Abstract
The role of plant–pollinator interactions in the rapid radiation of the angiosperms have long fascinated evolutionary biologists. Studies have brought evidence for pollinator-driven diversification of various plant lineages, particularly plants with specialized flowers and concealed rewards. By contrast, little is known about how this crucial interaction has shaped macroevolutionary patterns of floral visitors. In particular, there is currently no empirical evidence that floral host association has increased diversification in bees, the most prominent group of floral visitors that essentially rely on angiosperm pollen. In this study, we examine how floral host preference influenced diversification in eucerine bees (Apidae, Eucerini), which exhibit large variations in their floral associations. We combine quantitative pollen analyses with a recently proposed phylogenetic hypothesis, and use a state speciation and extinction probabilistic approach. Using this framework, we provide the first evidence that multiple evolutionary transitions from host plants with accessible pollen to restricted pollen from ‘bee-flowers’ have significantly increased the diversification of a bee clade. We suggest that exploiting host plants with restricted pollen has allowed the exploitation of a new ecological niche for eucerine bees and contributed both to their colonization of vast regions of the world and their rapid diversification.
Highlights
The importance of plant–pollinator interactions for speciation has been widely described in the literature, but so far focused on radiation of flowering plants with pollinators mainly being considered as the driving vehicle for their evolutionary success [1,2,3,4]
Increased specificity of floral visitors that are associated with specialized flowers has promoted speciation of plant lineages via floral isolation, other isolating factors contributed to their speciation [3,6]
Our results show a phylogenetic distribution pattern of pollen hosts, where early-diverging lineages are almost exclusively associated with accessible pollen, and recently diverging lineages associated with restricted pollen, either exclusively or in addition to accessible pollen
Summary
The importance of plant–pollinator interactions for speciation has been widely described in the literature, but so far focused on radiation of flowering plants with pollinators mainly being considered as the driving vehicle for their evolutionary success [1,2,3,4]. There is empirical evidence for increased diversification rates in plants with zygomorphic flowers and long corolla tubes or nectar spurs that conceal floral rewards [3,4,5]. Increased specificity of floral visitors that are associated with specialized flowers has promoted speciation of plant lineages via floral isolation, other isolating factors contributed to their speciation [3,6]. Because many bees take nectar from the same flowers that provide them with pollen, LT bees had presumably evolved to exploit both nectar and pollen from bee-flowers Fossil evidence support this assumption by showing that the appearance of LT bees in the family Apidae coincide with the rapid radiation of more specialized floral configurations, such as bilaterally symmetric zygomorphic flowers [16]. Namely flower buzzing [22] and utilization of floral oils [23], were not significantly associated with increased bee diversification, they contributed to increase species richness in the former and habitat occupancy in the latter
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