Development of inflorescences and flowers in Fabaceae subfamily Dialioideae: an evolutionary overview and complete ontogenetic series for Apuleia and Martiodendron

The goal of this study was to better understand the origin and development of flowers and inflorescences in the newly established subfamily Dialioideae, an unusual and morphologically variable clade of Fabaceae due to its varied levels of floral reduction. We present here the complete ontogenetic series for two species characterizing different levels of floral reduction: Apuleia leiocarpa, an andromonoecious species with trimerous flowers; and Martiodendron fluminense, a species lacking the inner whorl of stamens. We also performed a literature review and herbarium specimen survey of the inflorescence and floral morphology of the other 15 genera in Dialioideae. Among the exclusive traits of Apuleia found here are the absence of two sepals and petals from initiation, the simultaneous initiation of the sepals (never before documented for Dialioideae), the absence of carpel initiation in staminate flowers and the formation of the carpel in the staminal whorl of monoclinous flowers, with the presence of a nectariferous hypanthium in both flower types. In Martiodendron the two exclusive traits are the heteromorphic development of stamens of the outer whorl, with the abaxial one being the last to elongate, and the possible initiation of an inner staminal whorl, which stops developing immediately thereafter and is no longer visible at anthesis. Among the potential synapomorphies for the subfamily are the absence of bracteoles and a pair of bracts subtending a triad of flowers or inflorescence axes, the distichous anthotaxy of the thyrsoid inflorescences, the bidirectional initiation of the sepals and the simultaneous initiation of the stamens.