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Gynodioecy in structured populations: understanding fine-scale sex ratio variation in Beta vulgaris ssp. maritima

Authors

  • I. DE CAUWER,

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    • Present address: Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK

  • M. DUFAY,

    1. Laboratoire de Génétique et Évolution des Populations Végétales, FRE CNRS 3268, Bâtiment SN2, Université des Sciences et Technologies de Lille – Lille 1, F-59655 Villeneuve d’Ascq Cedex, France
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  • B. HORNOY,

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    • Present address: UMR CNRS 6553 ECOBIO, Université de Rennes 1, France

  • A. COURSEAUX,

    1. Laboratoire de Génétique et Évolution des Populations Végétales, FRE CNRS 3268, Bâtiment SN2, Université des Sciences et Technologies de Lille – Lille 1, F-59655 Villeneuve d’Ascq Cedex, France
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  • J.-F. ARNAUD

    1. Laboratoire de Génétique et Évolution des Populations Végétales, FRE CNRS 3268, Bâtiment SN2, Université des Sciences et Technologies de Lille – Lille 1, F-59655 Villeneuve d’Ascq Cedex, France
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Isabelle De Cauwer, Fax: +44 (0)1142220002; E-mail: i.decauwer@sheffield.ac.uk

Abstract

Natural selection, random processes and gene flow are known to generate sex ratio variations among sexually polymorphic plant populations. In gynodioecious species, in which hermaphrodites and females coexist, the relative effect of these processes on the maintenance of sex polymorphism is still up for debate. The aim of this study was to document sex ratio and cytonuclear genetic variation at a very local scale in wind-pollinated gynodioecious Beta vulgaris ssp. maritima and attempt to elucidate which processes explained the observed variation. The study sites were characterized by geographically distinct patches of individuals and appeared to be dynamic entities, with recurrent establishment of distinct haplotypes through independent founder events. Along with substantial variation in sex ratio and unexpectedly low gene flow within study sites, our results showed a high genetic differentiation among a mosaic of genetically distinct demes, with isolation by distance or abrupt genetic discontinuities taking place within a few tens of metres. Overall, random founder events with restricted gene flow could be primary determinants of sex structure, by promoting the clumping of sex-determining genes. Such high levels of sex structure provide a landscape for differential selection acting on sex-determining genes, which could modify the conditions of maintenance of gynodioecy in structured populations.

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