Anatomical conservatism in the wood and bark of the species-rich Byrsonimoid clade (Malpighiaceae)

Abstract The byrsonimoids include approximately 152 species of trees and shrubs native to the Neotropical region. The clade is monophyletic and consists of three genera: Byrsonima, Blepharandra, and Diacidia. Byrsonima woods are among the most well-known within the Malpighiaceae due to their economic importance. Nonetheless, here for the first time we explore the interspecific stem anatomical diversity of Byrsonima, its similarities and differences to Blepharandra and Diacidia, and how the previously proposed taxonomic circumscriptions for the genus Byrsonima are reflected or not in their wood and bark anatomy. Using a newly reconstructed phylogeny for the Malpighiaceae focusing on the clade Byrsonimoid and a broad selection of species, we conducted phylogenetic comparative analyses to determine how wood and bark traits have evolved across evolutionary time. Our results indicate stem anatomical homogeneity within the clade, particularly in wood traits. On one hand, the presence of septate fibres and prismatic crystals in the rays distinguishes Byrsonima from other members of the clade, but on the other hand, wood and bark traits do not support the classical infrageneric classifications for the genus. The bark in byrsonimoids displays the entire range of variation known for sclerenchyma in this tissue, from cells that can be considered true fibres to fibre-sclereids and typical true sclereids. Ancestral character state reconstructions revealed that radial arrangement, septate fibres, thin heterocellular rays, marginal parenchyma, and prismatic crystals are ancestral traits within the clade, with homoplasy common within the group. These observations demonstrate anatomical conservatism within the Byrsonimoid clade, which might be either related to their proposed recent diversification or a highly effective set of positively selected features.