Genetic exchange versus genetic differentiation in a medium-sized inversion of Drosophila: the A2/Ast arrangements of Drosophila subobscura

Chromosomal inversion polymorphism affects nucleotide variation at loci associated with inversions. In Drosophila subobscura, a species with a rich chromosomal inversion polymorphism and the largest recombinational map so far reported in the Drosophila genus, extensive genetic structure of nucleotide variation was detected in the segment affected by the O(3) inversion, a moderately sized inversion at Muller's element E. Indeed, a strong genetic differentiation all over O(3) and no evidence of a higher genetic exchange in the center of the inversion than at breakpoints were detected. In order to ascertain, whether other polymorphic and differently sized inversions of D. subobscura also exhibited a strong genetic structure, nucleotide variation in 5 gene regions (P236, P275, P150, Sxl, and P125) located along the A(2) inversion was analyzed in A(st) and A(2) chromosomes of D. subobscura. A(2) is a medium-sized inversion at Muller's element A and forms a single inversion loop in heterokaryotypes. The lower level of variation in A(2) relative to A(st) and the significant excess of low-frequency variants at polymorphic sites indicate that nucleotide variation at A(2) is not at mutation-drift equilibrium. The closest region to an inversion breakpoint, P236, exhibits the highest level of genetic differentiation (F(ST)) and of linkage disequilibrium (LD) between arrangements and variants at nucleotide polymorphic sites. The remaining 4 regions show a higher level of genetic exchange between A(2) and A(st) chromosomes than P236, as revealed by F(ST) and LD estimates. However, significant genetic differentiation between the A(st) and A(2) arrangements was detected not only at P236 but also in the other 4 regions separated from the nearest breakpoint by 1.2-2.9 Mb. Therefore, the extent of genetic exchange between arrangements has not been high enough to homogenize nucleotide variation in the center of the A(2) inversion. A(2) can be considered a typical successful inversion of D. subobscura according to its relative length. Chromosomal inversion polymorphism of D. subobscura might thus cause the genome of this species to be highly structured and to harbor different gene pools that might contribute to maintain adaptations to particular environments.     

Molecular population genetics of the rp49 gene region in different chromosomal inversions of Drosophila subobscura.

Nucleotide variation at the ribosomal protein 49 (rp49) gene region has been studied in 75 lines of Drosophila subobscura belonging to four chromosomal arrangements (Ost, O3+4, O3+4+8, and O3+4+23). The location of the rp49 gene region within the inversion loop differs among heterokaryotypes: it is very close to one of the breakpoints in heterozygotes involving Ost chromosomes, while it is in a more central position in all other heterokaryotypes. The distribution of nucleotide polymorphism in the different arrangements is consistent with a monophyletic origin of the inversions. The data also provide evidence that gene conversion and possibly double crossover are involved in shuffling nucleotide variation among gene arrangements. The analyses reveal that the level of genetic exchange is higher when the region is located in a more central position of the inverted fragment than when it is close to the breakpoints. The pairwise difference distributions as well as the negative values of Tajima''s and Fu and Li''s statistics further support the hypothesis that nucleotide variation within chromosomal arrangements still reflects expansion after the origin of the inversions. Under the expansion model, we have estimated the time of origin of the studied inversions.     

Nucleotide variation at the yellow gene region is not reduced in Drosophila subobscura: a study in relation to chromosomal polymorphism

In contrast to Drosophila melanogaster and Drosophila simulans, the yellow (y) gene region of Drosophila subobscura is not located in a region with a strong reduction in recombination. In addition, this gene maps very close to the breakpoints of different inversions that segregate as polymorphic in natural populations of D. subobscura. Therefore, levels of variation at the y gene region in this species relative to those found in D. melanogaster and D. simulans may be affected not only by the change in the recombinational environment, but also by the presence of inversion polymorphism. To further investigate these aspects, an approximately 5.4-kb region of the A (=X) chromosome including the y gene was sequenced in 25 lines of D. subobscura and in the closely related species Drosophila madeirensis and Drosophila guanche. The D. subobscura lines studied differed in their A-chromosomal arrangements, A(st), A(2), and A(1). Unlike in D. melanogaster and D. simulans, levels of variation at the y gene region of D. subobscura are not reduced relative to those found at other genomic regions in the same species (rp49, Acp70A, and Acph-1). This result supports the effect of the change in the recombinational environment of a particular gene on the level of neutral variation. In addition, nucleotide variation is affected by chromosomal polymorphism. A strong genetic differentiation is detected between the A(1) arrangement and either A(st) or A(2), but not between A(st) and A(2). This result is consistent with the location of the y gene relative to the breakpoints of inversions A(1) and A(2). In addition, the pattern of nucleotide polymorphism in A(st)+A(2) and A(1) seems to point out that variation at the y gene region within these chromosomal classes is in the phase transient to equilibrium. The estimated ages of these arrangements assuming a star genealogy indicate that their origin cannot predate the D. madeirensis split. Therefore, the present results are consistent with a chromosomal phylogeny where Am(1), which is an arrangement present in D. madeirensis but absent in current populations of D. subobscura, would be the ancestral arrangement.     

