Specialized and Generalized Pollen-Collection Strategies in an Ancient Bee Lineage

Abstract

Iconic examples of insect pollination have emphasized narrowly specialized pollinator mutualisms such as figs and fig wasps and yuccas and yucca moths. However, recent attention by pollination ecologists has focused on the broad spectra of pollinated plants by generalist pollinators such as bees. Bees have great impact for formulating hypotheses regarding specialization versus generalization in pollination mutualisms. We report the pollination biology of six northern European species of an extinct tribe of pollen-basket-bearing apine bees, Electrapini, of early-middle Eocene age, examined from two deposits of 48 and 44 million years in age. These bees exhibit a pattern of generalized, incidental pollen occurring randomly on their heads, thoraces, and abdomens, obtained from diverse, nectar-bearing plants. By contrast, a more restricted suite of pollen was acquired for metatibial pollen baskets (corbiculae) of the same bee taxa from a taxonomically much narrower suite of arborescent, evergreen hosts with uniform flower structure. The stereotyped plant sources of the specialist strategy of pollen collection consisted of pentamerous, radially symmetrical flowers with a conspicuous gynoecium surrounded by prominent nectar reward, organized in structurally similar compound inflorescences. Pollen specialization in bees occurs not for efficient pollination but rather in the corbiculate Electrapini as food for bee larvae (brood) and involves packing corbiculae with moistened pollen that rapidly loses viability with age. This specialist strategy was a well-developed preference by the early Eocene, providing a geochronologic midpoint assessment of bee pollen-collection strategies.

Assessment of pollen rewards by foraging bees

“…; Wappler et al . ). However, among modern bees, only a few truly monolectic species remain, as over evolutionary time increases in the breadth of pollen diets have been more common than restrictions (Müller ; Danforth, Conway & Ji ).…”

“…Pollen is diverse in form and the proportions of key nutrients vary considerably, which is likely to make foraging choices and the assessment of profitability a more complex task. One solution would be to establish foraging selectivity by specializing on pollen of particular plants or plant families, and indeed the majority of early bees were oligolectic (Michez et al 2008;Wappler et al 2015). However, among modern bees, only a few truly monolectic species remain, as over evolutionary time increases in the breadth of pollen diets have been more common than restrictions (Müller 1996;Danforth, Conway & Ji 2003).…”

An Early Miocene bumble bee from northern Bohemia (Hymenoptera, Apidae)

“…Varied extinct lineages representing stem groups or breaking the otherwise long branches between our modern corbiculates have been discovered from the Paleogene (Cockerell 1908, Engel 1998a, 2001a, Wappler and Engel 2003, Patiny et al 2007, Engel et al 2013, 2014), and some of these reveal that the bombine habitus is overall generalized and plesiomorphic for the Corbiculata ( e.g ., Engel 2001a). These extinct clades are also the fossils for which the most information has been accumulated regarding their pollen-collecting behaviors (Wappler et al 2015, Grímsson et al 2017). While controversy remains regarding their relationship to either Meliponini or Meliponini + Apini ( e.g ., Michener 1990, Schultz et al 1999, 2001, Engel 2000a, 2001b, Noll 2002, Cardinal and Packer 2007, Kawakita et al 2008, Kwang et al 2017), the 263 extant species of Bombini are likely a comparatively young, monophyletic crown group at the apex of an otherwise older lineage diverging from a common ancestor with meliponines and apines sometime in the latest Cretaceous (Engel 2000, 2001a), leaving a ghost record of stem groups between this divergence and perhaps the Early to mid-Eocene.…”

Taxonomic description of in situ bee pollen from the middle Eocene of Germany

“…Both angiosperm groups include many wind-pollinated modern genera and species, and, in the case of Fagales, which account for over 20% of the LM-diagnosed pollen taxa at both localities, are predominately or partly wind-pollinated. In addition, eusocial bees harvest pollen within a limited range of their dwellings, and the extinct species may have had collection preferences similar to some of their modern-day relatives (Wappler et al 2015), almost all of whom are polylectic (e.g. Apini, Meliponini, Bombini).…”