by Mathew Sharples, Erin Tripp
Whereas the eastern North American–eastern Asian floristic connection represents one of the most widely studied biogeographical relationships in flowering plant evolution, connections between western North America and Asia have been comparatively rarely investigated, especially through genetic approaches. Stellaria irrigua Bunge is one of several plants that has been treated as an exceptionally dramatic example of a disjunction between floristically similar, high alpine biotas of the southern Rocky Mountains and south-central Siberia. We here employ numerous new field collections and ddRADseq data to test the hypothesis that Stellaria irrigua—a species that has been known for over 180 years—represents a long-distance disjunction between the southern Rocky Mountains and central Asia. Extensive fieldwork, review and perusal of herbarium materials, and phylogenomic analyses indicate that Stellaria irrigua is broadly distributed across an amphi-Beringian arc extending from southern and central Asia, east through Beringia, and south throughout mountainous regions of western North America. Sampled Asian populations formed two clades, and North American individuals all formed a clade embedded within this broader Asian lineage. Stellaria irrigua is, however, rendered non-monophyletic by a lineage that is embedded within the North American populations and is ecologically and morphologically distinctive from S. irrigua. The identity of this newly recognized lineage, which was in prior works attributed to S. irrigua, has been confused since plants of the former were first collected in the San Juan Mountains of Colorado in the late 1800s under Arenaria and Alsine. We provide a new name for this taxon, Stellaria sanjuanensis M. Sharples & E. Tripp, a charismatic starwort of dry alpine scree slopes of the southern Rocky Mountains. Additionally, two lectotypes are designated, one holotype and one isotype are identified, and two new synonymies are proposed, to help stabilize the taxonomy and nomenclature of this long-confused species complex. A key to the starworts of the southern Rocky Mountains is also provided, and Stellaria alsine Grimm is reported as new to the region.