Karyotypic Evolution of the Common and Silverleaf Sunflower Genomes

Silverleaf sunflower (Helianthus argophyllus Torrey and Gray) has been an important source of favorable alleles for broadening genetic diversity and enhancing agriculturally important traits in common sunflower (H. annuus L.), and, as the closest living relative of H. annuus, provides an excellent model for understanding how apparently maladaptive chromosomal rearrangements became established in this genus. The genomes of H. annuus and H. argophyllus were comparatively mapped to identify syntenic and rearranged chromosomes and develop genomic blueprints for predicting the impact of chromosomal rearrangements on interspecific gene flow. Syntenic chromosomal segments were identified and aligned using 131 orthologous DNA marker loci distributed throughout the H. annuus genome (299 DNA marker loci were mapped in H. argophyllus). We identified 28 colinear chromosomal segments, 10 colinear chromosomes, and seven chromosomal rearrangements (five nonreciprocal translocations and two inversions). Four H. argophyllus chromosomes carrying non-reciprocal translocations apparently arose from the duplication of two chromosomes, and three H. argophyllus chromosomes apparently arose from end-to-end or end-to-opposite-end fusions of chromosomes or chromosome segments. Chromosome duplication may reduce the initial fitness costs of chromosomal rearrangements, thereby facilitating their establishment. Despite dramatic differences in chromosome architecture, a significant fraction of the H. argophyllus genome appears to be accessible for introgression into H. annuus.