Hybridization as an evolutionary driver for speciationA case in the Southern European Erysimum species

Abstract

The principal objective of this Ph.D. thesis was to disentangle the role of hybridization and polyploidization in the evolution of plants, using a set of Southern European Erysimum species as a study case, namely, E. nevadense and E. baeticum, E. mediohispanicum and E. bastetanum, E fitzii, and E. popovii. These species inhabit the Baetic Mountains, sometimes in sympatry. Our central hypothesis was that hybridization and polyploidization might have had a significant influence on the evolution of these species, leaving a signature in their genomes. This study was addressed considering that these and other evolutionary processes take place at the population level and that therefore intraspecific variability had to be explicitly incorporated in all analyses. Additionally, we hypothesized that the hybridization signal might be more patent in species with purple corollas, as this phenotype might have been favored by adaptive introgression. First, we have studied to which degree ITS sequences were homogenized by concerted evolution in this group (Chapter I). We found that these nuclear sequences were not thoroughly homogenized, suggesting a recent hybridization signature for these Erysimum species. Second, we assembled the chloroplast genomes of these species from RNA-Seq data, demonstrating the reliability of this strategy to provide complete cpDNA genomes (Chapter II). Then, we provided new genomic resources for Erysimum spp., like de novo assembled and annotated transcriptomes. Following this methodology, we assembled and annotated the chloroplast genomes, reconstructing a time-calibrated phylogeny (Chapter III). Finally, we studied the general hybridization scenario in the presence of ILS for these species by combining nuclear and chloroplast data (Chapter IV). We found that introgression was ubiquitous in the species studied here and that even diploid species showed a signature of introgression in their genomes. These results provide insights into the importance of hybridization and polyploidization for plant evolution in general and Erysimum in particular.