**1. Introduction**

*Fagus* is a genus of deciduous trees in the family Fagaceae (order Fagales), native to temperate Europe, Asia, and North America. The genus is divided into two subgenera, Engleriana and Fagus [1] and comprises 11 species (without ssp., var. and f.) [2]. As a naturally growing forest tree, European beech (*F. sylvatica* L.; subgenus Fagus) is among the most important and widespread tree species in Central Europe and is widely managed for its versatile hard wood. Because of the significance of the genus *Fagus*, genomic sequence resources are highly desired for this genus.

Mishra et al. [3] published a 542 Mb nuclear draft genome sequence of an up to 300-year-old *F. sylvatica* individual (Bhaga) from an undisturbed stand in the Kellerwald-Edersee National Park in Central Germany. The assembly comprised 6451 scaffolds that are not yet assigned to any chromosomes. Complete chloroplast genomes are publicly available for *F. sylvatica* [4], *F. engleriana* [5], *F. crenata* [6], and *F. japonica* var. *multinervis* [7]. No complete mitochondrial genome sequence is available for any *Fagus* species to date [8]. In the entire order of the Fagales, only two assembled mitochondrial genome sequences are publicly available: one from *Quercus variabilis* (unverified sequence; GenBank MN199236) [9] and one from *Betula pendula* (GenBank LT855379.1) [10]; however, this sequence is not annotated so far.

Plant mitochondrial genomes are much larger than those of animals and highly variable in size [8,11–16]. They have low mutation rates, but have such high rearrangement rates that there is virtually no conservation of synteny [17–22]. The plasticity of plant mitochondrial genomes, leading to genome expansion, arises primarily from repeat sequences (including nontandem repeats of 50 bp and up), intron expansion, and incorporation of plastid and nuclear DNA [12,15,17–19,22–28]. The mitochondrial genome sequences of angiosperms generally have one or more pairs of large nontandem repeats (interspersed repeats) that can act as sites for inter- and intramolecular recombination, leading to multiple alternative arrangements (isoforms; including subgenomic forms) within a species [19]. Although plant mitochondrial genomes are often assembled and displayed as circular maps, plant mitochondrial DNA (mtDNA) does most likely not exist as one large circular DNA molecule but mostly as a complex and dynamic collection of linear DNA with combinations of smaller circular and branched DNA molecules [12,14,15,17,19,26,29–34].

DNA barcoding is an effective technique in molecular taxonomy. Sequences suggested to be useful in DNA barcoding include mtDNA (e.g., *cox1* for animals), chloroplast DNA (e.g., *rbcL*, *matK*, *trnL*-*trnF*, *ndhF*, and *atpB*), and nuclear DNA (ITS and house-keeping genes, e.g., GAPDH; [35–39] among others). Chloroplast and mitochondrial genes are preferred over nuclear genes because most of the genes lack introns, and they are generally haploid [40]. Furthermore, each cell has many chloroplasts and mitochondria, and each one can contain several copies of the respective genome [41–43]. Thus, when sample tissue is limited, the chloroplast and the mitochondrion offer relatively abundant sources of DNA.

Advances in high-throughput sequencing (short read and single molecule long read sequencing) have promoted the assembly of complete DNA sequences of chloroplast and mitochondrial genomes and allowed for extending barcoding from single loci to whole genomes [44,45]. Especially, complete chloroplast genome (plastome) sequences provide valuable data sets to resolve complex evolutionary relationships in plastome phylogenies and improve resolution at lower taxonomic levels (e.g., [46,47]). Based on whole plastome alignments, genetic markers for species identification were developed in several studies (e.g., [44,48]).

The lower nucleotide substitution rate in mitochondrial compared to chloroplast genomes of land plants [17–22] often provides not enough variation at the species level, although a few mitochondrial markers for potential species differentiation were developed (e.g., [49]). However, nucleotide variants in mitochondrial genomes may provide promising targets for the development of DNA markers that are specific for higher taxonomic levels, such as genus, family or order.

European beech is often used in particle boards [50] and not always declared as it should be due to the European Timber Regulation that came into force in 2013 [51]. Although, species of the genus *Fagus* have not yet been added to the IUCN list [52] as "endangered" species (*F. longipetiolata* and *F. hayatae* classified as "vulnerable"), the specific ecosystems they form are in danger. Due to deforestation, old-growth forests are vanishing all over the world. Especially in Eastern Europe, the protected old forests consisting of oak, spruce, and beech have been declining during the last decades mostly because of human impact, i.e., mainly poor management practices and illegal logging of old valuable trees [53]. However, these forests play a key role for sustaining biodiversity and in climate change because they store a high amount of carbon for long time periods [53].

As a contribution to the preservation of valuable forest ecosystems, molecular markers can help to identify non-declared genera or species in different wood composite products. Identification of timber genera or species from solid wood products is much easier than from composite products as it may contain wood from many different genera or species. All the more important are molecular markers that are specific for a tree genus of interest ensuring a 100% classification probability ("golden markers"), and thus allowing for a doubtless identification of the related genus in wood composite products. To further increase the identification confidence in a tree genus, additional markers specific for higher taxonomic levels are highly desired. All potential taxon-specific markers should be validated in as many other tree species as possible (including species common in wood composite products).

This study aimed at sequencing and annotating the complete mtDNA sequence of a representative *F. sylvatica* individual, which was used as a source for the development of molecular markers suitable to identify the genus *Fagus*. The developed markers (for the genus *Fagus*, the family Fagaceae, and the order Fagales) provide a useful tool set to verify the declaration "genus *Fagus*" using wood from tree tissues or wood products, including composite wood.
