Genetic differentiation across the social transition in a socially polymorphic sweat bee, Halictus rubicundus

Authors

  • A. SORO,

    1. School of Biological Sciences, Queen’s University Belfast, 97 Lisburn Road, Belfast BT9 7BL, UK
    2. Animal Physiological Ecology, University of Tübingen, Konrad-Adenauer Str. 20, D-72072 Tübingen, Germany
    3. Department of Entomology, Cornell University, Ithaca, NY 14853, USA
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  • J. FIELD,

    1. School of Life Sciences, University of Sussex, John Maynard Smith Building, Brighton BN1 9QG, UK
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  • C. BRIDGE,

    1. School of Life Sciences, University of Sussex, John Maynard Smith Building, Brighton BN1 9QG, UK
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  • S. C. CARDINAL,

    1. Department of Entomology, Cornell University, Ithaca, NY 14853, USA
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  • R. J. PAXTON

    1. School of Biological Sciences, Queen’s University Belfast, 97 Lisburn Road, Belfast BT9 7BL, UK
    2. Department of Entomology, Cornell University, Ithaca, NY 14853, USA
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A. Soro, Fax: +1 607 255 0939; E-mail: a.soro@qub.ac.uk and as2426@cornell.edu

Abstract

Eusociality is widely considered a major evolutionary transition. The socially polymorphic sweat bee Halictus rubicundus, solitary in cooler regions of its Holarctic range and eusocial in warmer parts, is an excellent model organism to address this transition, and specifically the question of whether sociality is associated with a strong barrier to gene flow between phenotypically divergent populations. Mitochondrial DNA (COI) from specimens collected across the British Isles, where both solitary and social phenotypes are represented, displayed limited variation, but placed all specimens in the same European lineage; haplotype network analysis failed to differentiate solitary and social lineages. Microsatellite genetic variability was high and enabled us to quantify genetic differentiation among populations and social phenotypes across Great Britain and Ireland. Results from conceptually different analyses consistently showed greater genetic differentiation between geographically distant populations, independently of their social phenotype, suggesting that the two social forms are not reproductively isolated. A landscape genetic approach revealed significant isolation by distance (Mantel test r = 0.622, < 0.001). The Irish Sea acts as physical barrier to gene flow (partial Mantel test r = 0.453, < 0.01), indicating that geography, rather than expression of solitary or social behaviour (partial Mantel test r = −0.238, = 0.053), had a significant effect on the genetic structure of H. rubicundus across the British Isles. Although we cannot reject the hypothesis of a genetic underpinning to differences in solitary and eusocial phenotypes, our data clearly demonstrate a lack of reproductive isolation between the two social forms.

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