Sexing pollen reveals female bias in a dioecious plant.

• Information on angiosperm sex ratios has largely been restricted to surveys of flowering individuals. These often deviate from equality, with male bias more commonly reported. Female-biased sex ratios are concentrated in a few taxa and have been linked to the possession of heteromorphic sex chromosomes and bias introduced during the gametophytic stage of the life cycle. It has been proposed that differences in gamete quantity and quality could give rise to female bias, although there is no direct evidence with which to evaluate this possibility. * Here, we use flow cytometry to investigate microgametophytic 'sex ratios' in a flowering plant. We demonstrate that differences in DNA content between the sexes in Rumex nivalis, a species with heteromorphic sex chromosomes, make it possible to distinguish female- vs male-determining pollen nuclei. * We found a small but significant female bias in microgametophytes produced by males (mean 0.515) with significant variation among family means (range 0.463-0.586), and 18 of 22 families averaging > 0.50. * The observed female bias at the gametophytic stage of the life cycle is consistent with the direction of bias previously reported for seeds and vegetative and reproductive plants in wild populations of R. nivalis, but is insufficient to fully explain the degree of bias.