Natural hybridization between prairie milkweeds, Asclepias sullivantii and Asclepias syriaca:: Morphological, isozyme, and hand-pollination evidence

In rare species that co-occur with widespread congeners, hybridization can be an integral force because it can lead to an infusion of novel and beneficial genes through introgression, or it may result in genetic swamping of the rare plant species. At a disturbed prairie remnant in central Ohio, the potential for hybridization was examined within sympatric populations of a rare prairie species, Asclepias sullivantii ( Sullivant's milkweed), and the more widespread Asclepias syriaca ( common milkweed). Measurements of eight leaf, 15 flower, and seven pollinarium traits showed the two species to be distinct in all but one trait, with putative hybrids exhibiting intermediate values for most characters. A multivariate analysis of the morphological traits showed the parental species separated in two-dimensional ordination space, with the hybrids situated between them. The parental taxa overlapped broadly in flowering time, peaking during the first week of July. Isozyme analysis of 30 individuals of each parental taxon and eight apparent hybrids revealed three variable loci, one of which included species-specific alleles that were both present in the hybrid. These results reinforce our hypothesis that the morphologically intermediate plants at this site are hybrids resulting from recent crossing between two genetically distinct species. Experimental interspecific hand-pollinations showed the potential for hybrid fruit set in A. syriaca (26% fruit set) but not A. sullivantii (0%), although fruit set was reduced compared with conspecific pollinations (63% and 13%, respectively). Hybrid pollinia were essentially inviable, but the observation of numerous naturally set fruits borne by hybrids indicates that they may not be evolutionary dead ends but could serve as bridges to gene flow between the species. This hybridization record is the first for this pair of species, and it reinforces recent suggestions that local hybridization between milkweeds is more common than previously stated.