Patterns of sex ratio variation and genetic diversity in the dioecious forest perennial Mercurialis perennis Katrien Vandepitte Æ Olivier Honnay Æ Tim De Meyer Æ Hans Jacquemyn Æ Isabel Rolda ´n-Ruiz Received: 11 March 2009 / Accepted: 17 June 2009 Ó Springer Science+Business Media B.V. 2009 Abstract In small populations of plant species with separate sexes, it can be expected that besides the local environment also stochastic events influence population sex ratios. Biased sex ratios may in turn negatively affect genetic diversity due to increased genetic drift and, in clonal plants, due to reduced sexual reproductive output. Empirical evidence for these processes is scarce, however. We investigated the pattern of sex ratio variation and the distribution of genetic variation of the dioecious clonal forest herb Mercurialis perennis using AFLP markers. Analysis of molecular variance indicated a pro- nounced genetic structure. Overall within-population genetic diversity was moderate and local sex ratios were slightly male biased. The proportion of male to female plants in large populations slightly increased with increasing light penetration to the herb layer. Small populations, on the contrary, displayed high variability in sex ratios, unrelated to the local light environment. Genotypic diversity decreased with more male-biased sex ratios. We conclude that stochastic events related to small population size and the local forest environment, related to canopy closure, affect the proportion of female plants and indirectly influence local genotypic diversity, likely through the degree of sexual reproduction. This is one of the first studies to report a clear association between gender proportions and genetic diversity of a dioecious plant species in a fairly large survey. Keywords Mercurialis perennis Á Sex ratio Á Chance events Á AFLP Á Genetic differentiation Á Genotypic diversity Introduction Dioecious plant species, which make up ca. 4% of all plant species (Ainsworth 2000), often show variation in the local proportion of male-to-female plants. Sex ratios frequently covary with environmental condi- tions, a phenomenon recorded in over 20 plant families (Bierzychudek and Eckhart 1988). Environ- ment-dependent differences between the sexes in survival and growth rates can result in variable gender proportions along environmental gradients (Bierzychudek and Eckhart 1988; Popp and Reinartz K. Vandepitte (&) Á O. Honnay Á H. Jacquemyn Laboratory of Plant Ecology, University of Leuven, Arenbergpark 31, 3001 Heverlee, Belgium e-mail: katrien.vandepitte@bio.kuleuven.be I. Rolda ´n-Ruiz Unit Plant, Institute for Agricultural and Fisheries Research, Caritasstraat 21, 9090 Melle, Belgium T. De Meyer Laboratory for Bioinformatics and Computational Genomics, Ghent University, Coupure Links 653, 9000 Ghent, Belgium 123 Plant Ecol DOI 10.1007/s11258-009-9627-y