983 to 1987, in Languedoc populations have also been documented.     

Comparative evolution of P-M system and infection by the sigma virus in French and Spanish populations of Drosophila melanogaster.

In 1983, an extensive survey of populations of D. melanogaster was started in a southern French region (Languedoc) in two non-Mendelian systems: the P-M system of transposable elements and the hereditary Rhabdovirus sigma. Unexpectedly fast-evolving phenomena were observed and interesting correlations were noted, giving similar geographical pattern to the region in both systems. For these reasons, the analysis was continued and extended towards the north (Rh?ne Valley) and the south (Spain). In the P-M system, all the Languedoc populations evolved from 1983 to 1991 towards the Q type which is characteristic of the Rh?ne Valley populations. In contrast, M' strains are currently observed in the southernmost French populations and in all Spanish ones, so that there is a clear pattern in their geographical distribution. The frequency of flies infected by the sigma virus dramatically increased from 1983 to 1988 in Languedoc; this increase was clearly correlated with some viral characteristics. But, in northern France, similar characteristics did not trigger any increase in the frequency of infected flies. The data presented here show that the distinctive features of Languedoc extend northwards through the Rh?ne Valley up to Lyon and disappears southwards before the Spanish border.     

Selection affecting enzyme polymorphisms in laboratory populations of Drosophila melanogaster

Summary Gene frequencies at nine polymorphic enzyme loci were followed in 22 cage populations of D. melanogaster all founded from the same stock but subsequently maintained in different environments. The three factors distinguishing the different environments were temperature and the alcohol and yeast in the medium and for each factor each population experienced either a constant or variable condition. The variable conditions were either coarse-grained (period between changes longer than the generation time) or fine-grained (the period less than the generation time).Six polymorphisms were found to be unaffected by any experimental condition. However, the various constant temperatures tested differed in their effects on Alcohol dehydrogenase (Adh), Amylase (Amy) and Isocitrate dehydrogenase (Idh-NADP) gene frequencies. Furthermore, coarse-grained variation between high and low temperatures increased Amy and Idh-NADP heterozygosity and decreased Amy heterozygosity while fine-grained variation between such temperatures promoted Amy heterozygosity. In addition, joint coarse-grained variation for the type of alcohol and yeast promoted Amy and Idh-NADP heterozygosity as well as heterozygosity averaged over all nine loci.In general, the results suggested two major modifications to the hypothesis of a positive relationship between environmental and genetic variability. First, they indicated that the relationship is not always positive. It varies both between loci and between environments. Second however, they indicated that the relationship is more likely to be positive when the environmental variability is coarse- rather than fine-grained.     

Polymorphism of the Drosophila melanogaster — sigma virus system

In natural populations of D. melanogaster, a minority of flies are usually infected by the sigma Rhabdovirus, which is not contagious but vertically transmitted. A few parameters have been identified that affect the evolution of the Drosophila — sigma system. Various sets of values of these parameters in males mutually adapted to one another have been established in different populations. This study analyses female characteristics that were previously reputed not to vary much between populations. The newly collected data show that these characteristics (mainly transovarial transmission here) may also be polymorphic in the wild, and may contribute to the diversity of equilibria established between the virus and its host. The observations presented here, obtained from various populations, confirm the complexity of this system of two coevolving organisms: as already noted there seem to be as many different sets of values of parameters as there are populations.     

Evolution of natural populations in the Drosophila melanogaster sigma system II. Northern and Central France

A survey of French natural populations of Drosophila melanogaster has been systematically performed, concerning their status of infection by the sigma virus and the characteristics of viral clones. These investigations, which were not as extensive as those performed in the Languedoc region (Fleuriet et al., 1990) nevertheless give a good representation of the evolution of this system because of the long period involved (almost 20 years). Some trends were observed in all French populations such as (1) a decrease in the high efficiency of transmission by males (which is an important parameter for the viral invading ability); (2) high frequency of a best adapted viral Type. These high frequencies might be due to a recent invasion which is expected to spread to other European populations. However, the frequency of infected flies remained low in northern and central France, unlike in Languedoc. The complexity of this, apparently simple, system of two well-known coevolving organisms should once again be stressed. It is impossible with the known parameters to arrive at a general interpretation of observations made in Languedoc and the rest of France. These data may also throw some light on the structure of French wild populations of D. melanogaster which appear to be subdivided into local populations between which gene flow might be low.     

Geographical distribution and diversity of bacteria associated with natural populations of Drosophila melanogaster

Drosophila melanogaster is one of the most widely used model systems in biology. However, little is known about its associated bacterial community. As a first step towards understanding these communities, we compared bacterial 16S rRNA gene sequence libraries recovered from 11 natural populations of adult D. melanogaster. Bacteria from these sequence libraries were grouped into 74 distinct taxa, spanning the phyla Proteobacteria, Bacteroidetes, and Firmicutes, which were unevenly spread across host populations. Summed across populations, the distribution of abundance of genera was closely fit by a power law. We observed differences among host population locations both in bacterial community richness and in composition. Despite this significant spatial variation, no relationship was observed between species richness and a variety of abiotic factors, such as temperature and latitude. Overall, bacterial communities associated with adult D. melanogaster hosts are diverse and differ across host populations.     