Evidence of Extensive Genetic Exchange in the Rp49 Region among Polymorphic Chromosome Inversions in Drosophila Subobscura

Restriction map variation in 107 lines extracted from two natural populations of Drosophila subobscura was investigated with seven four-nucleotide-recognizing enzymes in a 1.6-kb region including the rp49 gene, that is located very close to the proximal breakpoint of inversion O3. Fourteen restriction site and 8 length polymorphisms, resulting in 73 haplotypes, have been identified. Estimated heterozygosity per nucleotide, pi = 0.0045, is comparable to the average nucleotide variation observed in Drosophila melanogaster. Because of the location of the rp49 region in D. subobscura, variation in three different gene arrangements-Ost, O3 + 4 and O3 + 4 + 8-has been compared. Out of 14 restriction site polymorphisms, 3 are shared by Ost, O3 + 4 and O3 + 4 + 8, and 3 additional ones are shared by Ost and O3 + 4, evidencing extensive genetic exchange among these polymorphic inversions. In agreement with previous data, the higher level of variation of O3 + 4 (as measured by haplotype diversity and nucleotide variation) suggests that O3 + 4 may be ancestral in relationship to extant gene arrangements.     

Changes in chromosomal polymorphism and global warming: The case of Drosophila subobscura from Apatin (Serbia)

In this study, chromosomal inversion polymorphism data for a natural population of Drosophila subobscura from a swampy region near the town of Apatin (Serbia) were compared with data for the same population collected approximately 15 years earlier. The pattern of chromosomal inversion polymorphism changed over time. There were significant increases in the frequency of characteristic southern latitude ("warm" adapted) chromosomal arrangements and significant decreases in the frequency of characteristic northern latitude ("cold" adapted) chromosomal arrangements in the O and U chromosomes. The chromosomal arrangements O(3+4) and O(3+4) (+) (22) (derived from the O(3+4) arrangement) showed significant increases in 2008 and 2009 with regard to the 1994 sample. There was also a significant increase (～50%) in the U(1) (+) (2) arrangement, while U(1+8) (+) (2) (a typical southern arrangement) was detected for the first time. Since the Apatin swampy population of D. subobscura has existed for a long time in a stable habitat with high humidity that has not been changed by man our results indicate that natural selection has produced chromosomal changes in response to the increase in temperature that has occurred in the Balkan Peninsula of central southeastern European.     

Genetic loads and coadaptivity of chromosomal inversions I. Inversion polymorphism and genetic load on chromosome O in a Drosophila subobscura population from Petnica

Inversion polymorphism on chromosome O and polymorphism for the viability of determining genes have been studied in a natural population of Drosophila subobscura from Petnica (Serbia). The range of inversion polymorphism and the abundance of particular gene arrangements in the study population agree with a general pattern of inversion polymorphism of D. subobscura in Europe. The data obtained on the amount of genetic loads show that the D. subobscura population from Petnica displays a moderate degree of that polymorphism, compared to the other studied populations of these species. Therefore, the D. subobscura population from Petnica could be tentatively classified as an ecologically central population. Examination association of chromosomal, thus, inversion polymorphism with gene polymorphism, in the form of genetic loads show that differences exist in the mean viability among certain gene arrangements. The distribution of deleterious genes among chromosome O gene arrangements were non-random.     

