Hybridization and Introgression in Buckeyes (Aesculus: Hippocastanaceae): A Review of the Evidence and a Hypothesis to Explain Long-Distance Gene Flow

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Abstract
Evidence from morphology, distribution patterns, allozyme variation, and meiotic irregularities associated with decreased pollen germinability confirms the existence of a broad hybrid zone involving three parapatric species of Aesculus in the southeastern United States. The overall hybrid zone involving the three species is at least 200 km in width and probably represents the overlap of two hybrid zones: one between A. pavia and A. sylvatica and the other between A. flava and A. sylvatica. Both zones are highly asymmetrical, with hybrid populations occurring primarily in the Piedmont, where A. sylvatica is native. Detailed analyses of the hybrid zone involving A. pavia and A. sylvatica showed that hybrid populations consistently lack one or both of the putative parental species. Morphology and allozyme variation provide similar estimates of the position of the hybrid zone, but allozymes allow the detection of a larger zone than apparent on the basis of morphology. All available evidence is consistent with the hypothesis that extensive introgression has occurred among these species. Nevertheless, allozymic differentiation between these species is insufficent to reject hypotheses other than introgression that could generate the genetic structure observed in hybrid populations. Observations of pollinator activity in populations of A. pavia, A. sylvatica, A. flava, and their hybrids showed that these species share a number of important pollinators, including several species of bumblebees (Bombus) and the ruby-throated hummingbird (Archilochus colubris). Spring migration patterns of the ruby-throated hummingbird coincide closely with the flowering phenology of each of the Aesculus species, suggesting that hummingbirds could be vectors of long-distance pollen dispersal. The lifespan of pollen under field conditions is sufficient to permit transport of pollen in this fashion over distances of tens or hundreds of kilometers. Directional migration and arrival of hummingbirds after peak flowering would enforce a directional pattern of gene flow and could generate an asymmetrical hybrid zone of unusually great width.