Goran Zivanovic

Seasonal changes in chromosomal inversion polymorphism in a Drosophila subobscura 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 O3 + 4 + 5 arrangements, not found in other populations from the region analyzed. Within the region studied, the E1 + 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 Est and O3 + 4 + 7 chromosomal arrangements, Est and O3 + 4 + 7, being more frequent in spring and in summer, respectively, were found. The changes in the J chromosome (Jst/Jst), E chromosome (Est/E1 + 2 + 9 and E8/E1 + 2 + 9), and O chromosome (Ost/O3 + 4 + 22; O3 + 4/O3 + 4; and O3 + 4 + 1/O3 + 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.

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 O3+4 and O3+4+22 (derived from the O3+4 arrangement) showed significant increases in 2008 and 2009 with regard to the 1994 sample. There was also a significant increase (∼50%) in the U1+2 arrangement, while U1+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.

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 colleptions 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.

Inbreeding and thermal adaptation in Drosophila subobscura

Using a well-adapted Drosophila subobscura population (Avala, Serbia), a drastic experiment of inbreeding was carried out to assess whether the expected level of homozygosity could be reached or if other evolutionary forces affected the process. In general, no significant changes of inversion (or arrangement) frequencies were detected after 12 brother-sister mating generations. Furthermore, no significant differences were obtained between observed and expected (under the inbreeding model) karyotypic frequencies. Thus, these results seemed to indicate that the main evolutionary factor in the experiment was inbreeding. However, in the G12 generation, complete chromosomal fixation was reached only in two out of the eight final inbred lines. In these lines, the chromosomal compositions were difficult to interpret, but they could be likely a consequence of adaptation to particular laboratory conditions (constant 18 °C, food, light period, etc.). Finally, in a second experiment, the inbred lines presented higher fertility at 18 °C than at 13 °C. Also, there was a significant line effect on fertility: inbred line number 6 (A1, J1, U1+2; U1+2+6, E8, and O3+4+7) presented the highest values, which maybe the result of an adaptation to laboratory conditions. Thus, the results obtained in our experiments reflect the adaptive potential of D. subobscura inversions.

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.

Medium-term changes in Drosophila subobscura chromosomal inversion polymorphism: a possible relation with global warming?

Drosophila subobscura is a species with a rich chromosomal polymorphism for inversions. Evidence demonstrates that it is adaptive. In the present research, we studied whether it is possible to detect changes in the inversion chromosomal polymorphism of D. subobscura in a medium-term period of time. The Serbian population of Avala was selected and its inversion composition in 2004 and 2011 (a seven year period) was compared. Significant variation was found in the U chromosome. This result was due to a significant increase of U1+2 (warm) and a decrease of Ust (cold) and U1+2+6. Further, minimum, maximum and mean temperatures increased (although not significantly). Thus, U chromosome seems to be able to react in a medium-term to temperature changes in the way expected by the global warming.

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.