STRUCTURE AND POPULATION GENETICS OF THE BREAKPOINTS OF A POLYMORPHIC INVERSION IN DROSOPHILA SUBOBSCURA

Drosophila subobscura is a paleartic species of the obscura group with a rich chromosomal polymorphism. To further our understanding on the origin of inversions and on how they regain variation, we have identified and sequenced the two breakpoints of a polymorphic inversion of D. subobscura—inversion 3 of the O chromosome—in a population sample. The breakpoints could be identified as two rather short fragments (～300 bp and 60 bp long) with no similarity to any known transposable element family or repetitive sequence. The presence of the ～300‐bp fragment at the two breakpoints of inverted chromosomes implies its duplication, an indication of the inversion origin via staggered double‐strand breaks. Present results and previous findings support that the mode of origin of inversions is neither related to the inversion age nor species‐group specific. The breakpoint regions do not consistently exhibit the lower level of variation within and stronger genetic differentiation between arrangements than more internal regions that would be expected, even in moderately small inversions, if gene conversion were greatly restricted at inversion breakpoints. Comparison of the proximal breakpoint region in species of the obscura group shows that this breakpoint lies in a small high‐turnover fragment within a long collinear region (～300 kb).     

Seasonal changes of chromosomal inversion polymorphism in a Drosophila subobscura natural population from southeastern European continental refugium of the last glaciation period

Chromosomal inversion polymorphism was studied in a natural population of Drosophila subobscura from Djerdap gorge of Danube river, that represents one of southeastern European continental refugiums during last glaciation period. The existence for changes of chromosomal inversion polymorphism during different periods of time was tested. Observed very high abundance of chromosomal arrangements and lower IFR value grouped this population to central European populations. It was characterized with O7 and O(3+4+5) arrangements, not found in other populations from the region analyzed. Except in this population from warmer habitat, observed E(1+2+9+12) chromosomal arrangement was found, in the region analyzed, only in one population from Mediterranean refugial area. Many similarities have been observed between population from Djerdap gorge and other populations from the region analyzed, indicating that process of post-glacial migration may have started from this southeastern European refugium area. Seasonal changes in the frequencies for Est and O(3+4+7) chromosomal arrangements where Est was more frequent during the spring period and O(3+4+7) during the summer period were found. The changes for J-chromosome (Jst/Jst), E-chromosome (Est/E(1+2+9); E(8)/E(1+2+9) and O-chromosome (Ost/O(3+4+22); O(3+4)/O(3+4); O(3+4+1)/O(3+4+7) whose karyotypic combination frequencies were associated with seasonal changes were also found. These changes most probably represent associations with seasonal climatic changes in the population analyzed, supporting adaptivity of chromosomal inversions in D. subobscura.     

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.     

Excess of nonsynonymous polymorphism at Acph-1 in different gene arrangements of Drosophila subobscura

Nucleotide variation in the Acph-1 gene region was analyzed in a natural population of Drosophila subobscura from Bizerte (Tunisia). The lines studied differed in their gene arrangement for segment I of the O chromosome: 21 lines were O3+4+8, 21 were O3+4+23, and 3 were O3+4. According to chromosomal phylogenies, O3+4 is a central arrangement from which O3+4+8 and O3+4+23 originated. Strong genetic differentiation at Acph-1 was detected among the different arrangements, which is reflected in strong linkage disequilibrium between the variants at informative polymorphic sites and the type of arrangement. Estimates of silent nucleotide diversity are slightly lower within O3+4+23 (pisilent = 0.0166) than within O3+4+8 (pisilent = 0.0228) or O3+4 (pisilent = 0.0234). In contrast, nonsynonymous nucleotide diversity estimates (around 0.1%) are similar in the three arrangements. Most nonsynonymous rare variants are singletons, which results in highly significant Tajima's neutrality tests within the young O3+4+8 and O3+4+23 arrangements. This test is not significant for nonsynonymous mutations within a large Spanish O3+4 sample. In addition, a significant and marginally significant excess of nonsynonymous polymorphism was detected by the McDonald and Kreitman test within O3+4+23 and O3+4+8, respectively. This excess results in a rather high neutrality index (NI = 5.25) when both arrangements are jointly analyzed, in contrast to its value within the old O3+4 arrangement (NI = 1.74). The pattern of variation at Acph-1 within the young arrangements is unusual for nuclear genes and has the same characteristics previously detected in most genes of the mitochondrial genome. Assuming that most nonsynonymous mutations at Acph-1 are under weak negative selection, a smaller effective size of the young arrangements relative to O3+4 might explain the observed results. The relatively low frequency of O3+4+8 and O3+4+23 in the distribution area of D. subobscura, the more recent origin of these arrangements relative to O3+4 and the suppression of recombination in heterokaryotypes might contribute to the relatively small effective size of the young arrangements. Therefore, present results indicate that the differences in effective size and recombination caused by chromosomal arrangements are modulating nonsynonymous variation at Acph-1.     

