Relationship between chromosomal polymorphism and wing size in a natural population of Drosophila subobscura

Chromosomal polymorphism and wing size (as a measure of body size) were analysed simultaneously in two samples of Drosophila subobscura from Barcelona, Spain. The very rich chromosomal polymorphism of this species makes it difficult to detect the relationship of this polymorphism with any phenotypical character. However, a positive significant regression of wing size on the percentage of the autosome length with standard arrangement was found. Furthermore, for each polymorphic chromosome, except for the J chromosome, an association between the most frequent arrangements and wing size was observed. This trend, which was the same in the two samples, was that expected according to the latitudinal clines of both characters.     

Rapid evolution of wing size clines in Drosophila subobscura

Parallel latitudinal clines across species and continents provide dramatic evidence of the efficacy of natural selection, however little is known about the dynamics involved in cline formation. For example, several drosophilids and other ectotherms increase in body and wing size at higher latitudes. Here we compare evolution in an ancestral European and a recently introduced (North America) cline in wing size and shape in Drosophila subobscura. We show that clinal variation in wing size, spanning more than 15 degrees of latitude, has evolved in less than two decades. In females from Europe and North America, the clines are statistically indistinguishable however the cline for North American males is significantly shallower than that for European males. We document that while overall patterns of wing size are similar on two continents, the European cline is obtained largely through changing the proximal portion of the wing, whereas the North American cline is largely in the distal portion. We use data from sites collected in 1986/1988 (Pegueroles et al. 1995) and our 1997 collections to compare synchronic (divergence between contemporary populations that share a common ancestor) and allochronic (changes over time within a population) estimates of the rates of evolution. We find that, for these populations, allochronically estimated evolutionary rates within a single population are over 0.02 haldanes (2800 darwins), a value similar in magnitude to the synchronic estimates from the extremes of the cline. This paper represents an expanded analysis of data partially presented in Huey et al. (2000).     

Patterns of puffing activity and chromosomal polymorphism in Drosophila subobscura

Summary The effect of inbreeding on the puffing patterns of polytene chromosomes of Drosophila subobscura was analysed. Puffing activity was studied in two strains of D. subobscura: one which had been subject to inbreeding for 288 generations, and in the hybrids from a cross between them. A strong overall decrease in puffing activity was found in the inbred line. In general, hybrids behaved in a similar way to the inbred line or showed activity intermediate between the two lines. The fertility and viability of the two homozygous lines and of the hybrids were also determined. These parameters of fitness are very low in the inbred line. Hybrids displayed intermediate behaviour.Publication No. 109, Departamento de Genética, Universidad de Valencia, Spain     

Assortative Mating and Fertility in two Drosophila subobscura Strains with different Mitochondrial DNA Haplotypes

The mating pattern and female fertility on the two main mitochondrial DNA haplotypes (I and II) of Drosophila subobscura were studied, in an attempt to find possible differences between them in relation to sexual selection or isolation that could explain the populational dynamics and the co-existence of these two strains in nature. The mating pattern indicated an assortative mating in population cages, where couples of the same haplotype, mainly those of haplotype I, mated more often. However, the significations detected in laboratory conditions disappeared in wild populations, where random mating was the rule. The female fertility also showed differences in the laboratory compared to the wild, since couples with haplotype I males were more efficient in the laboratory populations. These results, together with others that we previously obtained, either point to selection acting directly on the mtDNA or to the presence of some kind of cytonuclear co-adaptation in these two haplotypes, although this must be modulated by other factors that change with the seasons and time. The end result could well be a balance of opposite forces acting on both haplotypes.     

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock

Summary Survival time following a high temperature shock of Drosophila subobscura adults in dry air has shown great variability. This experimental condition involved desiccation as the first cause of death. Here survival is studied under saturation humidity, so that the mortality may be imputed only to thermic stress. We analyze the influence of culture temperature and crowding on resistance for different sex and age of the adults. The results show strong influences of these environmental factors on heat shock resistance and show interactions with the age and sex of the adults. We suggest that these facts could be due to acclimatization and/or to adaptation. The acclimatization would occur during development and would affect physiological processes related to aging of the flies. The adaptation would take place for selection, acting through differential mortality before the heat shock. Of course, other processes could be significant. Whatever the causal explanation, it will be necessary in any future research related with heat shock resistance to take these factors into account.     

