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 in creased selfing, was evaluated in the common morning glory Ipomoea pur purea. A field experiment was conducted to estimate selfing rates and outcross success using small arrays of plants with large or small anth er-stigma distance (ASD). To evaluate the effect of genotypic composit ion on the mating-system parameters, arrays were composed of five diff erent frequencies of small- and large-ASD genotypes. While the selfing rates of genotypes with small ASD were consistently higher than genot ypes with large ASD regardless of the genotypic frequency, outcross su ccess was negatively frequency dependent. The genotype that was at low er 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 ex treme ASD-morph and maintaining a mixed mating system in I. purpurea.