The Wood Anatomy of Sapindales: Diversity and Evolution of Wood Characters

The Sapindales are a large order comprised of trees, shrubs, lianas and rarely herbs. This lineage is mostly pantropical with important temperate lineages, inclusing some of the most diverse, highly prized woods in the world, such as mahogany (Swietenia). In this study, we characterized the wood anatomy across eight of the nine Sapindales families, and leverage phylogenetic comparative methods to explore the evolution of wood traits in the order. We delimited 23 characters and reconstructed them onto the most recent time-calibrated phylogeny for the group. We found that ring-porosity is derived within the order, coinciding with the occupation of more seasonal climates; marginal parenchyma is ancestrally present, but largely lost in Anacardiaceae-Burseraceae-Kirkiaceae; vessels in radial chains are ancestrally absent but gained many times; scanty paratracheal parenchyma was ancestrally present with multiple evolutions of more abundant parenchyma. Anacardiaceae-Burseraceae-Kirkiaceae share tyloses and large vessel-ray pits. Radial ducts are exclusive to Anacardiaceae-Burseraceae, while traumatic ducts are exclusive to Meliaceae-Rutaceae-Simaroubaceae. Rays are generally 2-4 cells wide, heterocellular, but with multiple lineages evolving homocellular narrow rays or more heterocellular wide rays. Prismatic crystals are commonly located in rays in Anacardiaceae-Burseraceae while in the other families they are mainly in axial parenchyma. Silica bodies are abundant in Burseraceae, but also present in Anacardiaceae and Meliaceae. Lianas are exclusively in Anacardiaceae and Sapindaceae, with Sapindaceae displaying an enormous diversity of cambial variants. Our work unravels several potential synapomorphies of Sapindales major clades, and evolutionary patterns for the enormous wood anatomical diversity of the order. In addition, our work highlights variable characters worth of more detailed studies within individual families of the Sapindales.