Chromosomal Evolution and Patterns of Introgression in Helianthus

The genus Helianthus is an established model for studying the role of chromosomal divergence in the evolution of reproductive isolation. Knowledge of the nature and extent of karyotypic differences between species provides insight into the evolutionary history of the genomes in question and, in the case of closely related species, the potential for genetic exchange between taxa. We constructed high-density genetic maps of the silverleaf sunflower (Helianthus argophyllus; ARG) and Algodones Dune sunflower (H. niveus ssp. tephrodes; NIV) genomes and compared them to a consensus map of cultivated sunflower (H. annuus; ANN). Comparative analyses revealed greater divergence between ANN and NIV (13 inversions, 18 translocations) than between ANN and ARG (10 inversions, eight translocations), consistent with their known phylogenetic relationships. Marker order was conserved across the genome, with 83% and 64% of the ARG and NIV genomes, respectively, being syntenic with ANN. Population genomic analyses between ANN and ARG, which are sympatric across a portion of the natural range of ANN, revealed significantly elevated genetic structure (estimated as F_ST) in rearranged portions of the genome, indicating that such rearrangements are associated with restricted gene flow between these two species. Despite the large number of rearrangements, there is sufficient synteny to access favorable wild alleles from across much of the ARG and NIV genomes for improvement of cultivated sunflower.