Taxonomy, morphology and cytology of Micranthes virginiensis (Michaux) Small (Saxifragales, Saxifragaceae), new chromosome counts for Micranthes and description of a new Micranthes species from the south-eastern USA

<i>Micranthes virginiensis</i> (Saxifragaceae) is an herbaceous, flowering plant species native to eastern North America with a range extending from the Gulf of Mexico into Canada. This broad range, known from previous studies to contain individuals with varying chromosome numbers and morphological variation outside of the current formal description, indicates the need for a re-examination of the taxonomy of this species. Some populations in south-eastern Appalachia display intermediate traits between <i>M. virginiensis</i> and the peripatric congener <i>M. careyana</i> and have unresolved phylogenetic placement, raising the possibility of hybridisation. This study explored hypotheses of hybridisation and undescribed taxa within <i>M. virginiensis</i>, based on morphometric and chromosome data collected from samples across Eastern North America. Floral, fruit and leaf measurements were analysed to investigate morphological variation across the species' range. Chromosome counts from south-eastern U.S.A. congener populations of <i>M. careyana</i>, <i>M. palmeri</i> and <i>M. petiolaris</i> all showed diploidy (2<i>n</i> = 20), representing the first known chromosome counts for these species. Tetraploidy and unique floral morphology in two populations of the Blue Ridge Escarpment of SC, where the distribution of <i>M. virginiensis</i> and <i>M. careyana</i> abut, indicate an undescribed species, possibly of hybrid origin. Other tetraploid populations in the south-eastern USA showed no morphological differences from diploid <i>M. virginiensis</i>, suggesting autopolyploidy. In addition, we document a trend of decreased reproductive investment with increasing elevation within <i>M. virginiensis.</i> Overall, the taxonomic boundaries across the broad range of <i>M. virginiensis</i> proved intact, aside from the escarpment tetraploid species. Here, we describe the new escarpment species, <i>Micranthes scopularum</i> Hall, Lanning & Mathews and provide a complete list of synonyms of <i>M. virginiensis</i>, in which we designate two lectotypes and one neotype.