Gaining a global perspective on Fagaceae genomic diversification and adaptation

The family Fagaceae is one of the most comprehensively studied and species rich groups of trees in the world. It provides a powerful model lineage for the general advance of tree science (Petit et al., 2013) and contributes enormously to global ecosystem services (Cavender-Bares, 2016). Biodiversity hotspots for Fagaceae are found in both temperate and tropical forests – which is unusual among trees – where they have undergone numerous independent and parallel diversifications (Kremer et al., 2012). Major genera provide possibilities for comparative studies within and across biomes and life histories, such as modes of pollination. Northern temperate groups including oak (Quercus) and beech (Fagus) are deciduous and wind-pollinated, and produce simple nuts exemplified by the classic oak acorn. In terms of biomass, these trees often dominate the forests in which they are found. These temperate groups are the best studied in the family while not being representative of Fagaceae biology. By contrast, the subtropical and tropical groups, primarily stone oaks (Lithocarpus) and tropical chestnuts (Castanopsis), are insect-pollinated and produce a variety of nut types. They are scattered across mega-diverse forests, contributing only a small proportion to overall biomass. By expanding our studies of Fagaceae trees to include the entire diversity across the family, we would create a powerful comparative framework for the study of tree adaptation and response to global environmental changes (Table 1). Because of their evolutionary characteristics, the family also provides a good model for comparative biogeography (Petit et al., 2002; Chen et al., 2012).While glacial periods restricted European and North American oaks to refugia, glacial periods had little impact on Mexican oaks (Gonz alez-Rodr ıguez et al., 2004), Californian oaks (Grivet et al., 2006; Gugger et al., 2013), and Southeast Asian stone oaks (Cannon & Manos, 2003). Additionally, frequent hybridization and genetic introgression among species leads to shorter coalescent times within communities, reinforcing biogeographical patterns. These characteristics allow a detailed study of the impact of past climate changes on the patterns of genetic diversification and adaptation found within each group, across several biomes. Given that we cannot understand the future of forests without understanding their past (Petit et al., 2008), a global comparative study of Fagaecae would allow simultaneous interpretations of major shifts across biomes with contrasting responses to glacial-driven changes.