Hybridization in Helianthus : the genomic profiles of potential and confirmed sunflower hybrid species

Hybridization is an important evolutionary force that acts in both constructive and destructive ways. It can both swamp out rare species and create new ones. To better understand these effects I studied hybridization within the sunflower genus Helianthus from three angles. First, I used a rich literature of artificial crossing experiments in Helianthus and Madiinae to ask how fast reproductive isolation evolves and what features affect its accumulation. I show that hybrid sterility can evolve quickly and is faster in annuals than in perennials. I then examine a classic case of introgression involving Helianthus bolanderi. I use modern genomic tools to show that it is not of hybrid origin and likely not a separate species from its congener H. exilis. We do however find introgression with the invading species, H. annuus. In agreement with theory, we find that gene flow is mainly into the invading species. Lastly, I use transcriptomic data for three established homoploid hybrid species, H. anomalus, H. deserticola, and H. paradoxus, and their parents H. annuus and H. petiolaris to map the genomic composition of hybrid species. I show that composition is even or biased towards H. petiolaris. Hybrid genomes are highly recombined but are more similar in genomic composition than expected by chance, suggesting the work of selection. Furthermore, although analyses of genetic distance between the hybrid species and their parents suggests that the hybrids are older than previously appreciated, they do not appear to be fully stabilized. Lastly two of the species, H. anomalus and H. deserticola, may share a common origin. Future directions include mapping introgression in H. annuus, and modeling parental block size to determine the number of loci and strength of selection during hybrid speciation.Science, Faculty ofBotany, Department ofGraduat