Monitoring of the genetic structure of natural populations: change of the effective population size and inversion polymorphism in <Emphasis Type="Italic">Drosophila subobscura</Emphasis>

We analyzed changes in the genetic structure and effective population size of two ecologically distinct populations of Drosophila subobscura over several years. Population sizes of D. subobscura in beech and oak wood habitats for a period of 6 years were estimated by the capture-mark-release-recapture method. Inversion polymorphism parameters were also assessed in the same populations for a period of 3 years. Significant differences in the numbers of individuals were observed between sexes. This affected the effective population sizes between particular years. The ratio of the effective size over the cenzus dropped significantly in beech wood in 2 years. Although overall heterozygosity remained unchanged during the years in both habitats, frequencies of gene arrangements on five chromosomes show variability. After the bottleneck, some complex chromosomal arrangements appeared for the first time in both populations. Standard gene arrangements of chromosome A increased in frequency over the years in each habitat, while the complex arrangements remain rather stable and specific for each population. The results obtained indicate that the population structure may significantly change if the effective size of D. subobscura population is reduced, which is mostly related to microclimatic changes in habitats. Based on the results to date, monitoring of microevolutionary changes by using D. subobscura and its relatives seems a promising way to study the effects of global climate changes.     

Chromosomal inversion polymorphism of Drosophila subobscura from south‐eastern part of Europe

Five Drosophila subobscura populations from south‐eastern Europe were investigated with respect to chromosomal inversion polymorphism. They belong to central European populations and their gene arrangement frequencies are in accordance with latitudinal variation observed in this species. The new complex gene arrangement A₂₊₈₊₉, found in one of the populations analysed, may be specific for the southern Balkan refugium. A general tendency of gene arrangement frequencies to exhibit some southern characteristics was found for some of the autosomal standard arrangements.     

Chromosomal inversion polymorphism of Drosophila subobscura populations from Jastrebac Mountain shows temporal and habitat-related changes

Inversion polymorphism in populations of D. subobscura from a beech forest on Jastrebac mountain was studied in June 1990, 1993, and 1994, respectively. The same analysis was performed in 1990 for D. subobscura populations in a beech forest and an oak forest in the same region. Statistically significant differences in the frequencies of the gene arrangements of A₁, J and U chromosome were observed during the period of investigation. A tendency towards a decrease in the frequency of the standard gene arrangements was found for all chromosomes, but was particularly evident with chromosomes A and J. The frequency of the gene arrangements A₁ A₂, J₁ and U_1–2+6 increased at the same time. Differences in the frequency of the gene arrangements of A, J and U chromosomes were also observed when the populations from two ecologically different habitats (beech and oak forest) were compared in 1990.     

Studies of chromosome structure polymorphism in Asia Minor and Persian populations of Drosophila subobscura Coll

Summary By means of the male sample method samples of D. subobscura from the north and south coast of Asia Minor and from the south coast of the Caspian Sea were examined with regard to the gene arrangements contained in them. The samples were judged by the following three criteria: (1) the number of different gene arrangements present, (2) the frequencies in which these gene arrangements occur, and (3) the rate of zygotic combinations.The samples from Asia Minor were found to be highly polymorphic. In this respect, they are comparable with other samples from the northern border of the Mediterranean region.The Persian populations are poor as regards number of gene arrangements. Commonly, some gene arrangements are highly frequent and, accordingly, high rates of homozygous combinations dominate. Presumably, these are marginal populations.Comparisons between samples from the north and south coast of Asia Minor confirm the findings from other parts of the species' range. As a rule, the frequencies of various gene arrangements vary according to the latitude in which they occur. Since climate and vegetation are also largely correlated with latitude, it is assumed that this is due to adaptive reaction to ecological factors.Comparing populations from the Western and Eastern Mediterranean region, similar differences were recorded. Some gene arrangements seem to be confined to either the western or eastern part of the area, and other gene arrangements differ in frequency. Since the ecological conditions in these areas appear to be comparable, such differences might be due to the history of the formation and dispersal of gene arrangements.In general, the frequencies of the gene arrangements in the chromosomes A, U, E and O are most equal along the northern margin of the Mediterranean area and, correspondingly, the frequencies of the heterokaryotypes are highest. It may be assumed that many heterokaryotypes reach their highest degree of heterotic supremacy in this area, and/or that the different homokaryotypes are uniform with respect to adaptive values. The reasoning rests on the additional assumption that in this area the biological conditions are the least extreme, or, in other words, that D. subobscura is best adapted to this environment. As it is, the region appears to lie geographically near the center of the species' range. In the I-chromosome the most equal frequencies of gene arrangements along with the highest frequency of the heterokaryotypes are to be found in Central Europe.On the other hand, in various populations lying supposedly at the borders of the species' range, the frequencies of gene arrangements of at least some chromosomes are strongly unequal and, accordingly, the frequencies of the homokaryotypes are high.In populations where the polymorphism is high, the character of the polymorphism varies slightly from chromosome to chromosome. In the O-chromosome usually one gene arrangement is abundant, while a large number of others are rare; two gene arrangements tend to dominate in the I- and U-chromosome, whereas in the E-chromosome several gene arrangements are apt to share similar frequencies.     

