Evolution and systematic value of fruit and seed characters in Adoxaceae (Dipsacales)

This study presents a description of the morphology and anatomy of the fruits and seeds of Adoxa, Sambucus, and Viburnum. A phylogenetic study included Adoxa moschatellina, Sinadoxa corydalifolia, Sambucus, Tetradoxa omeiensis, and Viburnum. All major clades of Caprifoliaceae, the sister family of Adoxaceae, were sampled and used as outgroup. Maximum parsimony and Bayesian inference optimality criteria were used to analyze three datasets, molecular data (ITS, trnK, matK), morphological data (21 fruit and seed characters), and a combined dataset. We used parsimony optimization to study character evolution. Results of the phylogenetic analyses were congruent with previously published results. Viburnum is the basalmost lineage in the family and sister to subfamily Adoxoideae. In Adoxoideae Sambucus is sister to the Adoxina clade (i.e., Adoxa, Sinadoxa, and Tetradoxa). Our results also confirmed the sister relationship between Sinadoxa corydalifolia and Adoxa plus Tetradoxa. For the first time, clear autapomorphies are described for sambucus, such as a long, slender, cylindrical embryo, sclereids with a u-shaped sclerification pattern, a rugose endocarp surface, and numerous, small calcium oxalate crystals in the endocarp sclereids. Character evolution in Sambucus, however, is complex and a broader sampling is required to draw firm conclusions. The evolution toward Viburnum is marked by a number of shifts such as the development of small, amorphous crystals in endosperm cells and a well-developed, parenchymatous seed coat. Based on our results and previous studies we hypothesize a paedomorphic event took place at the origin of Adoxina. Evidence supporting this hypothesis is the presence of green fruits, weakly sclerified endocarp sclereids, small seeds, and semi-inferior ovaries in Adoxina. Finally, our results indicate that the first Adoxaceae were characterized by tricarpellate ovaries with an endocarp composed of an outer layer of sclereids and an inner layer of fibers. A thorough study of the fruits and seeds of Sinadoxa and Tetradoxa is vital to fully understand fruit and seed evolution in Adoxaceae.