Structure and development of flowers and inflorescences in Peraceae and Euphorbiaceae and the evolution of pseudanthia in Malpighiales

Pseudanthia are reduced and compact inflorescences which apparently had independent evolution in Euphorbiaceae and Peraceae within Malpighiales. In order to analyze the hypothesis that the different pseudanthia found in Malpighiales have non-homologous developmental steps, we studied the inflorescence and flower development in the three Malpighiales genera that present this type of inflorescence– Dalechampia (Acalyphoideae/Euphorbiaceae), Euphorbia (Euphorbioideae/Euphorbiaceae), and Pera (Peraceae)–and compared them to that of Joannesia (Crotonoideae/Euphorbiaceae), which does not present a pseudanthium. Inflorescences and flowers were analyzed using light microscopy and scanning electron microscopy. Dalechampia and Euphorbia have protogynic bisexual pseudanthia, with unisexual perianthed flowers in Dalechampia , and achlamydeous flowers in Euphorbia . Pera has unisexual pseudanthia and the male flowers have a vestigial calyx and the female flowers are achlamydeous. Joannesia flowers are very distinct when compared to the pseudanthia flowers, as they are composed of all the whorls and there is no reduction. In the early stages of development, the first structures to be formed in the pseudanthia are the different series of bracts, including outer, involucral and involucel bracts. The floral primordia are initiated almost simultaneously with the involucre. Although the different morphology, the early inflorescence followed the same branching pattern in all studied genera, and the number and elongation of the branches were affected by the early female flower development in the terminal position. We suggest that the different pseudanthia evolved via process of floral whorl reduction and reorganization of flowers in the inflorescence axes, especially the position of female and male flowers and elongation or shortening of the branches. The sex of the terminal flower is a developmental key, i . e ., the protogynic development deeply affects the pseudanthia growth, reducing the ramification and elongation of the axes.