A mid-cretaceous origin of sociality in xylocopine bees with only two origins of true worker castes indicates severe barriers to eusociality.

Sandra M Rehan,Michael P Schwarz,Remko Leys,Gro V Amdam

doi:10.1371/journal.pone.0034690

Sandra M Rehan, Michael P Schwarz + Show 2 more

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https://doi.org/10.1371/journal.pone.0034690

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Abstract

The origin of sterile worker castes, resulting in eusociality, represents one of the major evolutionary transitions in the history of life. Understanding how eusociality has evolved is therefore an important issue for understanding life on earth. Here we show that in the large bee subfamily Xylocopinae, a simple form of sociality was present in the ancestral lineage and there have been at least four reversions to purely solitary nesting. The ancestral form of sociality did not involve morphological worker castes and maximum colony sizes were very small. True worker castes, entailing a life-time commitment to non-reproductive roles, have evolved only twice, and only one of these resulted in discrete queen-worker morphologies. Our results indicate extremely high barriers to the evolution of eusociality. Its origins are likely to have required very unusual life-history and ecological circumstances, rather than the amount of time that selection can operate on more simple forms of sociality.

Highlights

The evolution of life has been marked by a number of major but very infrequent transitions, such as the origins of eukaryotes, multicellularity, and sexual reproduction [1]
All analyses indicated that the Xylocopini comprise the most basal tribe and Manueliini was the next-most basal tribe (Figure 2 and Figures S1, S2, S3 electronic supplementary material)
We varied the set age of the root node to 120, 100 and 90 Mya to explore the effect of uncertainty in age of this node (Table 1)

Summary

Introduction

The evolution of life has been marked by a number of major but very infrequent transitions, such as the origins of eukaryotes, multicellularity, and sexual reproduction [1]. The question of why eusociality has been so successful, yet its origins have been so rare, has been a major puzzle in evolutionary biology. One approach to this issue has involved identifying socalled ‘pre-adaptations’ or ‘conditions’ for eusociality – combinations of genetic, life-history and ecological features that facilitate the evolution of strong forms of altruism [2,3,4,5]. Common factors might be identifiable, but whether or not they are sufficient is less straightforward

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