Assessing the threat from hybridization to the rare endemic Physaria bellii Mulligan (Brassicaceae)

Hybridization of a rare endemic with a more common congener can endanger the rarer species through gene swamping. This study used a combination of genetic, morphological and cytological analyses to assess the threat to Bell's twinpod (Physaria bellii), a rare endemic restricted to the Colorado Front Range, from hybridization with its more widespread congener Physaria vitulifera. Two populations of P. bellii at the southernmost end of its range were suspected of containing hybrid plants, based on field observations of intermediate leaf morphology. Inter-simple sequence repeat (ISSR) marker analysis confirmed that suspect individuals contained alleles from both P. bellii and P. vitulifera, with most hybrids genetically closer to P. vitulifera. Morphological data from leaf measurements showed hybrids to be intermediate for several characters, with the exception of the leaf sinus trait that was statistically closer to P. vitulifera. The morphometric data supported the genetic data with Canonical Discriminant Analyses of both data sets showing similar patterns. The extent of the threat to P. bellii populations from hybridization with the more common P. vitulifera was assessed through cytological analysis of existing populations and through field and greenhouse crosses. While naturally occurring hybrids are tetraploid, diploid P. vitulifera populations do not occur close enough to the current range of diploid P. bellii to permit gene flow. Interspecific crosses yielded fewer normal seeds than intraspecific crosses, regardless of the ploidy levels of the parents. It does not appear that P. bellii is currently threatened by hybridization with P. vitulifera, and efforts to conserve this globally imperiled species should focus on the threat from potential habitat loss rather than from gene swamping.