Heritable variation in seed sex ratio of the stinging nettle (Urtica dioica).

Male and female flowering plants of the dioecious Urtica dioica occur in approximately equal numbers in our study area on the coastal sand dunes of Meijendel. The seed sex ratio (SSR, fraction of males) collected from female plants in the field varied between 0.05 and 0.76, and differed significantly between maternal parents. After one generation of selection for either high or low SSR, female plants produced seed batches with sex ratios as extreme as 0.08 and 0.73. Natural populations of U. dioica harbour considerable genetic variation in SSR.