Floral ontogeny reveals synapomorphies for Senegalia sect. Monacanthea p.p. (Leguminosae)

Abstract

The genus Senegalia was recently described as non-monophyletic; however, its sections exhibit robust monophyletic support, suggesting a potential reclassification into separate genera. Senegalia sect. Monocanthea p.p. is the largest section and contains 164 species of pantropical distribution and includes all of the current 99 neotropical species of Senegalia; however, no morphological characteristics are available to differentiate this section. To characterize this section, we examined floral developmental traits in four species of Senegalia sect. Monocanthea p.p. These traits were previously considered as potentially distinguishing features within Acacia s.l. and include the onset patterns of the androecium, the timing of calyx union, the origin of the staminal disc, and the presence of stomata on the petals. Furthermore, we analyzed previously unexplored traits, such as corolla union types, inflorescence development, and micromorphological features related to the indumentum, as well as the presence and location of stomata. All these characters were analyzed in the context of the relationships among the studied species. The characteristics proposed as potential synapomorphies of the group include the postgenital fusion of the corolla and the presence of a staminal disc formed at the base of the filaments. The other analyzed floral characteristics were not informative for the characterization of the group and exhibited limited correlation with the phylogenetic position of the Senegalia species. Future studies of floral ontogeny will help to establish more precise patterns, mainly whether corolla union and staminal tube formation occur similarly in African and Asian sections of Senegalia.