Monitoring chromosomal polymorphism in Drosophila subobscura over 40 years

The inversion chromosomal polymorphism of Drosophila subobscura is considered to be adaptive as a result of its responses at different time scales to temperature changes. This work reports the longest‐term study of chromosomal polymorphism for a single population of D. subobscura with climatic data from the collecting site itself. The chromosomal analysis of D. subobscura samples collected six times over a 40‐year period at the same location and in the same seasonal interval has revealed the continuous presence of 16 common and six moderately rare chromosomal arrangements through the period. This analysis also corroborates the previously detected negative relationship between the frequencies of the standard (cold‐climate) arrangement on each of its five chromosomes and temperature, as well as between a comprehensive measure of cold adaptation (the total autosomal proportion of standard arrangement) and temperature. These and previous results would support that species harboring cold‐ and warm‐adapted polymorphic chromosomal arrangements, like D. subobscura, can rapidly respond to environmental changes.     

Geographic pattern of allozyme and inversion polymorphism on chromosome O of Drosophila subobscura and its evolutionary origin

The chromosome O of Drosophila subobscura was studied with respect to genetic variability at three enzyme loci (Odh, Me, and Lap-4) and with respect to inversion polymorphism. Population samples were taken from seven localities along a north-south gradient from Sweden and Scotland to Tunisia.The chromosomal analysis revealed clinal frequency changes for gene arrangements from north to south. With the enzyme loci Odh and Me allele frequencies are similar throughout the distribution range. Both loci are located outside the common inversion complex O₃₊₄. On the other hand, frequency changes parallel to those of the gene arrangements were observed for the alleles of the Lap-locus. Nonrandom associations between Lap-alleles and the superimposed gene arrangements O_ST, O₃₊₄, O₃₊₄₊₈, and O₃₊₄₊₂₃ were found. These gene arrangements differ from each other with respect to allele frequencies at the Lap-locus but for a given gene arrangement the relative frequencies of Lap-alleles remain relatively constant along the north-south gradient. Thus allele frequencies at the Lap-locus can be predicted from inversion frequencies.These observations can be interpreted in such a way that the pattern of allozyme variation within gene arrangements is due to founder effects caused by the unique origin of inversions. The gene blocks in the different inversions seem to represent more or less separated gene pools. In polymorphic populations the coexistence of genetically differentiated inversions presumably gives rise to heterotic interaction.     

THE COLONIZATION OF DROSOPHILA SUBOBSCURA IN CHILE. II. CLINES IN THE CHROMOSOMAL ARRANGEMENTS

Drosophila subobscura is a Palearctic species that was first detected in the New World in Puerto Montt (Chile) in February 1978. Since that time, it has spread over a broad area and increased in population density. The South American populations exhibit a high level of chromosomal polymorphism: 20 different arrangements exist, distributed among five chromosomes. Chromosomal arrangement heterozygosity varies from 0.55 to 0.61 in the nine populations examined. Incipient clines in the frequencies of the arrangements are appearing; these clines follow the same latitudinal direction as in the Old World. Wing length significantly decreases with latitude, as it does in Europe. The colonization of South America by D. subobscura appears to be a major natural experiment with outcomes that duplicate the distributional patterns—in chromosomal polymorphism and in wing length—observed in the Old World, thereby strongly supporting the adaptive significance of these patterns. The data show a very rapid effect of natural selection promoting genetic differentiation among natural populations.     

Population structure of Drosophila subobscura: Non-random microdispersion of inversion polymorphism on a mountain slope

On a forested mountain slope in the Swiss Alps gene arrangements of D. subobscura were sampled from altitudes between 600 and 1 900 m above sea level during the years 1977 and 1979. In both years the standard arrangement of chromosome I increased in frequency with altitude. Judging from the pattern of gene arrangement frequencies the population was spatially structured on a fine scale within altitude and on a coarser scale between altitudes. There were temporal changes between years. In spite of the fact that D. subobscura is at its ecological margin at the timberline no evidence of reduced variability was found there.     

