New insights on the chromosomal inversion thermal adaptation in insects: American populations of Drosophila subobscura

Drosophila subobscura is an excellent model species for testing genetic variation in thermal adaptation due to its rich inversion chromosomal polymorphism. In Palearctic populations, the inversions of this species can be classified as “cold”, “warm” or “nonthermal” adapted. Based on this classification the Chromosomal Thermal Index (CTI) was developed, which allows measuring the thermal adaptation of populations and monitoring changes over time. Here, we aim to use this index in American colonizing populations of D. subobscura for the first time to obtain new information on the species thermal adaptation. Thus, thermal adapted inversions (“cold” and “warm”) were defined for the American continent and CTI was computed in South American (1981 and 1999 samples) and North American (1985 and 2004 samples) populations of D. subobscura. In general, both American populations showed an inverse relationship between CTI values and latitude, with CTI values decreasing when latitude increases. When comparing populations sampled in different years, an increase in CTI values was detected in four out of six temporal comparisons (only one was significant) of South America (1981 and 1999) and in six out of seven populations of North America (1985 and 2004). A global analysis using a one-way repeated measures anova of CTI values in both American hemispheres showed a trend of increase for “warm” adapted inversions in Chile and North America, but this increase was only significant for the latter. Overall, these results are in agreement with global warming expectations, although natural selection acted differently in the colonized hemispheres.