Reproductive isolation and interspecific hybridization in the threatened species, Sidalcea nelsoniana

Abstract

In addition to its longstanding recognition as an influential evolutionary process, interspecific hybridization is increasingly regarded as a potential threat to the genetic integrity and survival of rare plant species, manifested through gamete wasting, increased pest and disease pressures, outbreeding depression, competitive exclusion, and genetic assimilation. Alternatively, hybridization has also been interpreted as a theoretically beneficial process for rare species suffering from low adaptive genetic diversity and accumulated genetic load. As such, interspecific hybridization, and the underlying pre- and post-mating reproductive barriers that influence its progression, should be considered fundamental components of conservation planning for many rare species, particularly those predisposed to hybridization by various ecological, genetic, and anthropogenic risk factors. In this study I evaluate the nature and efficacy of preand post-mating hybridization barriers in the threatened species, Sidalcea nelsoniana, which is sympatric (or nearly so) with three other congeners in the scarce native grasslands of the Willamette Valley in western Oregon. These four perennial species share a high risk of hybridization due to their mutual proximity, common occupation of disturbed habitats, susceptibility to anthropogenic dispersal, predominantly outcrossing mating systems, their capability of longlived persistence and vegetative expansion, and demonstrated hybridization tendencies among other members of the family and genus. Results show S. nelsoniana is reproductively isolated from all three of its congeners by a complex interplay of pre- and post-mating barriers. Although S. nelsoniana overlaps with S. campestris in the ecological attributes of flowering time, fine-scale geographic distribution, and pollinators, interspecific hybridization is discouraged through the post-mating barrier of sexual incompatibility (expressed primarily as reduced seed set). Hybridization between S. nelsoniana and S. virgata is limited by the premating barrier of asynchronous flowering (temporal isolation) and the supplemental post-mating barrier of sexual incompatibility, though the completeness of the latter varies in relation to crossing direction and S. virgata phenotype. Lastly, although S. nelsoniana and S. cusickii exhibit full interspecific sexual compatibility and produce fully fertile hybrids, hybridization in this species pair is discouraged by the pre-mating barrier of geographic isolation (the two species are narrowly parapatric). Additional findings of this study show that 1) pollinators transferred significantly more S. nelsoniana pollen to the heterospecific flowers of S. virgata and S. cusickii than to conspecific flowers (or those of S. campestris) in a mixed species array, 2) anthropogenic disturbance is ubiquitous across extant S. nelsoniana study populations so cannot be conclusively linked with the breakdown of spatial reproductive barriers, 3) polyploidy is confirmed in the species group and appears to directly influence the observed patterns of interspecific sexual compatibility and the chromosome numbers of hybrids, and 4) Fl hybrids exhibit a mosaic of parental, intermediate, and transgressive phenotypic characters that render hybrid discrimination difficult (especially in light of pronounced parental phenotypic variability). Ultimately, despite the presence of pre- and post-mating reproductive barriers, hybridization between Sidalcea nelsoniana and its local congeners is still possible through human intervention. Preserving the species' current genetic integrity will require prevention of Sidalcea dispersal that could lead to the breakdown of spatial, temporal, and sexual barriers between species. Development of useful molecular markers will be needed to help recognize hybrids in the wild, as traditional morphological methods appear inadequate for this endeavor.