Genomics of natural populations: Evolutionary forces that establish and maintain gene arrangements in Drosophila pseudoobscura

The evolution of complex traits in heterogeneous environments may shape the order of genes within chromosomes. Drosophila pseudoobscura has a rich gene arrangement polymorphism that allows one to test evolutionary genetic hypotheses about how chromosomal inversions are established in populations. D. pseudoobscura has >30 gene arrangements on a single chromosome that were generated through a series of overlapping inversion mutations with >10 inversions with appreciable frequencies and wide geographic distributions. This study analyses the genomic sequences of 54 strains of Drosophila pseudoobscura that carry one of six different chromosomal arrangements to test whether (i) genetic drift, (ii) hitchhiking with an adaptive allele, (iii) direct effects of inversions to create gene disruptions caused by breakpoints, or (iv) indirect effects of inversions in limiting the formation of recombinant gametes are responsible for the establishment of new gene arrangements. We found that the inversion events do not disrupt the structure of protein coding genes at the breakpoints. Population genetic analyses of 2,669 protein coding genes identified 277 outlier loci harbouring elevated frequencies of arrangement‐specific derived alleles. Significant linkage disequilibrium occurs among distant loci interspersed between regions with low levels of association indicating that distant allelic combinations are held together despite shared polymorphism among arrangements. Outlier genes showing evidence of genetic differentiation between arrangements are enriched for sensory perception and detoxification genes. The data presented here support the indirect effect of inversion hypothesis where chromosomal inversions are favoured because they maintain linked associations among multilocus allelic combinations among different arrangements.     

Molecular population genetics of the OBP83 genomic region in Drosophila subobscura and D. guanche: contrasting the effects of natural selection and gene arrangement expansion in the patterns of nucleotide variation

Chromosomal inversion polymorphism play a major role in the evolutionary dynamics of populations and species because of their effects on the patterns of genetic variability in the genomic regions within inversions. Though there is compelling evidence for the adaptive character of chromosomal polymorphisms, the mechanisms responsible for their maintenance in natural populations is not fully understood. For this type of analysis, Drosophila subobscura is a good model species as it has a rich and extensively studied chromosomal inversion polymorphism system. Here, we examine the patterns of DNA variation in two natural populations segregating for chromosomal arrangements that differentially affect the surveyed genomic region; in particular, we analyse both nucleotide substitutions and insertion/deletion variations in the genomic region encompassing the odorant-binding protein genes Obp83a and Obp83b (Obp83 region). We show that the two main gene arrangements are genetically differentiated, but are consistent with a monophyletic origin of inversions. Nevertheless, these arrangements interchange some genetic information, likely by gene conversion. We also find that the frequency spectrum-based tests indicate that the pattern of nucleotide variation is not at equilibrium; this feature probably reflects the rapid increase in the frequency of the new gene arrangement promoted by positive selection (that is an adaptive change). Furthermore, a comparative analysis of polymorphism and divergence patterns reveals a relaxation of the functional constraints at the Obp83b gene, which might be associated with particular ecological or demographic features of the Canary island endemic species D. guanche     

Selection in heterogeneous environments maintains the gene arrangement polymorphism of Drosophila pseudoobscura

Chromosomal rearrangements may play an important role in how populations adapt to a local environment. The gene arrangement polymorphism on the third chromosome of Drosophila pseudoobscura is a model system to help determine the role that inversions play in the evolution of this species. The gene arrangements are the likely target of strong selection because they form classical clines across diverse geographic habitats, they cycle in frequency over seasons, and they form stable equilibria in population cages. A numerical approach was developed to estimate the fitness sets for 15 gene arrangement karyotypes in six niches based on a model of selection-migration balance. Gene arrangement frequencies in the six different niches were able to reach a stable meta-population equilibrium that matched the observed gene arrangement frequencies when recursions used the estimated fitnesses with a variety of initial inversion frequencies. These analyses show that a complex pattern of selection is operating in the six niches to maintain the D. pseudoobscura gene arrangement polymorphism. Models of local adaptation predict that the new inversion mutations were able to invade populations because they held combinations of two to 13 local adaptation loci together.     

Chromosomal polymorphism in natural populations of Drosophila subobscura near Zürich,Switzerland: a contribution to long-term comparisons

The chromosomal inversion polymorphism in natural populations of D. subobscura was investigated near Zürich, Seitzerland, in 1986 and 1987. These data are compared with earlier data from 1963, 1964 and 1984, collected in the same region. In all five acrocentric chromosomes, significant differences in the proportions of the dominant gene arrangements occurred; in the chromosomes A, I, O and U the standard order decreased from the sixties to the eighties, whereas the arrangements I₁, O₃₊₄, O₃₊₄₊₈ and U₁₊₂ became more frequent.     

