CARYOPHYLLACEAE

CARYOPHYLLACEAE View in CoL

SABULINA : A new species of Sabulina ( Caryophyllaceae ) from the eastern United States and the typification of Arenaria patula Michx.

Primary author: Mason Brock

The identity of a Sabulina patula -like entity in the rugged calcareous barrens and cliffs in the mid-south has long perplexed botanists. These populations resemble S. patula (Michx.) Small ex Rydb. in many respects, and most herbarium specimens are currently identified under historical synonyms, viz., Arenaria patula Michx. , Minuartia patula (Michx.) Mattf. , and Mononeuria patula (Michx.) Dillenb. & Kadereit. However , the S. patula -like populations are distinct in their sepal and seed morphology, inflorescence and growth habit, and habitat. They are described here as a new species: Sabulina diffusa .

This new taxa was first brought to my attention during a collection trip to an area of limestone cliffs in Estill County Kentucky in 2019. The Sabulina growing at this location was highly aberrant from the S. patula I was accustomed to seeing around the disturbed flat limestone glades of the Nashville Basin. The branches were divaricate-spreading, forming a bush-like mound, and the plant in general was more robust. In addition, the sepals had three nerves, as opposed to the five nerves of the S.patula with which I was familiar.This problem inspired me to visit the Missouri Botanical Garden (MO) in 2020, where I investigated this issue more deeply and began mapping populations from herbarium specimens.

In both the regional Atlas of Kentucky ( Campbell & Medley 2012) and in Bassett Maguire’s 1951 conspectus of Arenaria ( Maguire 1951) this eastern Sabulina was treated as disjunct populations of what is now known as S. muscorum (Fassett) E.E.Schill. [syn. Arenaria patula var. robusta (Steyerm.) Maguire , Minuartia muscorum (Fassett) Rabeler , Mononeuria muscorum (Fassett) Dillenb. & Kadereit ]. This eastern mapping of S. muscorum is also currently being adopted by Weakley’s Flora of the Southeastern United States (Weakley & Southeastern Flora Team 2022a). The primary reason these populations have been called S. muscorum by some researchers is understandable: the sepals have three nerves as in S. muscorum instead of five found in S. patula .

However, as noted by Rabeler et al. (2005), a close examination of the seed morphology reveals a problem with these plants simply being S. muscorum : They have the tuberculate surface texture typical of S. patula , distinctly unlike the pebbled-papillose surface texture typical of S. muscorum ( Rabeler et al. 2005) . In addition, the general habit of these plants is more diffuse with a divaricate-spreading inflorescence, in contrast with the more sparingly flowered and erect-spreading inflorescence of the western S. muscorum .

A recent phylogenetic study of the stichwort complex provided additional genetic evidence of this taxon’s distinctiveness ( Schilling et al. 2022). As part of a study that used molecular data of populations of Caryophyllaceae to determine generic delimitations within the family, they revealed that Sabulina patula as it is currently understood is polyphyletic, with two specimens from eastern Tennessee (from Claiborne and Knox counties, TENN herbarium) appearing more closely related to S. michauxii (Fenzl) Dillenb. & Kadereit [syn. Arenaria stricta Michx. , Alsine michauxii Fenzl , Minuartia michauxii (Fenzl) Farw. ] than other populations of S. patula . This molecular data also showed that these populations were distinct from the clade containing S. muscorum .

An examination of these specimens on the online SERNEC database revealed that they indeed are specimens of the unusual and difficult-to-place entity in question. This unexpected placement of the atypical Sabulina as sister to neither S. patula nor S. muscorum confirms what other lines of evidence has already indicated: These populations represent an undescribed species.