Variance component analysis of bristle characters in local populations of Drosophila melanogaster.

The genetic variabilities of sternopleural and abdominal bristle numbers existing in local natural populations were assessed. Using second chromosome lines of Drosophila melanogaster extracted from three natural populations in Japan (the Ishigakijima, Ogasawara and Aomori populations), experiments were conducted to estimate the components of genetic variances, additive and dominance variances. The following results were obtained: For both sternopleural and abdominal bristle numbers, the additive genetic variances (sigma 2A) were much larger than the dominance variances (sigma 2D) especially in the southern populations. For example, in the Ishigakijima population, for females sternopleural bristle numbers of the inversion-free chromosome group, the additive and dominance variances were estimated to be 1.255 +/- 0.2034 and 0.0552 +/- 0.0180, respectively. The magnitudes of the estimates of additive genetic variances were nearly equal from north to south. By comparing the additive genetic variances of the inversion-free chromosome group with those of the In(2L)t-carrying chromosome group, it was inferred that sufficient number of generations to achieve the equilibrium state has not passed since the introduction of a single or a small number of the In(2L)t-carrying chromosomes to the Ishigakijima population.     

Microsatellite variation in North American populations of Drosophila melanogaster.

Computer database searching for microsatellites can be particularly effective for organisms like Drosophila melanogaster for which there are extensive sequence data. Here we demonstrate that 17 out of 18 such microsatellites are also highly polymorphic in natural populations of Drosophila, and that this variation is easily scorable with PCR followed by electrophoresis on high-resolution agarose. This form of variation is likely to be of great value in studies of the genomic distribution of polymorphism, population structure, the relation between intraspecific polymorphism and interspecific divergence and the mutation rate and pattern of mutations of microsatellites. In this preliminary survey of 15 lines, we find that the variance in repeat count is most strongly correlated with the maximum count, that perfect repeats are significantly more variable than imperfect repeats and that repeats which are split by an imperfection have unexpectedly low variance given the size of the perfectly repeated portion.     

Drosophila S virus, a hereditary reolike virus, probable agent of the morphological S character in Drosophila simulans.

Isometric reolike virions were found in all the examined Drosophila simulans flies from two strains (SimES-st and Israel-st) presenting the S phenotype, a maternally inherited morphological trait (abnormalities of bristles). Normal flies of both strains appeared virus-free. Virions were found in the cytoplasm of male and female gonads and epidermal cells, including the bristle-forming cells, which appeared disorganized. Steps of virogenesis were described. A positive correlation was demonstrated between expressivity of the S phenotype and degree of viral infection. This hereditary reolike virus seems to be responsible for the S character of D. simulans and was named DSV (Drosophila S virus).     

The recent spread of a vertically transmitted virus through populations of Drosophila melanogaster

The sigma virus is a vertically transmitted pathogen that commonly infects natural populations of Drosophila melanogaster. This virus is the only known host-specific pathogen of D. melanogaster, and so offers a unique opportunity to study the genetics of Drosophila-viral interactions in a natural system. To elucidate the population genetic processes that operate in sigma virus populations, we collected D. melanogaster from 10 populations across three continents. We found that the sigma virus had a prevalence of 0-15% in these populations. Compared to other RNA viruses, we found that levels of viral genetic diversity are very low across Europe and North America. Based on laboratory measurements of the viral substitution rate, we estimate that most European and North American viral isolates shared a common ancestor approximately 200 years ago. We suggest two explanations for this: the first is that D. melanogaster has recently acquired the sigma virus; the second is that a single viral type has recently swept through D. melanogaster populations. Furthermore, in contrast to Drosophila populations, we find that the sigma viral populations are highly structured. This is surprising for a vertically transmitted pathogen that has a similar migration rate to its host. We suggest that the low structure in the viral populations can be explained by the smaller effective population size of the virus.     

Geographical variation in AdhF and alcoholic resource utilization in Indian populations of Drosophila melanogaster

Indian geographical populations of Drosophila melanogaster, collected at latitude 10–32.3 ^oN, longitude 76.1–80.1 ^oE and altitude 16–2050 m, were analysed for changes in Adh ¹ frequency and utilization potential of diverse alcoholic resources. Until now, there have been limited data on climatic association of correlated changes on these aspects in continental populations of D. melanogaster. Indian populations demonstrate an increase in Adh ^F frequency by 0.04 per degree latitude and significantly higher utilization of ethanol (9.0–15.2%), acetic acid (8.1–12.5%), n-propanol (2.55-4.25%), 2-propanol (1.5-3.5%), n-butanol (2.15 3.5%) and 2-butanol (0.68-1.2%). Data also revealed significant correlation with latitude as well as altitude. Regression analysis of climatic data from collection sites confirmed the observed clinal variation of Adh ^F as well as alcoholic utilization in D. melanogaster. On the Indian subcontinent, variation in Adh ^F frequency and alcoholic resource utilization along increasing latitude as well as altitude are due to climatic selection factors such as temperature and rainfall.