Introduction
The yam genus, Dioscorea L. (Dioscoreaceae) is a diverse group currently containing 631 accepted species (POWO, 2022) possessing underground storage organs and, in most, a climbing habit. Species with starchy tubers constitute a food staple for millions of people, resulting in seven to ten species being cultivated on a large scale (Asiedu and Sartie, 2010), including two (D. alata L. andD. cayenensis Lam.) which together are the most widely cultivated crops (Price et al ., 2016). More than 40 wild species are harvested as food sources (Martin and Degras, 1978). In addition, some yams have been used in traditional medicine and as a source of steroidal precursors (De Luca et al., 2012; Hua et al., 2017; Priceet al ., 2016). While most wild yam species are found in tropical regions (Caddick et al., 2002), a few species are distributed in temperate regions and exhibit unique morphological traits (Viruelet al., 2010). For example, only six species occur in the Mediterranean-Macaronesian region: two species of the Stenophora clade (D. balcanica Košanin native to Montenegro and Albania, and D. caucasica Lipsky, found in Georgia and Caucasian Russia), the Borderea clade, which contains two well-defined and narrow endemic species from the Pyrenean mountains (D. chouardii Gaussen and D. pyrenaica Bubani & Bordère ex Gren.), and the Tamus clade, which is defined by having berries rather than winged capsules and is more widely distributed across the Mediterranean Basin, Macaronesia and Atlantic Europe (Viruel et al., 2016).
The Tamus clade currently comprises two species (Wilkin et al.,2005): D. communis (L.) Caddick & Wilkin, distributed throughout the Mediterranean Basin and the Macaronesian Islands (Canary Islands and Madeira), and with infraspecific variation in ploidy (Viruel et al., 2019); and D. orientalis (J. Thiébaut) Caddick & Wilkin, restricted to Lebanon and Israel. However, like in many Dioscoreaclades (Viruel et al., 2010), the Tamus clade has had multiple previous taxonomic circumscriptions. The Tamus clade was considered as a separate genus, Tamus L., distinct from Dioscorea , until 2002 (Caddick et al ., 2002). Linnaeus (1753) recognized two species: T. communis L. with cordate leaves and a Mediterranean distribution, and T. cretica L, with trilobed leaves and typified with material from the Greek island of Crete. In the 19th and early 20th centuries, four Macaronesian endemic species were described (T. edulisLowe, T. parviflora Kunth, T. norsa Lowe and T. canariensis Willd. ex Kunth), while T. cirrhosa Hausskn. ex Bornm., T. cordifolia Stokes and T. racemosa Gouan were treated as distinct Mediterranean species. In the late 20th Century, T. cretica was placed as a subspecies in T. communis (T. communis subsp.cretica (L.) Nyman), and T. communis f. subtriloba(Guss.) O. Bolòs & Vigo was described as a variety with trilobed leaves found in the Balearic Islands and northeastern Spain (Catalonia). All these names were subsequently united under the currently acceptedD. communis (Caddick et al., 2002). The second species currently recognized in the Tamus clade, D. orientalis , was originally described as T. orientalis J.Thièbaut named after its eastern Mediterranean distribution.
From the above, it is clear that species concepts have undergone many changes since Linnaeus described two Tamus species using morphology, especially reproductive traits (De Queiroz, 2007). As for many other species in other plant genera and families, integrative taxonomic and systematic approaches combining genetic data, morphometrics and climatic envelope data have successfully helped to delimit species in challenging groups of plants (e.g., Frajman et al., 2019). The emergence of high-throughput sequencing (HTS) techniques and the production of thousands of molecular markers have massively increased our ability to resolve relationships between and within species, and subsequently redefine species boundaries (e.g., Fayet al., 2019; Escudero et al., 2020). Among these HTS methods, Hyb-Seq has become widely adopted across plant phylogenomic studies due to its ability to generate data from degraded herbarium materials (e.g., Brewer et al., 2019; Viruel et al., 2019) and to resolve relationships at different taxonomic scales (e.g., Villaverde et al., 2018). Hyb-Seq techniques rely on genome skim data and target capture probes either designed specifically for some genera or families (e.g., Soto Gomez et al., 2019) or more widely across larger groups, including all angiosperms (e.g., Johnson et al., 2019). In this study, we use a multidisciplinary approach combining genomic, morphometrics, and environmental niche modelling data generated from herbarium specimens to identify taxon boundaries in the challenging Tamus clade of Dioscorea , and to explore their phylogeographic patterns across the Mediterranean.