Proc Biol Sci. 2011 Dec 7;278(1724):3593-600. doi: 10.1098/rspb.2011.0365. Epub 2011 Apr 13.

Why do leafcutter bees cut leaves? New insights into the early evolution of bees.

Litman JR, Danforth BN, Eardley CD, Praz CJ.

Source

Department of Entomology, Cornell University, Ithaca, NY 14853, USA.

Abstract

Stark contrasts in clade species diversity are reported across the tree of life and are especially conspicuous when observed in closely related lineages. The explanation for such disparity has often been attributed to the evolution of key innovations that facilitate colonization of new ecological niches. The factors underlying diversification in bees remain poorly explored. Bees are thought to have originated from apoid wasps during the Mid-Cretaceous, a period that coincides with the appearance of angiosperm eudicot pollen grains in the fossil record. The reliance of bees on angiosperm pollen and their fundamental role as angiosperm pollinators have contributed to the idea that both groups may have undergone simultaneous radiations. We demonstrate that one key innovation--the inclusion of foreign material in nest construction--underlies both a massive range expansion and a significant increase in the rate of diversification within the second largest bee family, Megachilidae. Basal clades within the family are restricted to deserts and exhibit plesiomorphic features rarely observed among modern bees, but prevalent among apoid wasps. Our results suggest that early bees inherited a suite of behavioural traits that acted as powerful evolutionary constraints. While the transition to pollen as a larval food source opened an enormous ecological niche for the early bees, the exploitation of this niche and the subsequent diversification of bees only became possible after bees had evolved adaptations to overcome these constraints.

PMID:: 21490010; [PubMed - indexed for MEDLINE]
PMCID:: PMC3189370

Free PMC Article

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Figure 1.

(Opposite.) Fossil-calibrated maximum clade credibility tree for bee family Megachilidae. (a) Bayesian posterior probabilities and maximum-likelihood bootstrap values shown above and below nodes, respectively, for all clades older than 50 Myr. Terminals are labelled to tribe according to present taxonomic assignment, even if determined to be paraphyletic in the current analysis. Branch colours correspond to significant changes in diversification rate (black: diversification rate = 0.0164, relative extinction = 0.885; red: diversification rate = 0.0867, relative extinction = 0.848; blue: diversification rate = 0.315, relative extinction = 0.518). The node marked with a green star corresponds to the transition between building unlined nests and building nests using foreign material. There is no reversion to building unlined nests after this point. Photographs to the right of phylogeny from top to bottom: (1) Tribe Fideliini: Fidelia villosa using hind legs to excavate sand from a burrow (photo: Jerome G. Rozen [14], courtesy of the American Museum of Natural History); (2) Tribe Lithurgini: Lithurgus chrysurus entering nest in dead tree trunk (photo: Andreas Müller); (3) Tribe Anthidiini: Anthidium strigatum closing a nest cell of resin (photo: Albert Krebs); (4) Tribe Osmiini: nest of Osmia bicolor built in an abandoned snail shell (photo: Albert Krebs); (5) Tribe Megachilini: (top) Megachile parietina entering her nest made of mud (photo: Albert Krebs); (bottom) Megachile ligniseca using her mandibles to cut a leaf disc (photo: Felix Amiet). (b) Biogeographic reconstructions indicate a Gondwanan origin for Megachilidae, approximately 126 Ma (figure reprinted from [45], copyright 1988, with permission from Elsevier). (c) The ancestor of all Megachilidae built unlined nests in sandy soil, much like extant lineages Fidelia, Neofidelia and Pararhophites (nest of Fidelia villosa shown; picture: Jerome G. Rozen [14], courtesy of the American Museum of Natural History). (d) Host plants of Fideliini (electronic supplementary material, table S4). Top row (left to right): Nolana sp. (Solanaceae; host of Neofidelia longirostris; photo: Michael O. Dillon), Calandrinia sp., Trichocereus sp. (Portulacaceae and Cactaceae, respectively; hosts of N. profuga; photos: Joshua R. McDill, Scott Zona); centre row (left to right): Sesamum sp. (Pedaliaceae; host of Fidelia friesei; photo: Jessica Litman), Psilocaulon sp. (Aizoaceae; host of F. villosa, F. kobrowi, F. paradoxa; photo: Jessica Litman), Sisyndite spartea (Zygophyllaceae; host of F. pallidula; photo: Tomas Hajek); bottom row (left to right): Grielum sp. (Neuradaceae; host of F. hessei, F. major, F. fasciata; photo: Serban Proches), Berkheya fruticosa (Asteraceae; host of F. braunsiana; photo: Henry Brisse), Convolvulus trabutianus (Convolvulaceae; host of F. ulrikei; photo: Pierre-Marie Roux). Not shown: Tribulocarpus dimorphanthus (Aizoaceae; host of F. ornata). Note that all flowers are characterized by radial symmetry and exposed anthers.

Why do leafcutter bees cut leaves? New insights into the early evolution of bees

Proc Biol Sci. 2011 December 7;278(1724):3593-3600.