The effect of different concentrations of lead on inversion polymorphism in Drosophila subobscura

Drosophila subobscura is a wild Drosophila species that is spread over almost all of Europe. It possesses an uniquely rich inversion polymorphism on all five long chromosomes. This polymorphism is to a certain degree associated with the variation and dynamics of ecological factors in space and time. We analyzed the changes of inversion polymorphism components of Drosophila subobscura flies maintained on media with different concentrations of lead in laboratory conditions. The effects of lead on inversion polymorphism were observed by cytological analysis of gene arrangements on all of the five acrocentric chromosomes, as well as by cytological analysis of karyotypes on all of the four autosomes. The frequencies of particular gene arrangements on the four autosomes changed significantly in the samples maintained on medium not supplemented with lead. The frequencies of some gene arrangements on all of the five acrocentric chromosomes changed significantly in the flies maintained on media supplemented with lead. The length of exposure to different lead concentrations results in a significant change in the frequency of a few gene arrangements on two autosomes. However, the results show that different concentrations of lead, as well as the length of exposure, do not affect major parameters of inversion polymorphism. The results suggest that some gene arrangements could be linked with adaptive processes in evolving heavy metal resistance.     

Global warming and chromosomal inversion adaptation in isolated islands: Drosophila subobscura populations from Madeira

Global warming is an environmental phenomenon to which species must adapt to survive. Drosophila subobscura presents an adaptive capacity due to its chromosomal inversion polymorphism. Until now, the impact of global warming on this polymorphism has been studied in D. subobscura populations located either on a continental mainland or on islands not far from a continent. In this context, gene flow could be a relevant mechanism allowing the movement of thermally adapted inversions between populations. Our aim was to sample and study the chromosomal polymorphism on Madeira, a small isolated island in the Atlantic Ocean. We compared our findings with those reported in the same location approximately four and five decades ago. Moreover, we studied whether global warming has occurred on this island by analyzing mean, maximum and minimum temperatures over a 55‐year period. All atmospheric parameters have increased significantly, consistent with climate change expectations. Frequencies and chromosomal thermal index values of thermal adapted inversions remained quite stable over years. Furthermore, J, U and O chromosomes are almost fixed for “warm” adapted inversions. Thus, if there is little genetic variability remaining and temperatures continue increasing, island populations of D. subobscura might be on the threshold of endangerment. However, apart from selection, genetic drift and inbreeding, other processes, such as phenotypic plasticity or thermoregulatory behavior, could be involved in the survival of the species’ populations. Finally, although in danger, D. subobscura is a generalist that lives in humanized environments, and this fact could favor its persistence on Madeira Island.     

PATTERNS OF PUFFING ACTIVITY AND CHROMOSOMAL POLYMORPHISM IN DROSOPHILA SUBOBSCURA. IV. EFFECT OF INVERSIONS ON GENE EXPRESSION

We have observed that, contrary to a common assumption, the puffing patterns manifest in the salivary chromosomes of Drosophila subobscura are modified by chromosomal inversions as well as by genic content. An inversion effect is apparent in the E and A chromosomes of five strains coming from four different natural populations. An effect due to the geographical location of the populations is also detected in the J and O chromosomes. The chromosomal and geographic effects are distinguishable but not contradictory. Indeed, a statistical test using the DK² coefficient of distance shows that, for a given chromosomal arrangement, strains of different geographic origin exhibit puffing patterns significantly different; these patterns are, however, more similar to each other than they are to those of strains carrying different chromosomal arrangements of the same chromosome.     

New gene arrangements in natural populations of Drosophila subobscura

Four new gene arrangements were detected in natural populations of Drosophila subobscura near Zürich, Switzerland. Three of them are the result of inversions not recorded before, and one is a new combination of known gene arrangements. Another arrangement, I₃₊₄, was recorded for the first time in Switzerland. It may have been imported or possibly may have arisen de novo. It is suggested that inversion mutations are not so infrequent as so far supposed, if the presence of mobile elements is taken into account in D. subobscura.     

