Evolution of pollinator generalization in bumblebee‐pollinated plants

Abstract To understand the relationships between floral morphologies and the reproductive strategies of plants, the contributions of multiple pollinators to plant reproductive success need to be examined. To outline recent advances on this topic, we review studies of five bumblebee‐pollinated plants in Japan. These plant species are pollinated by one to three bumblebee species differing in proboscis length: Bombus consobrinus , Bombus diversus and Bombus honshuensis. These pollination systems allow us to compare the reproductive strategies of plants pollinated by single and multiple bumblebee species. First, we address the effects of morphological correspondence between flowers and pollinators on the reproductive success of plants. In plants with narrow tubular flowers, Isodon umbrosus and Clematis stans , pollination effectiveness depended on the morphological correspondence between floral tube length and pollinator mouthparts. These flowers are subject to varied directions and intensities of selection by multiple bumblebee species, resulting in the maintenance of large variation in floral morphology within and between populations, and even within individuals. In contrast, pollination effectiveness in plants having flowers with a wide entrance, Melampyrum roseum and Hosta sieboldiana , did not differ among bumblebee species because of good correspondence between floral morphology and bumblebee body size, resulting in small morphological variations among flowers. Thus, generalized pollination systems with multiple pollinator species favor floral traits that enable the use of various pollinators. Second, we address the quantity and quality of the pollination service in bumblebee‐pollinated plants. Isodon umbrosus , with its generalized pollination system, suffered from pollen limitation because of low pollination quality. In contrast, Isodon effusus , which has a specialized pollination system, suffered from pollen limitation because of low pollination quantity. Isodon umbrosus produced large numbers of short tubular flowers to improve pollination quantity, whereas I. effusus produced small numbers of long tubular flowers to improve pollination quality, resulting in a clear trade‐off between the size and number of flowers within plants. In M. roseum , which has a generalized pollination system, three bumblebee species achieved sufficient pollination, but seed production was limited by an increased rate of self‐pollination. This is because this species invests more resources in cross‐pollinated seeds by selective abortion of selfed seeds. Our results demonstrate that the strategy of generalized plants is to improve pollination quantity, whereas the strategy of specialized plants is to improve pollination quality.