CLINAL PATTERNS OF CHROMOSOMAL INVERSION POLYMORPHISMS IN DROSOPHILA SUBOBSCURA ARE PARTLY ASSOCIATED WITH THERMAL PREFERENCES AND HEAT STRESS RESISTANCE

Latitudinal clines in the frequency of various chromosomal inversions are well documented in Drosophila subobscura . Because these clines are roughly parallel on three continents, they have undoubtedly evolved by natural selection. Here, we address whether individuals carrying different chromosomal arrangements also vary in their thermal preferences ( T _p) and heat stress tolerance ( T _ko). Our results show that although T _p and T _ko were uncorrelated, flies carrying "cold-adapted" gene arrangements tended to choose lower temperatures in the laboratory or had a lower heat stress tolerance, in line with what could be expected from the natural patterns. Different chromosomes were mainly responsible for the underlying genetic variation in both traits, which explains why they are linearly independent. Assuming T _p corresponds closely with temperatures that maximize fitness our results are consistent with previous laboratory natural selection experiments showing that thermal optimum diverged among thermal lines, and that chromosomes correlated with T _p differences responded to selection as predicted here. Also consistent with data from the regular tracking of the inversion polymorphism since the colonization of the Americas by D. subobscura , we tentatively conclude that selection on tolerance to thermal extremes is more important in the evolution and dynamics of clinal patterns than the relatively "minor" adjustments from behavioral thermoregulation.     

The evolutionary history of Drosophila buzzatii. XXXV. Inversion polymorphism and nucleotide variability in different regions of the second chromosome

Inversions are portions of a chromosome where the gene order is reversed relative to a standard reference orientation. Because of reduced levels of recombination in heterokaryotypes, inversions have a potentially important effect on patterns of nucleotide variability in those genomic regions close to, or included in, the inverted fragments. Here we report sequence variation at three anonymous regions (STSs) located at different positions in relation to second-chromosome inversion breakpoints in 29 isochromosomal lines derived from an Argentinean population of Drosophila buzzatii. In agreement with previous findings in Drosophila, gene flux (crossing over and/or gene conversion) between arrangements seems to appreciably increase as we approach the middle sections of inversion 2j, and patterns of nucleotide variability within, as well as genetic differentiation between chromosome arrangements, are comparable to those observed at the molecular marker outside the inverted fragments. On the other hand, nucleotide diversity near the proximal breakpoint of inversion 2j is reduced when contrasted with that found at the other regions, particularly in the case of derived inverted chromosomes. Using the data from the breakpoint, we estimate that the inversion polymorphism is approximately 1.63 N generations old, where N is the effective population size. An excess of low-frequency segregating polymorphisms is detected; mostly in the ancestral 2st arrangement and probably indicating a population expansion that predates the coalescent time of inversion 2j. Heterogeneity in mutation rates between the markers linked to the inversions may be sufficient to explain the different levels of nucleotide diversity observed. When considered in the context of other studies on patterns of variation relative to physical distance to inversion breakpoints, our data appear to be consistent with the conclusion that inversions are unlikely to be "long-lived" balanced polymorphisms.     

A long-term study on seasonal changes of gametic disequilibrium between allozymes and inversions in Drosophila subobscura

Seasonal variation (spring, early summer, late summer, and autumn) of gametic disequilibrium between gene arrangements (OST and O3+4) of the O chromosome and Lap, Pept-1, and Acph allozyme loci, located inside these inversions, has been recorded in a natural population of Drosophila subobscura during seven years over a 15-year period. The length of the study allowed us to investigate the temporal variation of the allozyme-inversion associations by statistical methods of time series analysis. Cyclic seasonal changes of allozyme-inversion associations for both Lap and Pept-1 are detected in the natural population. In both cases, the patterns of seasonal change are due to the seasonal change of frequency of Lap and Pept-1 allozymes occurring exclusively within the OST gene arrangement. In contrast, the allozyme frequencies at these loci within the O3+4 gene arrangement are stable along seasons. The patterns of temporal variation of allozyme-inversion associations for Lap and Pept-1 in the natural population are contrasted with those previously published that correspond to gene arrangements of the O chromosome and nucleotide polymorphism at the rp49 region located inside these inversions, suggesting that natural selection is operating on these allozyme-inversion associations.     

