Hybridization between the threatened herb Boltonia decurrens (Asteraceae) and its widespread congener, B. asteroides

Boltonia decurrens (Torr. & A. Gray) A. Wood is a federally threatened herb endemic to the Illinois River floodplain, and currently restricted to fewer than 25 populations. We tested for hybridization between this species and a widespread congener, Boltonia asteroides (L.) L’Hér., at two sites where they are known to occur in sympatry, by comparing morphological and genetic diversity with an allopatric population of each species. Morphological analyses identified significant differentiation between species. Discriminant classification analysis of the morphological data assigned most individuals to one or the other of the two parental species with high probability (>0.95), with only four plants having an intermediate probability of assignment (<0.90) indicative of hybrid origin. Similarly, likelihood-based methods based on genetic marker data (allozymes) failed to detect the presence of F1 hybrids in either sympatric population and revealed only low frequencies of F2 and backcross progeny. These findings demonstrate that combining morphological and genetic data increases the likelihood of detecting low-scale introgression between closely related species. Even with this combined approach, the low rate of recombination between these species together with their limited sympatry indicate that hybridization does not pose an immediate threat to the genetic integrity of B. decurrens.