Green 1929: 189 Pl. 2: 998. 1753 A subgeneric classification of Fagus (Fagaceae) and revised taxonomy of western Eurasian beeches Denk, Thomas Grimm, Guido W. Cardoni, Simone Csilléry, Katalin Schulze, Mirjam Kurz Ernst-Detlef Simeone, Marco Cosimo Worth, James R. P. Willdenowia 2024 2024-10-02 54 151 181 8VXN4 [189,256,745,769] Magnoliopsida Fagaceae Fagus Plantae Fagales 10 160 Tracheophyta genus   Molecular diagnosis— The subgenus differs consistently from allspecies of  Fagussubg. Englerianaein any sufficiently divergent nuclear marker sequenced so far ( Denk & al. 2002, 2005; Renner & al. 2016; Cardoni & al. 2022). Its ITS variants belong to Lineages II–IV as defined in Denk & al. (2005); the sequenced part of the Crabs Claw( CRC) gene (~1650 bps) and the 2 ndintron of the Leafygene ( LFY, up to ~1300 bps) include 14 subgenus-sorted SNPs ( CRCpos. 212 [C vs T in F.subg. Englerianae], 714 [C↔T], 904 [C↔A], 1148 [A↔T/Y] and 1329 [T↔G]; LFYpos. 241 [A↔C], 398, 402, 465 [all G↔A], 507 [T↔A], 529 [C↔T], 1031 [T↔G], 1206 [A↔G] and 1246 [T↔A]; according reference matrices are included in the SDA, file RefMatrixCRCLFYSpCons.nex). Additional subgenus-diagnostic SNPs can be found in 21 of the 28 nuclear loci sequenced by Jiang & al. (2022; cf. supplement to Cardoni & al. 2022). Plastomes are divergent but geographically sorted and reflect two independent origins: Lineage I in North America; sibling lineages Lineage IV in East Asiaand Lineage V in western Eurasia; see Fig. 1) The exception are populations of the Japanese species  F. crenatacomprising individuals that may carry near-private haplotypes of Lineage II (supplementary content, file Genotypification.xlsx, sheet PlstmDissim; including information from upcoming complete plastome data; Worth & al. 2021, work in progress).  Morphological diagnosis— Trees; buds sessile; leaves thick-chartaceous, abaxial leaf surface commonly smooth or papillate (  Fagus longipetiolata), wax ornamentation on abaxial leaf surface missing or present (  F. longipetiolata), size of stomata usually large, small in  F. hayataePalib.,  F. pashanicaC. C. Yang, and  F. grandifolia, subsidiary cells of stomata usually actinocytic to cyclocytic, anomocytic in  F. grandifolia, leaf margin smooth or serrate; cupule peduncle short to long; pollen usually large, intermediate in  F. hayatae, colpi usually short with more or less acute apex, or long and narrow with rectangular apex in  F. grandifoliaand occasionally in  F. longipetiolata.  Species— Twelve:  Fagus sylvatica,  F. orientalis,  F. hohenackeriana,  F. caspica sp. nov.in western Eurasia (west to east);  F. chieniiW. C. Cheng(†?),  F. crenata,  F. hayatae,  F. longipetiolata,  F. pashanica,  F. lucidain East Asia;  F. grandifolia,  F. mexicanaMartÍnezin North America.  Remarks— Members of  Fagussubg. Faguscan be traced in the fossil record based on their pollen and leaf-anatomical similarities with one or several modern-day species ( Denk & Grimm2009; Renner & al. 2016; Worth & al., work in progress). The oldest fossils representing this modern subgeneric lineage are cupules and leaves from the Eocene-Oligocene boundary, Northeast Asia ( Pavlyutkin & al. 2014; see also Denk & Grimm2009). Any molecular-phylogenetic tree analysis (e.g. Denk & al. 2005; Jiang & al. 2022) relying on sufficiently variable nuclear data will produce a prominent split with high (BS ≥ 70, PP ≥ 0.9) to unambiguous (BS = 100, PP = 1.0) support between the subgenera irrespective of the optimality criterion used for tree-inference. Newly sequenced individuals can be easily placed in either subgenus using e.g. the evolutionary placement algorithm implemented in RAxML 8 and its successor RAxML-ng. In contrast, any plastome data requires in-depth analysis and, in some cases, may fail to elucidate the subgeneric affinity.