Mitochondrial DNA effects on fitness in Drosophila subobscura

We tested different fitness components on a series of conspecific mtDNA haplotypes, detected by RFLPs in Drosophila subobscura. Additionally, haplotype VIII, endemic to the Canary Islands, was tested upon its own native nuclear DNA background and upon that of the rest of mtDNAs tested herein. We found that both nuclear and mitochondrial DNA can have a significant effect upon their hosts' fitness, and that negative selection is one of the mechanisms that can intervene in this species' mtDNA haplotype pattern. We discuss the importance of this mechanism in relation to genetic drift, in the form of periodic population bottlenecks, and how the latter can enhance the former. We also detected a significant positive effect of haplotype VIII upon fitness that could explain in part the dominance of this endemic haplotype on some of the Canary Islands, and a mitochondrial heterosis involving this haplotype when on a foreign nuclear DNA background.     

MDH-polymorphism in Drosophila subobscura: I. Selection and hitch-hiking in laboratory populations

Summary The interrelation of genic polymorphism at the Malate dehydrogenase (Mdh)-locus and chromosomal polymorphism for superimposed gene arrangements was studied in 5 experimental populations of Drosophila subobscura. It could be shown that chromosomal polymorphism is maintained by balancing selection in favour of the heterozygotes. In contrast, selection at the Mdh-locus itself seems to be of minor importance. The changes of frequency observed for the Mdh-alleles are most probably due to hitch-hiking on the gene blocks enclosed by the chromosomal structures or to selection for tightly linked genes rather than to fitness differences between Mdh-genotypes. The results may be considered as a model for the situation found in natural populations of D. subobscura.     

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock

Two replicate selection experiments to increase and decrease heat shock resistance of Drosophila subobscura adults were carried out maintaining control lines. In the present paper, the chromosomal-inversion and enzymatic polymorphism variation with selection is analyzed. The results indicate an erratic variation of chromosomal arrangement frequencies for practically all the chromosomes in the selected lines, showing a loss of the less frequent arrangements especially in sensitive lines. Only the A chromosome and the O+4 arrangement show a behaviour that may not be due to random effects, which points to the possible existence of heat shock factor(s) in these chromosomes. Similarly, an erratic variation of allele frequencies is observed for all the enzymes studied (Aph, Pept-1) except for the Hk-1 enzyme. We cannot establish the possible participation of this locus in heat shock resistance from the results obtained up to now. A significant decrease in heterozygosity is detected in sensitive lines from chromosomal-inversion polymorphism.     

Latitudinal differences in sex chromosome inversions,sex linked allozymes,and mitochondrial DNA variation in Drosophila subobscura

Mitochondrial DNA, sex linked allozymes, and chromosome A gene arrangement data, from eight European natural populations of Drosophila subobscura, were analyzed to determine the existence of latitudinal clines. Strong north-south correlations with latitude were found for gene arrangements and for the Hbdh and 6Pgdh allozymes. Gametic associations between the A₂ gene arrangement, the Hbdh ⁹⁶ and the 6Pgdh ⁹⁶ alleles, point out some kind of epistatic interaction. At mtDNA level, the Hae III, A variant did not show a previously found north-south clinal variation.     

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.     

Multiple mating and sperm displacement in a natural population of Drosophila melanogaster

Summary Mother-offspring data for alcohol dehydrogenase genotypes of a vineyard cellar population of D. melanogaster are best explained by a model that allows 21% of females in the population to mate twice with an 83% level of sperm displacement. A population model with multiple mating and sperm displacement is examined theoretically. A formula for the effective population size is derived under this model. Multiple mating increases the effective population size relative to single mating.     

