Long-term changes in the inversion chromosomal polymorphism: Drosophila subobscura population from Rasht (north of Iran)

Drosophila subobscura is characterized by a rich chromosomal polymorphism for inversions. Many inversions are adaptive to global warming and can be classified as ‘warm’ or ‘cold’ adapted. However, most studies were carried out from European populations located in the central area of the species distribution or from American colonizing populations. For this reason, we aimed to analyse the isolated and marginal Rasht population, located in the Hyrcanian forests area (Iran). The chromosomal polymorphism for inversions was compared with the previous Rasht samples (Rasht I and II) obtained 57 years ago. This polymorphism has changed based on the inversion composition and frequencies. Interestingly, the polymorphism for inversions was scarce and similar to that of Madeira, an isolated Atlantic island. Likely, this similarity is a consequence of the marginal location and isolation of the Rasht population. Also, the chromosomal thermal index (CTI) was 0.445, showing a significant increase over those from Rasht I (0.184) and II (0.210). All these observations were in agreement with the global warming expectations. Moreover, the CTI was also computed for Russian Caucasus and Turkish populations collected more than 40 years ago to better understand the adaptive potential of D. subobscura and to study the similarity between populations of different geographic areas. In summary, the inversions of D. subobscura also changed in marginal and isolated populations in agreement with the global warming expectations, and an open question is to know where is the threshold for this evolutionary change.