Format: ORAL

Vicky Beckers1,2, Mary Endress3, Pieter Baas1, Erik Smets1,2 and Frederic Lens1,2

1 Naturalis Biodiversity Center, Leiden, The Netherlands 2 Leiden University, Leiden, The Netherlands 3 University of Zurich, Zurich, Switzerland

Apocynaceae are a wonderfully diverse group, widely studied for their variable evolutionary processes. The stem is one of the most functionally important plant parts, yet a wood anatomical overview of this family is missing. Here, we present a wood anatomical overview of the Apocynaceae, including reconstructions of wood evolution. To do so, we revised over 200 published wood anatomical descriptions and made original sections and light microscopic descriptions for another ~50 species. Changes in wood anatomical characters through time were visualized with ancestral state reconstructions on a selection of 26 informative traits for 147 woody species included in the familys most recent molecular phylogeny. We performed tests for correlated evolution on a combined dataset of anatomical and published morphological traits to identify potential synnovations and key traits associated with growth form adaptations. The most important finding was a shift towards a syndrome of derived wood anatomical traits within the paraphyletic apocynoid grade that include an increased presence of vessel multiples, vessel dimorphism, laticifers, cambial variants, and paratracheal axial parenchyma. These traits separate the grade into early- and later diverging tribes, and continue in the monophyletic Periplocoideae, Secamonoideae, and Asclepiadoideae. When we compared these trait reconstructions with morphology traits evolution, we found that key morphological innovations co-occur with wood characters on consecutive nodes in the familys phylogeny. In contrast to these abrupt trait changes, vessel elements and fibers gradually shorten along the phylogenetic backbone of the family. This correlates to a general plant size reduction, regardless of the many transitions towards (phylogenetically) derived woodiness and a scandent or erect growth form. In conclusion, there are clear evolutionary transitions in the wood anatomy of Apocynaceae many associated with climbers representing structural adaptations that are part of a cascade of morphological evolutionary changes across the family.