Widespread functional androdioecy inMercurialis annuaL. (Euphorbiaceae)

Abstract

Abstract The widespread coexistence of male and monoecious (cosexual) plants in Spanish, Portuguese and Moroccan populations of Mercurialis annua , an annual wind-pollinated ruderal, represents an important case of functional androdioecy, a rare breeding system in plants and animals. In M. annua , both males and cosexes disperse fully competent pollen. Quantitative gender varies discontinuously between males and cosexes, with males producing a mean of 6.09 times as much pollen as cosexes. It appears that gender is determined by a simple developmental switch, with male and cosexual inflorescences differing markedly in morphology: staminate flowers are borne on erect peduncles in males and in tight spiral clusters around a subsessile pistillate flower in cosexes. Males do not differ from cosexes in their biomass, but they are significantly taller, principally as a result of their greater internode lengths. The cosexual inflorescence is strongly protogynous so that outcrossing is favoured in dense stands, but seed-set is assured in cosexes isolated from prospective mates because of their ability to self-fertilize. Males typically occur at frequencies of less than about 30% in androdioecious populations, in accordance with theoretical predictions for functional androdioecy. In the genus Mercurialis , dioecy is the ancestral condition and monoecy and androdioecy, which occur in polyploid populations of M. annua , are derived. I argue here that androdioecy is most likely to evolve in plants (1) from dioecy, (2) in wind-pollinated species, and (3) in species with a colonizing habit. These predictions are also consistent with the limited published data available for other species.