Summary:

The study of ecological niche evolution is fundamental to understand how the environment influences species distributions and the role it plays in adaptation to divergent environments and species formation. Drosophila americana is widely distributed in North America. It comprises two chromosomal forms distributed in a north-south cline (chromosomes X/4 fused and non-fused, respectively), which is maintained by selection. Here we present a study of the ecological niche evolution and thermal adaptation of D. americana and the two chromosomal forms with the aim of understanding how they respond to a variety of environmental conditions. For this, we have used Ecological Niche Modelling, phylogeographic analysis and performance (locomotor and developmental time) experiments. Temperature is the environmental factor that contributes most to the ecological niches, although the relevance of precipitation is also high in the model of the Southern populations. Thermal performance experiments show no difference in the locomotor activity across a temperature range of 15º to 38ºC between flies from the north and the south of its distribution. Finally, we have modelled the past distribution of the species; its range during the last glacial maximum (LGM) was reduced to the southernmost of North America, while they had a similar distribution during the last interglacial (LIG) as currently. Analysis of the demographic history, nevertheless, detects no bottlenecks during the LGM. These results suggest that D. americana displays a plastic adaptation to the different temperatures of their distribution and do not support niche conservatism in Drosophila.

Summary:

Developmental time (DT) is a variable and complex trait that is of great relevance to all organisms. Nevertheless, in Drosophila, the specific genes contributing to DT variation in natural populations remain largely unknown. Here, we make the first attempt to characterize the molecular basis of DT variation in Drosophila americana, a species of the virilis group of Drosophila that is distantly related to D. melanogaster. The comparison of divergent species can shed light on the generality of the molecular basis of DT differences. For this purpose, we developed a set of 43 indel markers equally spaced along the D. americana genome and conducted two F2 association experiments. Associations are found between markers in all chromosomes and DT but two regions stand out, namely, a region in the middle of Muller’s element C, and the Frost (Fst) gene region on Muller’s element E. Variation at the Frost gene explains 25.3 % of the variation in DT in a sample of unrelated individuals from the same population. Fst is known to be involved in the Drosophila cold response, but there is a growing body of evidence suggesting that it is also a developmental gene.