Genetic analysis of a broad hybrid zone in Aesculus (sapindaceae): is there evidence of long-distance pollen dispersal?

The genetic structure of a broad hybrid zone involving three hybridizing Aesculus species, Aesculus flava, Aesculus pavia, and Aesculus sylvatica, was examined. The objectives were to assess genetic variability, to test previously reported hypotheses on patterns of gene flow, and to infer the genetic structure and evolutionary processes in the hybrid zone. Samples from 24 populations within parental ranges and the hybrid zone were analyzed for variation at microsatellite and intersimple sequence repeat loci. The results indicated that genetic variability was similar among parental and hybrid populations, indicating no evident increased diversity in the hybrid zone. Most hybrid individuals were genetically more similar to A. sylvatica than to the other two species, and the overall genetic structure of the hybrid zone is asymmetrically biased toward A. sylvatica. Our analyses supported occasional recurrent long-distant gene flow from A. pavia and frequent gene flow from A. sylvatica into the hybrid zone, agreeing with results of a previous allozyme study. Collectively, the data from our study and previous allozyme and chloroplast DNA studies indicate that both historical localized gene flow and recurrent long-distant gene flow have contributed to the existence of the hybrid zone, that is, its origin via historical localized gene flow, while its maintenance involves ongoing long-distance pollen dispersal.