Floral synorganization in acmantheroid clade suggests hypotheses to explain elaiophore suppression in Malpighiaceae

Abstract

Elaiophores in angiosperms first appeared in Malpighiaceae and are fundamental for this family's sexual reproduction. However, they are absent in some lineages. The complete loss of calyx oil glands is an important deviation from the floral conservatism of the Malpighiaceae. Although the literature points to ontogenetic processes involved in the loss of anterior sepal glands in some lineages, the eglandular calyx remains unexplained. Thus, understanding how flowers are losing the main structure responsible for conserving their floral architecture is important to comprehend the processes that drive floral evolution in Malpighiaceae. We selected one species from each genus of the acmantheroid clade, which is the unique lineage with all the main character states related to the occurrence of elaiophores. Floral buds near anthesis were processed using usual techniques for light microscopy. The calyx vasculature is variable and forms vascular complexes shared between sepals, petals, and, in some species, stamens. For the first time, we describe the occurrence of a hypanthium in Malpighiaceae, indicating different levels of floral synorganization. Floral synorganization appears to be the key to understand how specific neotropical lineages broke the floral conservatism in Malpighiaceae with eglandular calyces. The presence of different vascular complexes between perianth parts allows us to infer the group's floral evolution. Thus, we propose two hypotheses that explain the absence of the elaiophores and indicate the ten-elaiophore condition as plesiomorphic for Malpighiaceae.