Non-destructive species identification of Drosophila obscura and D. subobscura (Diptera) using near-infrared spectroscopy

The vinegar flies Drosophila subobscura and D. obscura frequently serve as study organisms for evolutionary biology. Their high morphological similarity renders traditional species determination difficult, especially when living specimens for setting up laboratory populations need to be identified. Here we test the usefulness of cuticular chemical profiles collected via the non-invasive method near-infrared spectroscopy for discriminating live individuals of the two species. We find a classification success for wild-caught specimens of 85%. The species specificity of the chemical profiles persists in laboratory offspring (87–92% success). Thus, we conclude that the cuticular chemistry is genetically determined, despite changes in the cuticular fingerprints, which we interpret as due to laboratory adaptation, genetic drift and/or diet changes. However, because of these changes, laboratory-reared specimens should not be used to predict the species-membership of wild-caught individuals, and vice versa. Finally, we demonstrate that by applying an appropriate cut-off value for interpreting the prediction values, the classification success can be immensely improved (to up to 99%), albeit at the cost of excluding a considerable portion of specimens from identification.     

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     

Genetic variation in the spread of Drosophila subobscura from a nonequilibrium population

Abstract.— Drosophila subobscura was first identified in North America in the early 1980s, and a newer D. subobscura population in Utah appears to have been established more than 10 years later. In this study, we use nuclear microsatellite allele frequencies, mitochondrial restriction fragment length polymorphism (RFLP) allele frequencies, and computer simulations to investigate possible scenarios of how this species has spread across North America. Our method develops a 95% confidence interval for the maximum and minimum number of founders that could have colonized the new population given various scenarios for spread. Unlike many other methods, it may be applied to nonequilibrium source populations given certain conditions. We find that observed allele frequency differences between newer and older D. subobscura populations are consistent with very few inseminated females being transported east from the coast in a single step or with larger numbers of colonizers invading after several intermediate steps. They are not consistent with a large, panmictic population of D. subobscura colonizing Utah in a single step.     

Effect of rate of inbreeding on inbreeding depression in Drosophila melanogaster

Summary This experiment was designed to study the relationship between rate of inbreeding and observed inbreeding depression of larval viability, adult fecundity and cold shock mortality in Drosophila melanogaster. Rates of inbreeding used were full-sib mating and closed lines of N=4 and N=20. Eight generations of mating in the N=20 lines, three generations in the N=4 lines and one generation of full-sib mating were synchronised to simultaneously produce individuals with an expected level of inbreeding coefficient (F) of approximately 0.25. Inbreeding depression for the three traits was significant at F=0.25. N=20 lines showed significantly less inbreeding depression than full-sib mated lines for larval viability at approximately the same level of F. A similar trend was observed for fecundity. No effect of rate of inbreeding depression was found for cold shock mortality, but this trait was measured with less precision than the other two. Natural selection acting on loci influencing larval viability and fecundity during the process of inbreeding could explain these results. Selection is expected to be more effective with slow rates of inbreeding because there are more generations and greater opportunity for selection to act before F=0.25 is reached. Selection intensities seem to have been different in the three traits measured. Selection was most intense for larval viability, less intense for fecundity and, perhaps, negligible at loci influencing cold shock mortality.     

Genetic and environmental factors in the resistance of Drosophila subobscura adults to high temperature shock

Summary We have carried out two equivalent selection experiments to increase and decrease heat shock resistance of Drosophila subobscura adults, using an indirect selection method that avoids excessive consanguinity. Heat shock was 33±0.5 °C at saturation humidity. Control lines showed a rapid change of the physiological trait as a consequence of laboratory culture conditions, expressed as a decrease both in heat shock resistance and in the initial population variability for heat shock resistance. Thus, this reduction of variability seems to consist in the loss of those combinations of genes that confer high resistance to heat shock. After eight generations of selection, the selected lines were differentiated from their respective control lines, and the selection response obtained was similar in resistant and sensitive lines. Differences in survival of progeny of reciprocal crosses between selected lines suggest that inheritance of heat resistance may depend in part on the origin of egg cytoplasm.     

