Dichogamy and sexual dimorphism in floral traits in the andromonoecious Euphorbia boetica

Abstract

• Background and Aims: Euphorbia boetica (Euphorbiaceae) is a functional andromonoecious species that shows both intra- and interfloral dichogamy, hermaphrodite cyathia being protogynous. Sexual dimorphism of the cyathia of E. boetica is examined according to their gender and arrangement on the inflorescence. • Methods: Data were obtained from two natural populations, where the distribution of male and hermaphrodite cyathia in the inflorescence was recorded. The size, pollen production and viability, and nectar secretion were measured in both types of cyathia. • Key Results: Most cyathia were male at the first levels of the inflorescence, then hermaphrodite cyathia predominated at the successive levels, although at the last levels the proportion of male cyathia increased. Male cyathia at basal positions lack ovaries, whereas those at distal positions showed vestigial ovaries. The size of the cyathia varied significantly depending on the level of the inflorescence where they were produced: those of the last levels were usually smaller. The hermaphrodites were significant bigger than males; however, these differences were due to the differential distribution of each cyathium type in the inflorescence. Male cyathia produced significantly more pollen and nectar than hermaphrodites. • Conclusions: In Euphorbia boetica, basal male cyathia could be explained by the presence of protogyny, and apical male cyathia seem to respond to a preemption of resources. A true dimorphism affecting primary sexual characters and related to gender function appears at lower levels of the inflorescence, whereas an apparent size dimorphism due to positional effects occurs at upper positions. Longevity and distribution of cyathia, and their pattern of nectar production, could improve both male and female fitness. © The Author 2005. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.