Inbreeding reveals interpopulation differences in inversion polymorphism of Drosophila subobscura

The adaptive significance of inversion polymorphism of Drosophila subobscura is well established. However, little is known about gene combinations within inversions which are coadapted because of population-specific effects. We studied this aspect of Dobzhansky's coadaptation hypothesis, using the systematic inbreeding method. Differences in magnitude and quality of inversion polymorphism in two ecologically and topologically distinct habitats were compared after several generations of continuous full-sib inbreeding. Populations from the two habitats differ in frequency of homokaryotypes after third and fifth generation of inbreeding and in the levels of homozygosity of different gene arrangements. The effect of homozygosity appears population and chromosome specific.     

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.     

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 leads to extensive genetic structure: a multilocus survey in Drosophila subobscura

The adaptive character of inversion polymorphism in Drosophila subobscura is well established. The O(ST) and O(3+4) chromosomal arrangements of this species differ by two overlapping inversions that arose independently on O(3) chromosomes. Nucleotide variation in eight gene regions distributed along inversion O(3) was analyzed in 14 O(ST) and 14 O(3+4) lines. Levels of variation within arrangements were quite similar along the inversion. In addition, we detected (i) extensive genetic differentiation between arrangements in all regions, regardless of their distance to the inversion breakpoints; (ii) strong association between nucleotide variants and chromosomal arrangements; and (iii) high levels of linkage disequilibrium in intralocus and also in interlocus comparisons, extending over distances as great as approximately 4 Mb. These results are not consistent with the higher genetic exchange between chromosomal arrangements expected in the central part of an inversion from double-crossover events. Hence, double crossovers were not produced or, alternatively, recombinant chromosomes were eliminated by natural selection to maintain coadapted gene complexes. If the strong genetic differentiation detected along O(3) extends to other inversions, nucleotide variation would be highly structured not only in D. subobscura, but also in the genome of other species with a rich chromosomal polymorphism.     

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.     

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.     

Microdistribution and habitat selection in Drosophila subobscura Collin

Microdistribution and habitat selection were investigated in a population of Drosophila subobscura near Leeds, Yorkshire. Microdistribution was examined along a transect running through a deciduous woodland. Two main types of habitat were recognized, those with dry soil and a closed canopy of mature trees (dry and dark) and those with wet soil and an open canopy of mature trees (wet and light). Flies were much more frequent, judged by their attendance at baited traps, in the dry/dark habitat. Flies taken from both habitat types, marked with different coloured fluorescent dust and released at a point mid-way between the original habitats, tended to return to their area of initial capture.     

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.'     

Nucleotide polymorphism at the RpII215 gene in Drosophila subobscura. Weak selection on synonymous mutations

Nucleotide variation in an 8.1-kb fragment encompassing the RpII215 gene, which encodes the largest subunit of the RNA polymerase II complex, is analyzed in a sample of 11 chromosomes from a natural population of Drosophila subobscura. No amino acid polymorphism was detected among the 157 segregating sites. The observed numbers of preferred and unpreferred derived synonymous mutations can be explained by neutral mutational processes. In contrast, preferred mutations segregate at significantly higher frequency than unpreferred mutations, suggesting the action of natural selection. The polymorphism to divergence ratio is different for preferred and unpreferred changes, in agreement with their beneficial and deleterious effects on fitness, respectively. Preferred and unpreferred codons are nonrandomly distributed in the RpII215 gene, leading to a heterogeneous distribution of polymorphic to fixed synonymous differences across this coding region. This intragenic variation of the polymorphism/divergence ratio cannot be explained by different patterns of gene expression, mutation, or recombination rates, and therefore it indicates that selection coefficients for synonymous mutations can vary extensively across a coding region. The application of nucleotide composition stationarity tests in coding and flanking noncoding regions, assumed to behave neutrally, allows the detection of the action of natural selection when stationarity holds in the noncoding region.     

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.     

O chromosome inversion polymorphism in Northern and Atlantic Europe and its implications in the American colonization by Drosophila subobscura

The Western Mediterranean populations of D. subobscura have a very similar chromosomal inversion polymorphism to the American colonizing populations, with the exception of the O₅ inversion. This inversion, which is found in appreciable frequencies in colonizing populations and is distributed according to a significant latitudinal cline, both in North and South America, has never been reported in that Palearctic region. Therefore, an analysis of the distribution of this inversion, along a latitudinal cline, has been carried out in Europe. The O Chromosome inversion polymorphism has been studied in seven populations: Gävle and Lilla Edet (Sweden), Gesten (Denmark), Ter-Apel (Holland) and Crézy, Aizenay and Taulé (France). The O₅ inversion was only detected in three populations and in low frequencies: Gävle (3.7%), Lilla Edet (1.8 %) and Taulé (1.2 %). However, none of these three populations has all the inversions found in the American populations. The clinal distribution of some O chromosomal arrangements has also been studied according to latitude and the temporal changes of this polymorphism.     

