Gynodioecy in structured populations: understanding fine-scale sex ratio variation in Beta vulgaris ssp. maritima I. DE CAUWER, 1 M. DUFAY, B. HORNOY, 2 A. COURSEAUX and J.-F. ARNAUD Laboratoire de Ge ´ne ´tique et E ´ volution des Populations Ve ´ge ´tales, FRE CNRS 3268, Ba ˆtiment SN2, Universite ´ des Sciences et Technologies de Lille – Lille 1, F-59655 Villeneuve d’Ascq Cedex, France 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. Keywords: Founder events, gene flow, gynodioecy, pollen and seed dispersal, polymorphic mating systems, spatial genetic structure Received 14 July 2011; revision received 8 November 2011; accepted 17 November 2011 Introduction Populations of sexually polymorphic plants species typ- ically vary in sex ratio. In the particular case of gyno- dioecious plants, in which both hermaphroditic and male-sterile (female) individuals can be observed in nat- ural populations, sex ratio can differ considerably among localities (e.g. Medrano et al. 2005; Nilsson & Agren 2006; Dufay et al. 2009). It is generally acknowl- edged that variation in sex ratio among sexually poly- morphic plant populations arises from (i) differential selection on the phenotypes in varying environments, (ii) deterministic oscillations because of frequency- dependent selection and ⁄ or (iii) nonselective processes, such as random founder events or spatially restricted migration (reviewed in McCauley & Bailey 2009). Understanding the causes of variation in the relative frequencies of the co-occurring sexual phenotypes should thus help gain insight into the mechanisms behind the maintenance of females and hermaphrodites in natural populations. Although poorly understood in most sexually poly- morphic species, the genetic basis of sex determination is well known for several gynodioecious species. It has been shown that it generally involves epistatic interac- tions between cytoplasmic male sterility genes (CMS) Correspondence: Isabelle De Cauwer, Fax: +44 (0)1142220002; E-mail: i.decauwer@sheffield.ac.uk 1 Present address: Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK 2 Present address:: UMR CNRS 6553 ECOBIO, Universite ´ de Rennes 1, France Ó 2012 Blackwell Publishing Ltd Molecular Ecology (2012) 21, 834–850 doi: 10.1111/j.1365-294X.2011.05414.x