Taxonomic delimitations within the Penstemon scariosus Pennell (Plantaginaceae) complex

Authors: STEVENS, MIKEL - Brigham Young University; JOHNSON, ROBERT - Brigham Young University; STETTLER, JASON - Brigham Young University; MESERVEY, LINDSEY - Brigham Young University; Robbins, Matthew; ANDERSON, CHRIS - Brigham Young University; PORTER, SYDNEY - Brigham Young University; RICKS, NATHAN - Brigham Young University; HARRISON, SARAH - Brigham Young University

Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2022
Publication Date: 12/7/2022
Citation: Stevens, M.R., Johnson, R.L., Stettler, J.M., Meservey, L.M., Robbins, M.D., Anderson, C.D., Porter, S.J., Ricks, N.J., Harrison, S. 2022. Taxonomic delimitations within the Penstemon scariosus Pennell (Plantaginaceae) complex. Western North American Naturalist. 14(1):23-64. https://doi.org/10.3398/042.014.0102.
DOI: https://doi.org/10.3398/042.014.0102

Interpretive Summary: Currently, there are four botanical varieties in the species Penstemon scariosus, called albifluvis, cyanomontanus, garrettii and scariosus. It is important to taxonomically distinguish these four varieties, especially for variety albifluvis, which is being considered for listing under the Endangered Species Act. However, it is difficult using current taxonomic descriptions and methods to clearly delineate these varieties. In order to better characterize these varieties, we used a combination of ten molecular markers to find genetic differences between 66 plant populations collected across the known geographic range of these botanical varieties. We also examined ten morphological characteristics from 264 herbarium specimens to identify physical differences between varieties. From the genetic and physical characterization, we determined four major conclusions. First, the variety albifluvis is distinct from the other varieties and should become its own species, as it was historically. Second, the variety cyanomontanus has a much broader geographic range and the characteristic of glandular hairs is not a distinguishing characteristic of this variety as previously understood. Third, the varieties garrettii and scariosus cannot reliably by distinguished using either genetic or physical characteristics, and therefore, should be combined into the same variety. Fourth, we have identified plant populations in the region near Tabby Mountain, UT that are distinct from any of the four varieties. We describe the characteristics of this new taxon and provide a taxonomic key to distinguish it from the other members of the Penstemon scariosus species complex.

Technical Abstract: We examine the four recognized varieties of Penstemon scarious which comprise a complex of related taxa that share overlapping morphological characters; namely varieties albifluvis, cyanomontanus, garrettii and scariosus. Modern taxonomic descriptions and associated keys are not in complete agreement as how to clearly delineate these varieties. It is particularly important to understand the taxonomic circumscription of variety albifluvis, since it is being considered for listing under the Endangered Species Act. To address the taxonomic position of taxa in this species complex we examine the genetic structure of 66 accessions of P. scariosus representing the four known varieties across its entire known range using ten SSR (microsatellite) markers. We also examine plant morphology of these taxa from 264 herbarium specimens. The results of our molecular and morphological studies give rise to four conclusions. First, due to the genetic distinctiveness of P. scariosus var. albifluvis and its geographical isolation, we consider conserving its original status at the species level. Second, our molecular study suggests that the geographic area for var. cyanomontanus is much larger than previously understood, consisting of plants and populations with or without the characteristic glandular hairs that have been used to identify that taxon. Third, both our molecular and morphometric data suggest that varieties garrettii and scariosus are not reliably separable and should be considered the same taxon. Finally, our molecular data reveal a distinct genotype from the Tabby Mountain, UT area that has not previously been given taxonomic recognition. We describe this new taxon and provide a taxonomic key to separate this new variety from the other members of the species complex.