Journal Article

Exogenous selection shapes germination behaviour and seedling traits of populations at different altitudes in a <i>Senecio</i> hybrid zone

Rebecca I. C. Ross, J. Arvid Ågren and John R. Pannell

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 110, issue 7, pages 1439-1447
Published in print November 2012 | ISSN: 0305-7364
Published online October 2012 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcs211
Exogenous selection shapes germination behaviour and seedling traits of populations at different altitudes in a Senecio hybrid zone

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  • Ecology and Conservation
  • Evolutionary Biology
  • Plant Sciences and Forestry

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Background and Aims

The Senecio hybrid zone on Mt Etna, Sicily, is characterized by steep altitudinal clines in quantitative traits and genetic variation. Such clines are thought to be maintained by a combination of ‘endogenous’ selection arising from genetic incompatibilities and environment-dependent ‘exogenous’ selection leading to local adaptation. Here, the hypothesis was tested that local adaptation to the altitudinal temperature gradient contributes to maintaining divergence between the parental species, S. chrysanthemifolius and S. aethnensis.

Methods

Intra- and inter-population crosses were performed between five populations from across the hybrid zone and the germination and early seedling growth of the progeny were assessed.

Key Results

Seedlings from higher-altitude populations germinated better under low temperatures (9–13 °C) than those from lower altitude populations. Seedlings from higher-altitude populations had lower survival rates under warm conditions (25/15 °C) than those from lower altitude populations, but also attained greater biomass. There was no altitudinal variation in growth or survival under cold conditions (15/5 °C). Population-level plasticity increased with altitude. Germination, growth and survival of natural hybrids and experimentally generated F1s generally exceeded the worse-performing parent.

Conclusions

Limited evidence was found for endogenous selection against hybrids but relatively clear evidence was found for divergence in seed and seedling traits, which is probably adaptive. The combination of low-temperature germination and faster growth in warm conditions might enable high-altitude S. aethnensis to maximize its growth during a shorter growing season, while the slower growth of S. chrysanthemifolius may be an adaptation to drought stress at low altitudes. This study indicates that temperature gradients are likely to be an important environmental factor generating and maintaining adaptive divergence across the Senecio hybrid zone on Mt Etna.

Keywords: Senecio aethnensis; S. chrysanthemifolius; hybrid; germination; exogenous selection; early life history; temperature

Journal Article.  5947 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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