A new subspecies of Fagus orientalis (Fagaceae) in Turkey: F. orientalis subsp. nurensis, a relict close to the ancestor of modern European and Oriental Beech

Genetic diversity assessment is a key component of understanding biodiversity and conservation action. In this study, we used High-Throughput Sequencing (HTS) of the nuclear ribosomal 5S RNA gene arrays (5S-HTS), to investigate the small isolates of Oriental beech ( Fagus orientalis Lipsky) thriving at the south-easternmost edge of the genus range in Western Eurasia (Nur Mountains, the southern spur of the Taurus range, South Anatolia). We used data from 15 populations covering all four Western Eurasian beech species to assess geno-taxonomic affinities and evolutionary relationships. Distinct quantitative (number of exclusive) and qualitative (characteristic molecular features) composition of the detected 5S-IGS Operational Taxonomic Units (OTUs) document the Nur Mountains beeches as a unique genetic resource. Phylogenetic analyses using simultaneously generated 5S-IGS Amplified Sequence Variants (ASVs) and a host-associate framework revealed a peculiar evolutionary imprint, indicating their ancestral nature within the Euro-western Euxinian ( F. sylvatica L. + F. orientalis [s.str.]) and the Caucasian ( F. hohenackeriana Palib.) species lineages. Combined with fossil records, morphological evidence, and available genetic data (nuclear microsatellites and limited low-copy number gene sequences), our findings suggest that the Nur Mountains beeches are directly derived from the common ancestor of F. orientalis - F. sylvatica and retain sequence variants partly lost in all northwestern relatives. We propose a new taxonomic rank for these relict south-eastern Mediterranean beech outposts ( F. orientalis subsp. nurensis ) and call for adequate conservation strategies.