Achik Dorchin, Dafna Langgut, Frank Neumann, Nicolas Vereecken
Flowers with specialised pollination mechanism with concealed pollen, such as in the Fabaceae and Lamiaceae, are pollinated primarily by bees and often referred to as ‘bee flowers’. While some long-tongue bees show preference to bee flowers with restricted pollen, other species specialize on pollen accessible flowers or are floral generalists. Recent studies suggested that pollen is not an easy-to-use resource, such that changes in floral host preference would strongly depend on physiological and neurological constraints of bees. It can be therefore hypothesised that exploitation of restricted pollen from bee flowers consisted a significant event in bee evolution, opening a new ecological niche that has resulted in increased diversification of the associated bee lineages. So far, no empirical evidence for increased diversification rates has been demonstrated in bees in association with exploitation of restricted pollen from bee flowers.
This study uses phylogenetic inference and determination of pollen grains collected by ca. 390 females from ca. 80 species to trace the evolution of floral host preference in longhorn bees of the widely distributed genus Eucera. Based on the pollen spectrum collected, each species is assigned to one of the floral preference categories: 1. pollen accessible flowers; 2. pollen restricted flowers; or 3. generalist with regard to pollen accessibility. Reconstruction of ancestral floral preferences using Bayesian methods show that early diverging Eucera lineages are mostly associated with accessible pollen whereas more recently diverging lineages are capable of utilising both accessible and restricted pollen or prefer restricted pollen. We use different approaches, including character state dependent speciation-extinction (SSE), method-of-moments, and Phylogenetic Generalised Least Squares (PGLS), to test the hypothesis that a main switch event from utilising accessible pollen to restricted pollen form bee flowers has increased the rate of diversification in Eucera bees.