Wood anatomy helps solve challenging taxonomic relationships in the tropical Sapotaceae

The Sapotaceae (Ericales) are an average-sized pantropical family mostly known for hard and durable timber. The many impressively tall trees of this family are ecologically important in tropical forests, but ongoing (illegal) logging practices threaten many species with extinction. Conservation efforts for Sapotaceae are difficult since lineages have undergone many taxonomic changes over the past two centuries, and classifications based on morphological characters are heavily debated. Molecular phylogenies have improved our knowledge of species relationships to some extent, but a robust classification of the entire family remains lacking. Likewise, wood anatomical studies are often fragmented or unpublished, urging the need for a global study of the family. Hence, we present the first wood anatomical overview of Sapotaceae based on approx. 450 species, summarising earlier unpublished work combined with over 150 new descriptions. We found monophyletic clades can be recognised by wood anatomical traits, such as variation in mineral inclusion type (prismatic crystals, silica bodies, or crystal sand), silica body size, vessel-ray pit variation, ray width, fibre wall thickness, vessel arrangement, and axial parenchyma distribution. We used these traits to perform ancestral state reconstructions on the Chrysophylloideae and Sapotoideae, for which published molecular phylogenies are available with a limited number of chloroplast and/or nuclear markers. Likewise, we did the same on a new genus-level phylogeny made with an angiosperm target capture bait set that includes 353 low-copy nuclear markers. Several striking evolutionary patterns were revealed, such as complex evolutionary shifts in mineral inclusions and independent reversals from simple to scalariform vessel perforations. With this overview, we have summarised wood anatomical variation in the often-ignored Sapotaceae, and hope these results will aid taxonomic and evolutionary studies, as well as conservation efforts on these majestic trees.