Contrasting Histories of Three Gene Regions Associated with In(3l)payne of Drosophila Melanogaster

In the present report, we studied nucleotide variation in three gene regions of Drosophila melanogaster, spanning >5 kb and showing different degrees of association with the cosmopolitan inversion In(3-L)Payne. The analysis of sequence variation in the regions surrounding the breakpoints and the heat shock 83 (Hsp83) gene locus, located close to the distal breakpoint, revealed the absence of shared polymorphisms and the presence of a number of fixed differences between arrangements, indicating absence of genetic exchange. In contrast, for the esterase-6 gene region, located in the center of the inversion, we observed the presence of shared polymorphisms between arrangements suggesting genetic exchange. In the regions close to the breakpoints, the common St arrangement is 10 times more polymorphic than inverted chromosomes. We propose that the lack of recombination between arrangements in these regions coupled with genetic hitchhiking is the best explanation for the low heterozygosity observed in inverted lines. Using the data for the breakpoints, we estimate that this inversion polymorphism is around 0.36 million yr old. Although it is widely accepted that inversions are examples of balanced polymorphisms, none of the current neutrality tests including our Monte Carlo simulations showed significant departure from neutral expectations.     

Tracking changes in chromosomal arrangements and their genetic content during adaptation

There is considerable evidence for an adaptive role of inversions, but how their genetic content evolves and affects the subsequent evolution of chromosomal polymorphism remains controversial. Here, we track how life‐history traits, chromosomal arrangements and 22 microsatellites, within and outside inversions, change in three replicated populations of Drosophila subobscura for 30 generations of laboratory evolution since founding from the wild. The dynamics of fitness‐related traits indicated adaptation to the new environment concomitant with directional evolution of chromosomal polymorphism. Evidence of selective changes in frequency of inversions was obtained for seven of 23 chromosomal arrangements, corroborating a role for inversions in adaptation. The evolution of linkage disequilibrium between some microsatellites and chromosomes suggested that adaptive changes in arrangements involved changes in their genetic content. Several microsatellite alleles increased in frequency more than expected by drift in targeted inversions in all replicate populations. In particular, there were signs of selection in the O₃₊₄ arrangement favouring a combination of alleles in two loci linked to the inversion and changing along with it, although the lack of linkage disequilibrium between these loci precludes epistatic selection. Seven other alleles increased in frequency within inversions more than expected by drift, but were not in linkage disequilibrium with them. Possibly these alleles were hitchhiking along with alleles under selection that were not specific to those inversions. Overall, the selection detected on the genetic content of inversions, despite limited coverage of the genome, suggests that genetic changes within inversions play an important role in adaptation.     

Colonization of America by Drosophila subobscura: association between Odh gene haplotypes, lethal genes and chromosomal arrangements

The colonization of America by Drosophila subobscura has been a unique exper iment in nature that has allowed us to explore the effects of evolution on a continental scale. To analyze this evolutionary event, nucleotide sequences of the Odh (Octanol dehydrogenase) gene were obtained for 43 lethal chromosomal lines from colonizing populations of North America and 5 from South America, in addition to 5 chromosomal lines from Europe with different viabilities and 2 from laboratory marker stocks. Since 10 different Odh haplotypes were found in America, the minimum number of colonizers would be 5 (or 3 mated females). Only one Odh haplotype was found in American O(5) inversions confirming that only one copy of this inversion was included among the sample of colonizers. The same Odh haplotypes were detected in association with the same chromosomal arrangements and with identical lethal genes in both North and South America indicating that exactly the same chromosome types reached both hemispheres. These observations indicate that the two continental colonizations are not independent. They are derived from the same colonization event. The population from which the colonization started should contain the O(5) inversion, a non-negligible frequency of the O(3+4+7) arrangement and all other arrangements found in America. So far the only populations that fulfill all these requirements are those from Greece, indicating that these populations can be considered good candidates as a starting point for an in depth analysis of the origin of the American colonization by D. subobscura.     