The genetics of Drosophila subobscura populations. 3. Inversion polymorphism and climatic factors

Summary The natural population of Drosophila subobscura of Mt. Parnes has been followed for two successive years and the frequency of the gene arrangements determined. Some changes occured in the first year but do not explain the existence of the geographical clines for these frequencies. Within the second year no changes were observed. For chromosomes J and U there is evidence for a greater fitness of the heterozygotes.Experiments on temperature effect on viability of adult male flies, on their temporal sterility, on the viability of flies during all their biological cycle and experiments on the effect of dryness on the viability of adult male flies did not show an important change of gene arrangement frequencies in the resistant individuals. Nor did diapausing females have very different gene arrangements, except for chromosome O. Cage experiments with temperature as a selective agent did not elucidate the action of this factor. It is concluded that temperature or dryness do not affect very much the polymorphic system, nor explain the existence of these clines.This polymorphic system is much more stable in D. subobscura than in D. pseudoobscura, its nearctic relative. It is also richer. It is concluded that it is historically older and helps the populations to cope with many environmental changes.     

The evolutionary history of Drosophila buzzatti. XXVI. Macrogeographic patterns of inversion polymorphism in New World populations

Inversion polymorphisms in the second and fourth chromosomes of the cactophilic Drosophila buzzatti in the native distribution range of the species are described. Over 5,000 flies from 26 localities were scored revealing interesting geographic structuring of arrangement frequencies. Multiple regression and partial correlation approaches showed that the frequencies of second and fourth chromosome arrangements vary clinically along latitudinal and altitudinal gradients and to a lesser extent with longitude. Although many non selective explanations can account for this pattern, its resemblance to the clinal pattern described in recently established Australian populations of Drosophila buzzatii, strongly suggests a selective explanation. Additionally, the correlated variation observed between the frequencies of arrangements 2St on the second chromosome and 4St on the fourth suggests a pattern of interchromosomal association, which, when considering the vast area surveyed, might be explained as the result of epistatic interactions. The analysis of population structure revealed a significant regional pattern, concordant with previously described phytogeographic regions. F-statistics showed that the patterns of variation were different not only between the second and fourth chromosomes, but also between second chromosome arrangements, suggesting that selective differentiation might have contributed to population structure. Since D. buzzatii breeds and feeds on the decaying tissues of diverse cactus species present in different phytogeographic regions, it is difficult to distinguish the underlying causes of the geographic patterns observed. However, inversion heterozygosity is not correlated with the diversity of potential cactus hosts. The evidence presented suggests that differential selection may be the main cause for the population structure. It is also possible to conclude that the inversion polymorphism of D. buzzatti is flexible rather than rigid.     

Seasonal changes in chromosomal inversion polymorphism in a <Emphasis Type="Italic">Drosophila subobscura</Emphasis> natural population from a southeastern European continental refugium of the last glaciation period

Chromosomal inversion polymorphism was studied in a natural population of Drosophila subobscura from the Djerdap gorge of the Danube River, which is one of southeastern European continental refugia during the last glaciation period. The existence of changes in chromosomal inversion polymorphism during different periods of time was tested. The observed high abundance of chromosomal arrangements and lower IFR value allowed this population to be classified with central European populations. It was characterized by the O₇ and O_{3 + 4 + 5} arrangements, not found in other populations from the region analyzed. Within the region studied, the E_{1 + 2 + 9 + 12} chromosomal arrangement was found, apart from this population from a warm habitat, only in one population from a Mediterranean refugium. Many similarities have been observed between the population from the Djerdap gorge and other populations from the region analyzed, which indicates that postglacial migration may have started from this southeastern European refugium. Seasonal changes in the frequencies of the E_st and O_{3 + 4 + 7} chromosomal arrangements, E_st and O_{3 + 4 + 7}, being more frequent in spring and in summer, respectively, were found. The changes in the J chromosome (J_st/J_st), E chromosome (E_st/E_{1 + 2 + 9} and E₈/E_{1 + 2 + 9}), and O chromosome (O_st/O_{3 + 4 + 22}; O_{3 + 4}/O_{3 + 4}; and O_{3 + 4 + 1}/O_{3 + 4 + 7}), whose karyotypic combination frequencies were associated with seasonal changes, were also found. These changes are most probably associated with seasonal climatic changes in the population analyzed, which confirms that the chromosomal inversions in D. subobscura are adaptive. The text was submitted by the author in English.     

The genetics ofDrosophila subobscura populations

Summary The inversion polymorphism in a population ofDrosophila subobscura from Holland is examined. The high frequency of the Standard gene arrangement in all five chromosomes, the high IFR value, and the low mean number of inversions show a low polymorphism expected in submarginal northern populations within the area of the geographic distribution of the species. The study of this population further confirms the existence of North-South clines for inversion types in every chromosome of this species.