Individual inversions or their combinations: which is the main selective target in a natural population of Drosophila subobscura?

It is generally accepted that chromosomal inversions have been key elements in adaptation and speciation processes. In this context, Drosophila subobscura has been, and still is, an excellent model species due to its rich chromosomal polymorphism. In this species, many analyses from natural populations have demonstrated the adaptive potential of individual inversions (and their overlapped combinations, the so-called arrangements). However, little information is available on the evolutionary role of combinations generated by inversions located in homologous and nonhomologous chromosomes. The aim of this research was to ascertain whether these combinations are also a target for natural selection. For this objective, we have studied the inversion composition of homologous and nonhomologous chromosomes from a D. subobscura sample collected in a well-studied population, Mount Avala (Serbia). No significant deviation from H-W expectations was detected, and when comparing particular karyotypic combinations, likelihood ratios close to 1 were obtained. Thus, it seems that for each pair of homologous chromosomes inversions no deviation from randomness was detected. Finally, no linkage disequilibrium was observed between inversions located in different chromosomes of the karyotype. For all these reasons, it can be assumed that, at the cytological level, the individual inversions rather than their combinations in different chromosomes are the main target of selection.

Inversion polymorphism in two Serbian natural populations of Drosophila subobscura: analysis of long-term changes

To study whether inversions (or arrangements) by themselves or karyotypes are the main global warming adaptive target of natural selection, two Drosophila subobscura Serbian populations (Apatin and Petnica) were re-analyzed using different statistical approaches. Both populations were sampled in an approximately 15 years period: Apatin in 1994 and 2008 + 2009 and Petnica in 1995 and 2010. For all chromosomes, the four collections studied were in Hardy-Weinberg equilibrium. Thus, it seems that inversions (or arrangements) combined at random to constitute populations' karyotypes. However, there were differences in karyotypic frequencies along the years, although they were significant only for Apatin population. It is possible to conclude that inversions (or arrangements) are likely the target of natural selection, because they presented long-term changes, but combine at random to generate the corresponding karyotypic combinations. As a consequence, the frequencies of karyotypes also change along time.

O-chromosome lethal frequencies in Serbian and Montenegrin Drosophila subobscura populations

Lethal chromosomal frequencies were obtained from three Drosophila subobscura samples from the Mt. Avala (Serbia) population in September 2003 (0.218), June 2004 (0.204) and September 2004 (0.250). These values and those from other Balkan populations studied previously (Petnica, Kamariste, Zanjic and Djerdap) were used to analyze the possible effect of population, year, month and altitude above sea level on lethal chromosomal frequencies. According to ANOVAS no effect were observed. Furthermore, the lethal frequencies of the Balkan populations did not vary according to latitude. This is probably due to the relative proximity and high gene flow between these populations. From a joint study of all the Palearctic D. subobscura populations so far analyzed, it can be deduced that the Balkan populations are located in the central area of the species distribution. Finally, it seems that lethal chromosomal frequencies are a consequence of the genetic structure of the populations.

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.

Genetic load and coadaptation of chromosomal inversions. II. O-chromosomes in Drosophila subobscura populations

We have analysed the inversion polymorphism and genetic load of O-chromosomes in three populations of D. subobscura from southeastern Europe. As expected for a central populations the inversion polymorphism was extensive. In a like fashion, the genetic load, in particular the frequency of lethals, was heavy in all three populations. There were significant differences in the frequency of moderately deleterious genes. These differences in viability can be attributed to balancing selection. A comparison of these two kinds of genetic polymorphism indicates that there are differences in mean viability among different gene arrangements of O-chromosomes in the three populations. The differences observed are due to an unequal distribution of various viability classes among O-chromosome gene arrangements. We here show for the first time a specific distribution of lethal genes among these arrangements within the Palearctic distribution area of D. subobscura. The lethal allelism test showed lethals are non-randomly associated with the Ost gene arrangement. The amount of genetic load is heavy in gene arrangements with a high frequency, in comparison with the ones with a low frequency. Lethal genes may be protected in combinations of low and moderate frequency gene arrangements that harbor more lethal genes, as the Ost in the one population. Some arrangements that are less protected against recombination have a higher load than ones that are more protected against recombination. This can be taken as evidence for coadaptation.