Chromosomal inversion polymorphism in the southewestern range ofDrosophila subobscura distribution area

The level of chromosomal inversion polymorphism is much higher in three populations from South Morocco than in the nearby Canary and Madeira Islands. The composition of the chromosomal arrangements is also strikingly different in both areas. The similarity among the populations from the islands excludes drift or the founder principle as an explanation of these differences. A variety of reasons support the conclusion that the polymorphism in the islands is a relict polymorphism, corresponding to a less evolved stage than the one present in the mainland. The populations from South Morocco have a relatively low index of free recombination, like others from the southern range of the distribution ofD. subobscura. This contrasts with the high values found in the northern range. The populations from the North are relatively recently established ones, whereas those from the South are longer established and have become marginal only in relatively recent times. This difference in evolutionary history is sufficient to explain the differences in the index of free recombination.     

Long‐term changes in the chromosomal inversion polymorphism of Drosophila subobscura. II. European populations*

The chromosomal polymorphism of 13 European populations of Drosophila subobscura has been compared with that of the same populations collected 15–35 years ago. The chromosomal polymorphism of the old populations differs significantly from that of the new populations, mainly for chromosomes U and O. There is a very good agreement between the geographical space and the genetic space as shown by a graphical representation of the 26 statistical populations (13 old and 13 new) obtained by a principal coordinate analysis. This reflects both the existence of significant latitudinal clines for the frequencies of some chromosomal arrangements in the old and new samples and systematic changes that have taken place in these populations during the period that elapsed between the two surveys. An increase in the frequency of those arrangements typical of southern latitudes and a decrease for those common in northern latitudes is observed in all populations – Mediterranean, Atlantic and Central European. Furthermore, the genetic distances of the new populations to a southern population of reference have decreased in comparison with those of the old populations. These changes could be the result of climatic factors that are correlated with latitude. In particular, the assumption that global warming is responsible for all the changes observed appears rather likely. Whether these systematic changes of the chromosomal polymorphism are a consequence of local adaptations or have been produced by migration from the south remains an open question.     

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.     

Patterns of puffing activity and chromosomal polymorphism in Drosophila subobscura

The puffing patterns in polytene E chromosomes of Drosophila subobscura were followed in third-instar larvae and throughout the prepupa period. Two gene arrangements, E_st and E_1·2+9–12 were studied. A majority of puffs exhibit a similar pattern, but the puffs 61AC and 67AB behave differently in the two chromosomal arrangements, both in homozygotes and in heterozygotes. These two puffs are located at the end of the E₁₂ inversion. This position effect is an interesting phenomenon that probably is not due to a heterochromatinization effect.     

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     

Adaptive significance of amylase polymorphism in Drosophila I. The geographical pattern of allozyme polymorphism at the amylase locus in Drosophila subobscura

Allelic variation at the Amy locus was studied in eight natural populations from the central and northern range of D. subobscura, and the geographical pattern of Amy polymorphism over the range of this species was described. Even though regional and local differences in gene frequencies were found, in general the same alleles occur at high, intermediate and low frequencies, in nearly all populations. There are no significant differences in allele frequencies, but there is significant difference in the degree of heterozygosity among groups of populations from the northern, central and southern range. An analysis of population subdivision indicates that heterogeneity within populations is higher than between populations. Genetic distance values indicate that there is a variable degree of geographical differentiation between local populations. Variability within and between continental and insular populations is also discussed.     

Temporal changes of chromosomal polymorphism in natural populations of Drosophila subobscura

The behaviour of the chromosomal polymorphism of D. subobscura was analyzed in relation to temporal changes, daily, seasonal and annual. Firstly, chromosome analyses were carried out over a year in a natural population. Samples were taken at monthly intervals in Tibidabo, a locality close to Barcelona. In all the months except January, the number of individuals captured was enough to carry out a chromosome analysis of that population. The A, E and O chromosomes showed a great uniformity in the frequencies of gene arrangement over the year. However, significant changes occur in the frequencies of J and U chromosomes. The J₁ and U₁₊₂₊₈ arrangements showed a similar tendency, with two maxima, in June and February, and a minimum in September. These changes seem to be unrelated to the cyclical climatic changes. A chromosome analysis of the subsamples captured during the day, in the May sample, was done. In no case was the change in chromosome frequencies statistically significant. The behaviour of the U_st arrangement must be noted, the frequency of which decreased through the day. Finally, the data for the Tibidabo population were compared with samples captured in nearby localities over a period of 21 years. Significant differences were found in the frequencies of all the chromosomes, with the exception of J. The most differentiated population was the last sampled, from 1982. In this population the st arrangements tended to decrease in the A, E and O chromosomes and A₂, E_1+2+9+12 and O₃₊₄₊₇ increase, respectively.