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.