Genome scan of hybridizing sunflowers from Texas (Helianthus annuus and H. debilis) reveals asymmetric patterns of introgression and small islands of genomic differentiation

Although the sexual transfer of genetic material between species (i.e. introgression) has been documented in many groups of plants and animals, genome-wide patterns of introgression are poorly understood. Is most of the genome permeable to interspecific gene flow, or is introgression typically restricted to a handful of genomic regions? Here, we assess the genomic extent and direction of introgression between three sunflowers from the south-central USA: the common sunflower, Helianthus annuus ssp. annuus; a near-endemic to Texas, Helianthus debilis ssp. cucumerifolius; and their putative hybrid derivative, thought to have recently colonized Texas, H. annuus ssp. texanus. Analyses of variation at 88 genetically mapped microsatellite loci revealed that long-term migration rates were high, genome-wide and asymmetric, with higher migration rates from H. annuus texanus into the two parental taxa than vice versa. These results imply a longer history of intermittent contact between H. debilis and H. annuus than previously believed, and that H. annuus texanus may serve as a bridge for the transfer of alleles between its parental taxa. They also contradict recent theory suggesting that introgression should predominantly be in the direction of the colonizing species. As in previous studies of hybridizing sunflower species, regions of genetic differentiation appear small, whether estimated in terms of FST or unidirectional migration rates. Estimates of recent immigration and admixture were inconsistent, depending on the type of analysis. At the individual locus level, one marker showed striking asymmetry in migration rates, a pattern consistent with tight linkage to a Bateson-Dobzhansky-Muller incompatibility.