Sex‐specific effects of inbreeding in wild‐caught Drosophila melanogaster under benign and stressful conditions

In animal populations, sib mating is often the primary source of inbreeding depression (ID). We used recently wild‐caught Drosophila melanogaster to test whether such ID is amplified by environmental stress and, in males, by sexual selection. We also investigated whether increased ID because of stress (increased larval competition) persisted beyond the stressed stage and whether the effects of stress and sexual selection interacted. Sib mating resulted in substantial cumulative fitness losses (egg to adult reproduction) of 50% (benign) and 73% (stressed). Stress increased ID during the larval period (23% vs. 63%), but not during post‐stress reproductive stages (36% vs. 31%), indicating larval stress may have purged some adult genetic load (although ID was uncorrelated across stages). Sexual selection exacerbated inbreeding depression, with inbred male offspring suffering a higher reproductive cost than females, independent of stress (57% vs. 14% benign, 49% vs. 11% stress).     

Quantitative genetics of speciation: additive and non-additive genetic differentiation between <Emphasis Type="Italic">Drosophila madeirensis</Emphasis> and <Emphasis Type="Italic">Drosophila subobscura</Emphasis>

The role of dominance and epistasis in population divergence has been an issue of much debate ever since the neoDarwinian synthesis. One of the best ways to dissect the several genetic components affecting the genetic architecture of populations is line cross analysis. Here we present a study comparing generation means of several life history-traits in two closely related Drosophila species: Drosophila subobscura, D. madeirensis as well as their F ₁ and F ₂ hybrids. This study aims to determine the relative contributions of additive and non-additive genetic parameters to the differentiation of life-history traits between these two species. The results indicate that both negative dominance and epistatic effects are very important in the differentiation of most traits. We end with considerations about the relevance of these findings for the understanding of the role of non-additive effects in speciation.     

Open ground individuals and population structure in Drosophila subobscura Collin

The distribution of Drosophila subobscura adults within, and their movement through, open ground habitats has been investigated, along with the status of such individuals within populations as a whole. Movement occurs between open ground and woodland, to some extent as a result of passive transport by air currents, but flies can remain in open ground for at least several days at a time. There are no detectable differences in either the reproductive age-structure or the body-size distributions of flies collected from woodland and open ground habitats, although a significantly higher proportion of females are found in open ground than in woodland. The large number of gravid females in open ground provide the potential for breeding there, but circumstantial evidence suggests that this is unlikely to occur. Rather, their presence appears to reflect the foraging strategy adopted by females in response to their high nutritional requirements. The flux of flies through open ground between woodlands suggests that previous estimates of the genetically-important parameter, effective population size, have been seriously underestimated for natural D. subobscura populations.     

Evolutionary pace of chromosomal polymorphism in colonizing populations of Drosophila subobscura: an evolutionary time series

Abstract. Biologists have long debated the speed, uniformity, and predictability of evolutionary change. However, evaluating such patterns on a geographic scale requires time-series data on replicate sets of natural populations. Drosophila subobscura has proven an ideal model system for such studies. This fly is broadly distributed in the Old World, but was introduced into both North and South America just over two decades ago and then spread rapidly. Rapid, uniform, and predictable evolution would be demonstrated if the invading flies evolved latitudinal clines that progressively converged on those of the native populations. Evolutionary geneticists quickly capitalized on this opportunity to monitor evolutionary dynamics. Just a few years after the introduction, they surveyed chromosomal inversion frequencies in both North and South America. On both continents they detected incipient latitudinal clines in chromosome inversion frequencies that almost always had the same sign with latitude as in the Old World. Thus the initial evolution of chromosomal polymorphisms on a continental scale was remarkably rapid and consistent. Here we report newer samples of inversion frequencies for the colonizing populations: the time series now spans almost one decade for North America and almost two decades for South America. Almost all inversions in the New World continue to show the same sign of frequency with latitude as in the Old World. Nevertheless, inversion clines have not consistently increased in steepness over time; nor have they consistently continued to converge on the Old World baseline. However, five arrangements in South America show directional, continentwide shifts in frequency. Overall, the initial consistency of clinal evolutionary trajectories seen in the first surveys seems not to have been maintained.