Parallel floral evolution of Lilium species pollinated by pierid butterflies

Abstract

Although pollinators are often categorized into broadly defined functional groups, much of floral diversity may actually reflect adaptation to different subdivisions within such broad pollinator groups. In Lilium, some species have white or pink flowers of moderate size (the pierid type), in contrast to those with relatively large orange or red flowers that are pollinated by swallowtail butterflies (the swallowtail type). Here we test whether species of the pierid type are pollinated by relatively small butterflies and whether these species belong to a monophyletic clade or whether this pollination system originated through convergent evolution. We observed floral visitors, examined their pollination effectiveness and the breeding systems of four eastern Asian Lilium species, measured floral traits, and conducted a phylogenetic analysis involving all species of the pierid type and several of the swallowtail type. Each of the species were visited by a variety of insects, but pierids most frequently contacted stigmas. These butterflies carried pollen predominantly on their hindwings. Visitors other than pierids deposited few pollen grains per visit. The focal species are highly self-incompatible and rely completely on pollinators for seed production. They differ from the swallowtail type mainly in floral coloration and size. In particular, the difference in size between the two types can likely be attributed to matching butterflies of different sizes. Species of the pierid type have independently evolved at least three times and therefore their floral similarity reflects floral convergence. Thus, like other pollinator groups, butterflies comprise subdivisions in relation to distinct floral syndromes, which have contributed to the evolution of floral diversity of Lilium.