Frequency-dependent pollen discounting contributes to maintenance of a mixed mating system in the common morning glory Ipomoea purpurea

Pollen discounting, a reduction in outcross success associated with increased selfing, was evaluated in the common morning glory Ipomoea purpurea. A field experiment was conducted to estimate selfing rates and outcross success using small arrays of plants with large or small anther-stigma distance (ASD). To evaluate the effect of genotypic composition on the mating-system parameters, arrays were composed of five different frequencies of small- and large-ASD genotypes. While the selfing rates of genotypes with small ASD were consistently higher than genotypes with large ASD regardless of the genotypic frequency, outcross success was negatively frequency dependent. The genotype that was at lower frequency in the array had higher outcrossing success in three out of the four array types with unequal frequencies. This advantage-when-rare phenomenon can contribute to preventing the fixation of either extreme ASD-morph and maintaining a mixed mating system in I. purpurea.