Genome evolution of a bee-associated bacterium

The use of large-scale comparative genomics allows us to explore the genetic diversity and mechanisms of evolution of related organisms. This thesis has focused on the application of such approaches to study Lactobacillus kunkeei, a bacterial inhabitant of the honeybee gut. We produced 102 novel complete genomes from L. kunkeei, which were used in four large comparative studies. In the first study, 41 bacterial strains were isolated from the crop of honeybees whose populations were geographically isolated. Their genome sequences revealed differences in gene contents, including the mobilome, which were mostly phylogroup-specific. However, differences in strain diversity and co-occurrence between both locations were observed. In the second study, we obtained 61 bacterial isolates from neighboring hives at different timepoints during the summer. We observed that strain diversity seemed hive-specific and relatively constant in time. Surprisingly, the observed mobilome also showed hive specificity and was maintained through the summer. The novel genome data were combined with previously published genomes, allowing us to perform deep comparative analyses on the evolution of the species using a total of 126 genomes. We determined that, despite the large number of sequenced genomes, L. kunkeei has an open pangenome. Besides, we evaluated the effects of recombination on the species core genome, and concluded that it mainly evolves through mutation events. In the last study we described the mechanisms of evolution of a cluster of 5 giant genes (about 90 kb long in total) that are unique to L. kunkeei and the closest sister species. Their patterns of evolution do not reflect those of the species core genome. We concluded that they originated by duplication events, and have diverged by accumulation of both mutations and recombination events. We predicted a potential interaction between the proteins encoded by two of them, and we hypothesized a role in host-specific interaction for another protein. In conclusion, these studies have provided novel and cohesive knowledge on the composition and dynamics of different populations of L. kunkeei, and may have contributed to better understand its ecological niche