Summary:

Species with wide distributions may be highly differentiated across contrasting environments. While gene flow may help maintain similarities among populations, local adaptation may lead to their divergence, as well as promote reproductive isolation, further fostering evolutionary diversification. The interplay between these two processes is of major importance for Conservation, as it will determine if populations differentiate or become more similar in the long run, as a result of environmental changes such as those imposed by man. In Drosophila subobscura, populations are differentiated along a latitudinal gradient, but recent evidence indicates that northern populations are becoming more similar to southern ones. An important issue is how much populations from the extremes of the cline differ in mating preferences, as this might contribute to reduce genetic introgression when populations meet. To address this, we analyzed the evolutionary dynamics of reproductive isolation of D. subobscura populations derived from the extremes of the European cline, while adapting to a common, laboratorial environment. We show that mating performance increased during laboratory adaptation. In general northern populations had a better performance than southern ones, and this difference was sustained across generations. Moreover northern females preferred mates from their own populations while southern females preferred males from the north. The assortative mating of the northern populations was stable through time, while disassortative mating of the southern populations faded away during laboratory evolution. Overall this study suggests that reproductive barriers may slow down the genetic introgression due to migration to the north, an important finding in evolutionary and conservation terms.

Summary:

Chromosomal inversions are widespread in Drosophila with strong evidence supporting an adaptive cause for the evolution and maintenance of inversion polymorphisms. One emblematic example of the adaptive role associated with inversions is the repeatable clinal variation in inversions frequencies in different continents as happens in Drosophila subobscura. More recently, it has been found that these inversion polymorphisms are shifting as a response to global warming. Both local adaptation and gene flow may be involved, the latter possibly overcoming historical constraints. Nevertheless, it is still unclear what forces are important in shaping the evolution of inversion polymorphisms. An approach to this issue is to study the evolutionary dynamics of chromosomal inversions of populations initially differentiated along a cline, during adaptation to a novel, common environment in the absence of gene flow. We use this strategy analyzing laboratory adaptation in Drosophila subobscura founded from contrasting European latitudes. Will natural selection in the new environment overcome the initial historical differences, promoting convergence of inversion frequencies? During the first 25 generations of adaptation to a common environment we found that the polymorphism of chromosomal inversions was gradually reduced in all populations. We also found persistent differentiation between populations from contrasting latitudes, though these differences reduced with increasing generations in the new environment. While genetic drift seems to play an important role in inversion frequency changes, we also found consistent increase in frequency of specific inversions (initially in low frequencies) across replicate populations, which suggests that selection also played a role. Altogether this study indicates that, in the absence of gene flow, inversion polymorphism evolves under a balance of selection and genetic drift with historical constraints also playing an important role.

Summary:

Clinal variation for wing size and shape as well as for chromosomal inversion frequencies has been found in Drosophila subobscura from Europe as well as North and South America. Particularly it has been reported that standard arrangements increase in frequency towards higher latitudes, where flies are also bigger. In the New World the rapid evolution of body size clines as a follow up of clinal evolution of inversions, suggested that the wing traits cline had been driven by the inversion polymorphism. Previous studies, in favor of this hypothesis, found an intra-population association between wing traits (size and shape) and standard chromosomal inversions. Nevertheless, it was also found that the association (for shape) or details of it (for size) had opposite signs in one European population (Adraga) and one South American population (Puerto Montt). This is probably due to a bottleneck effect following the colonization of the Americas. Despite this interesting finding it is still unknown if this is a generalized difference between continents. To tackle this question we here tested for the consistency of the association between wing traits and inversions in three populations along the cline of Europe as well as South America. Surprisingly we found no clear association between wing size and the number of standard inversions in either continent. On the other hand, we confirmed that the previously reported negative association between wing shape and standard dose of Puerto Montt spread with latitude through other South America populations. This is not in agreement with the positive sign of the cline for both wing shape and inversions. Overall and contrary to previous indications, this study suggests that the inversion and wing clines in D. subobscura have evolved independently from each other.