Floral structure and development in Alchornea sidifolia (Acalyphoideae) and the evolution of wind pollination in Euphorbiaceae

Abstract

Most Euphorbiaceae are insect pollinated, but in Acalyphoideae a considerable number of species are wind pollinated and generally have inconspicuous monochlamydeous flowers with red styles and multifid or laciniate stigmata. Alchornea is within the basal alchorneoid clade, and its flowers are more conspicuous in the group. The aim of this study was to investigate the ontogeny, anatomy and vasculature of flowers of Alchornea sidifolia Müll. Arg., species from the Alchorneoids clade, a basal clade sister of the "core acalyphoid," in order to find structural aspects related to wind pollination. Flowers and buds of A. sidifolia were embedded in Paraplast and sectioned with a rotary microtome for analysis under light microscopy. SEM was performed for additional structural and ontogenetic data. Alchornea sidifolia is dioecious and has morphologically and functionally unisexual flowers which only share a monochlamydeous, tetramerous perianth, organized in a single whorl of sepals. In the male flower, calyx begins as a ring on the floral meristem in a congenital connation, but in the female one, the calyx has a late synsepaly. Pistillodes and staminodes are absent in the male and female flowers, respectively. The filaments of the stamens are connate at the base for a short extant and free toward the apex. The gynoecium is syncarpous, and the ovary is mostly synascidiate with a short symplicate zone and two long stigmata. Our results highlight the development of features of the inflorescence and flowers of A. sidifolia which in comparison with insect-pollinated Euphorbiaceae display a set of features which are typically associated in many other unrelated angiosperm lineages with the evolution of wind pollination, such as dioecy, production of a higher number of flowers in the male inflorescences than the female inflorescences, unisexual flowers with reduced perianth and no nectaries, male flowers with a variable number of stamens and female flowers with extensive stigmatic receptive surface. We also discuss the presence of two carpels in Alchornea, when most Euphorbiaceae and many other Malpighiales have three, and its relationship with the wind pollination.