Adaptation and selection in the Senecio (Asteraceae) hybrid zone on Mount Etna, Sicily

Authors

  • Adrian C. Brennan,

    1. Sir Harold Mitchell Building, School of Biology, University of St Andrews, St Andrews, Fife KY16 9TH, UK;
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    • *

      These authors contributed equally to this work.

  • Jon R. Bridle,

    1. School of Biological Sciences, University of Bristol, Bristol, BS8 1UG, UK;
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    • *

      These authors contributed equally to this work.

  • Ai-Lan Wang,

    1. Sir Harold Mitchell Building, School of Biology, University of St Andrews, St Andrews, Fife KY16 9TH, UK;
    2. Key Laboratory of Arid and Grassland Ecology, Lanzhou University, Lanzhou, Gansu 730000, China
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  • Simon J. Hiscock,

    1. School of Biological Sciences, University of Bristol, Bristol, BS8 1UG, UK;
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  • Richard J. Abbott

    1. Sir Harold Mitchell Building, School of Biology, University of St Andrews, St Andrews, Fife KY16 9TH, UK;
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Author for correspondence:
A. C. Brennan
Tel: +44 (0)1334 463377
Email: acb16@st-andrews.ac.uk

Summary

  • • Hybrid zone theory provides a powerful theoretical framework for measuring and testing gene flow and selection. The Senecio aethnensis and Senecio chrysanthemifolius hybrid zone on Mount Etna, Sicily, was investigated to identify phenotypic traits under divergent selection and to assess the contributions of intrinsic and extrinsic selection against hybrids to hybrid zone maintenance.
  • • Senecio samples from 14 sites across Mount Etna were analyzed for 24 quantitative traits classified into four groups (QTGs), six allozymes and seven simple sequence repeat (SSR) loci to describe patterns of variation throughout the hybrid zone.
  • • Narrower cline widths or shifts in cline centre position were observed for three QTGs relative to the molecular clines, indicating that these traits are likely to be under extrinsic environmental selection. Altitude was key to describing species distributions, but dispersal and intrinsic selection against hybrids explained patterns at smaller spatial scales. The hybrid zone was characterized by strong selection against hybrids, high dispersal rates, recent species contact and few loci differentiating QTGs based on indirect measures.
  • • These results support the hypothesis that extrinsic and intrinsic selection against hybrids maintains the hybrid zone and species distinctiveness despite gene flow between the two Senecio species on Mount Etna.

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