Diversity and leaflet anatomical characters evolution of Machaerium Pers. (Fabaceae – Faboideae)

Taxonomic and phylogenetic studies of Machaerium species have demonstrated overlapping morphological characters that delimit sections and/or clades, such as width, length, and overall shape of the leaflets and venation pattern. We conducted a comparative analysis of the morphoanatomical characters of Machaerium leaflets. We aimed to evaluate the usefulness of leaflet anatomical characters for Machaerium systematics, to assess whether these characters support the monophyly of the Lineata clade, to identify leaflet anatomical synapomorphies for the Lineata clade, and to evaluate secretory structures present in the leaflets regarding the type, location, distribution, and chemical nature of their exudates. Subterminal leaflets from 26 Machaerium species were examined by light and scanning electron microscopy. Phylogenetic and ancestral character state reconstruction analyses were performed for all members of the Lineata clade. Characters such as secondary venation pattern, type of stomata, presence of a subepidermal layer at the leaflet margin (hypodermis), trichome morphology, and the type and location of mucilaginous idioblasts are relevant at the species level. Secretory structures such as secretory-base trichomes, mucilaginous idioblasts, and phenolic idioblasts are confirmed for the species. Craspedodromous venation, papillose epidermis and marginal vascular bundles are shared by species of the Lineata clade and are considered a possible synapomorphy for the group. Our data reveal new useful characters for taxonomy of Machaerium species, reinforce the recognition of the monophyly of the Lineata clade, and support future studies focused on the systematics of the genus.