Interspecific hybridization and subsequent admixture often has important ecological and evolutionary consequences, especially when one of the species involved is relatively rare. Common milkweed (Asclepias syriaca) has been observed to hybridize with its uncommon congener A. sullivantii, but the extent to which hybridization occurs across their range is unknown. The purpose of this study was to (1) quantify the extent of admixture between A. syriaca and A. sullivantii in multiple co-occurring populations, and (2) assess population level differences in A. syriaca across its Midwestern United States distribution. Individuals were collected from sites ranging from Ohio to Iowa: 14 with isolated A. syriaca populations, and 13 with co-occurring populations of A. sullivantii, and A. syriaca. We are assessing leaf morphological variation and performing double digest RAD Sequencing (ddRADSeq) for 564 individuals. Additionally, we are assessing morphological variation in flower and pollinia traits for a subset of 264 individuals.
Results/Conclusions
Principal component analysis of morphological data for a representative subset of all collected samples indicates clear differentiation between A. syriaca and A. sullivantii, with floral characteristics being the most discriminating. Molecular analysis of 24 Ohio samples from one site where the two species co-occur revealed clear evidence of F1 and F2 putative hybrids, slight admixture between co-occurring species, and a set of highly discriminating species-specific alleles. Genetic structure (Fst) pairwise comparisons among a subset of four Ohio populations showed that A. sullivantii samples were more similar to co-occurring A. syriaca than they were to isolated populations. Pairwise Fst values were also suggestive of differentiation between northern and southern populations of A. syriaca within Ohio, although results were hindered by low sample size. To clarify these preliminary patterns, the full set of Midwest populations will be analyzed using ddRADSeq and morphological analysis. This will help to better illuminate the evolutionary relationship between these two species, identify loci under selection in hybrid lineages and elucidate intraspecific genetic variation within A. syriaca.