Sexual dimorphism in flower size

Sexual dimorphism in the secondary characters of unisexual flowers is often observed. Most prior reports indicate that male flowers have larger petals than female flowers. In this article we examine hypotheses regarding patterns of perianth (sepals and petals) size dimorphism. (1) Developmental associations between the corolla and the stamens constrain the independent evolution of these characters. (2) The role of the perianth in enclosing the reproductive structures in the bud results in a correlation between the size of the enclosed structures and the perianth parts. (3) In animal-pollinated species, the perianth serves to attract pollinators; fitness gains achieved through allocation to attractive structures differ between the sexes. To test these hypotheses, we compared 919 species with unisexual flowers; quantitative measurements of floral parts were made for 84 of these species. Unlike most previous studies, this study found that the petals of male flowers were larger than those of females in less than one-half of the cases, demonstrating that developmental associations are not strictly responsible for patterns of sexual dimorphism. Relative perianth size appears to be evolutionarily labile, even within genera. Patterns of perianth-size dimorphism differed between temperate and tropical species and between animal- and wind-pollinated species. Perianth size was strongly associated with the size of the reproductive structures enclosed in each flower, suggesting that the function of petals and sepals to protect developing reproductive structures may determine their size. However, the size of the reproductive structures was a better predictor of perianth size in wind-than in animal-pollinated species, which supports the attractive function hypothesis of the perianth. We conclude that the two functional roles of the perianth, protection and attraction, are the primary determinants of perianth size in unisexual flowers.