Taxonomic reassessment of Onosma gaubae (Boraginaceae) in the flora of Iran

Onosma gaubae was described by Bornmüller in 1937 as an endemic species of Iran. However, Khatamsaz treated this species as a synonym of Onosma elwendica in the Flora of Iran (2002), whereas Riedl recognized it as a distinct species in Flora Iranica (1967). Onosma elwendica was described by Wettstein in 1885, and its type specimen was collected from Mount Alvand near Zamanabad, Hamedan Province, Iran, by Polak. Given the taxonomic disagreement regarding the status of O. gaubae, the present study aims to reassess its taxonomic position and evaluate its taxonomic independence from O. elwendica based on morphological characters. In this study, we assessed a total of 104 specimens representing 71 populations of O. gaubae and O. elwendica using both quantitative and qualitative morphological features. Our research was based on field-collected samples and herbarium specimens stored in the HSBU, Iran, and D herbaria, as well as digital images of the holotype and isotype specimens. Detailed examination of the protologues, herbarium specimens, and type images revealed clear and consistent morphological differences between the two taxa. Onosma gaubae belongs to O. subsect. Haplotricha, whereas O. elwendica belongs to O. subsect. Heterotricha. The most important diagnostic characters distinguishing the two species include the quantitative and qualitative traits of the basal leaves, the type and structure of the indumentum, corolla morphology, the shape and size of the calyx lobes, and nutlet morphology. Furthermore, the two species exhibit distinct geographical distributions: O. gaubae is endemic to the southern slopes of the Central Alborz Mountains and parts of the Eastern Alborz, whereas O. elwendica occurs in central Iran and the Zagros Mountains of western and southwestern Iran. These morphological and biogeographical differences strongly support the recognition of O. gaubae as a distinct species in the flora of Iran.