Floral development and morphology of the mistletoe Antidaphne viscoidea: a case of extreme flower reduction in the sandalwood family (Santalaceae)

Flowers in the Antidaphne genus are among the smallest in family Santalaceae. We traced the development and morphology of flowers in A. viscoidea and compared them with those in other members of the family. Both staminate and carpellate inflorescences proliferate through collateral and serial buds, and become dimorphic when fully elongated. The former are short racemes with a terete axes and a protective, cup-like distalmost bract; the latter are spikes with flattened, furrowed and longer axes with a leafy, non-protective bract. The staminate flowers (traditionally described as apetalous) initiate from transversely flattened primordia, from which 3–5 stamens initiate opposite to variously reduced petals in an abaxial-to-adaxial direction. A massive, intrastaminal, lobed nectariferous disc, likely gynoecium-derived, is formed, with the lobes alternate to the stamens. The carpellate flowers initiate from radially symmetric primordia; they also possess three or four perianth organs. Floral monosymmetry of staminate flowers is likely due to mechanical constraint rather than to floral reduction. Our data support the current phylogenetic relationship between Antidaphne and the neotropical genera Eubrachion and Lepidoceras, all having unisexual flowers, lacking trichomes between the petals and stamens, sessile stigmas, and baccate fruits. The nectariferous disc is likely apomorphic for Santalaceae.