Long-term changes in the chromosomal inversion polymorphism of Drosophila subobscura. I. Mediterranean populations from southwestern Europe

The chromosomal polymorphism of seven Mediterranean populations of Drosophila subobscura has been compared with that of the same populations collected 26 to 35 years ago. Significant latitudinal clines for the frequencies of A(ST), E(ST), O(ST). and U(ST) chromosomal arrangements have been detected in the old and new samples. Standard gene arrangements are frequent in the north and decrease in frequency towards the south. Significant negative regression coefficients between latitude and transformed frequency have also been observed for the more frequent nonstandard gene arrangements. The pattern of the clines is practically the same in the old and new collections. Furthermore, the frequencies of gene arrangements of all chromosomes have changed significantly during this period in a systematic way: an increase in the frequency of those arrangements typical of southern latitudes and a decrease for those more common in northern latitudes is observed in all populations. These changes could be due to climatic factors that are correlated with latitude, making the chromosomal composition of this species more "southern.'     

Seasonal cycles of allozyme-by-chromosomal-inversion gametic disequilibrium in Drosophila subobscura

Allozyme loci are frequently found non randomly associated to the chromosomal inversions in which they are included in Drosophila. Two opposite views compete to explain strong allozyme-by-inversion gametic disequilibria: they result from natural selection or, conversely, merely represent remnants of associations accidentally established at the origin of inversions. Empirical efforts aimed at deciding between adaptive and historical scenarios have focused on the spatial distribution of disequilibria. Yet, the evolutionary significance of these associations remains uncertain. I report here the results of a time-series analysis of the seasonal variation of alleles at six allozyme loci (Acph, Lap, Pept-1, Ao, Mpi, and Xdh) in connection with the O chromosomal polymorphisms of D. subobscura. The findings were: (1) in the segment I of the O chromosome, Lap and Pept-1 allozymes changed seasonally in a cyclical fashion within the ST gene arrangement, but they changed erratically within the 3 + 4 gene configuration; (2) the frequencies of Lap1.11 and Pept-1(0.40) within ST dropped to their lowest values in early and late summer, respectively, when the seasonal level of the ST arrangement is lowest. Furthermore, Lap1.11 and Pept-1(0.40) covary with ST only within these seasons, yet in a fashion inconsistent with these alleles having a major influence on the dynamics of the inversion; (3) seasonal cycling of alleles within inversions were not detected at Acph, Ao, Mpi, and Xdh, yet these loci are nearly monomorphic at the study population, and/or their sampled series were shorter than those for Lap and Pept-1; and (4) simply monitoring allozyme frequencies separately for each inversion proved to be superior, for evidencing the seasonal cycles of the disequilibria, to the use of the D' coefficient of association. Observed seasonal cycles of allozymes within inversions likely reflect natural selection.     

The Genetic Content of Chromosomal Inversions across a Wide Latitudinal Gradient

There is increasing evidence regarding the role of chromosomal inversions in relevant biological processes such as local adaptation and speciation. A classic example of the adaptive role of chromosomal polymorphisms is given by the clines of inversion frequencies in Drosophila subobscura, repeatable across continents. Nevertheless, not much is known about the molecular variation associated with these polymorphisms. We characterized the genetic content of ca. 600 individuals from nine European populations following a latitudinal gradient by analysing 19 microsatellite loci from two autosomes (J and U) and the sex chromosome (A), taking into account their chromosomal inversions. Our results clearly demonstrate the molecular genetic uniformity within a given chromosomal inversion across a large latitudinal gradient, particularly from Groningen (Netherlands) in the north to Málaga (Spain) in the south, experiencing highly diverse environmental conditions. This low genetic differentiation within the same gene arrangement across the nine European populations is consistent with the local adaptation hypothesis for th evolutionof chromosomal polymorphisms. We also show the effective role of chromosomal inversions in maintaining different genetic pools within these inverted genomic regions even in the presence of high gene flow. Inversions represent thus an important barrier to gene flux and can help maintain specific allelic combinations with positive effects on fitness. Consistent patterns of microsatellite allele-inversion linkage disequilibrium particularly in loci within inversions were also observed. Finally, we identified areas within inversions presenting clinal variation that might be under selection.     

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