Comparative phylogeography of European bees with different level of diet specialization and range size
Past climate change is known to have strongly impacted current patterns of genetic variation of animals and plants in Europe. However, ecological factors also have the potential to influence demographic history and thus patterns of genetic variation. We investigated the impact of past climate, and also the potential impact of host plant species abundance, on intraspecific genetic variation in three codistributed and related specialized solitary bees of the genus Melitta with very similar life history traits and dispersal capacities but different host plants. We sequenced five independent loci (∼3750 bp) from 506 specimens. In a second study we also characterized the phylogeographic patterns displayed by five eusocial species of Bumblebees with overlapping ranges in Eurasia, but different levels of range fragmentation, range size and food specialization. Genetic variation was explored by using sequence variation of 368 specimens at three independent loci (∼2380 bp).
Our analyses revealed that the solitary species associated with the most abundant host plant (i.e. Melitta leporina) displays unusually high genetic variation, to an extent that is seldom reported in phylogeographic studies of animals and plants. Patterns of genetic variation in the two other solitary species indicated lower overall levels of diversity. Ecological niche modelling of the three Melitta species and their main host plant species suggested a strong reduction in range size during the last glacial maximum. Comparing observed sequence data with data simulated using spatially explicit models of coalescence suggests that M. leporina recovered a range and population size close to their current levels at the end of the last glaciation, and confirms recent range expansion as the most likely scenario for M. nigricans. As expected for the Bumblebees, stronger fragmentations of the species range are associated with a stronger geographic differentiation. Furthermore, diet specialisation appears to increase population structure at the landscape level, presumably due to the more heterogeneously distributed food resources. On the other hand, no clear association was highlighted between diet specialisation or overall range size and genetic diversity. Surprisingly, the two generalist and co-distributed species investigated displayed widely divergent patterns in terms of genetic diversity and population structure.
Overall, our results are compatible with a combined impact of two interacting parameters on intraspecific genetic variation: environment disturbances (e.g. past climate changes) and features specific to the organism, such as diet specialization. They thus further highlight the challenge of dissociating both parameters in future phylogeographic studies.