The role of inbreeding depression in maintaining the mixed mating system of the common morning glory, Ipomoea purpurea

Theoretical studies show that, although inbreeding depression (ID) will cou nterbalance the transmission advantage of selfing, it can only maintain a m ixed mating system in plants when at least one of the following two conditi ons is met: (1) there is a positive association between selfing rates and t he level of ID; and (2) ID is greater than 0.5 for the female component of fitness, while the average ID for male and female fitness is less than 0.5. This study tests whether these two conditions hold in the common morning g lory, Ipomoea purpurea, which has a mixed mating system with 30% self-ferti lization. Inbreeding depression was found in all by one fitness component m easured in two groups of plants with distinct anther-stigma distances (ASD) , a character that influences selfing rates. However, when examined separat ely, a negative association was found between selfing rates and ID; plants with large ASD (low-selfing-rate genotypes) tended to have higher ID than o nes with small ASD (high-selfing-rate genotypes). Furthermore, the overall lifetime ID for male (12.5%) and female (24%) components of fitness, averag ed across two ASD groups, were lower than what is necessary for ID to maint ain an evolutionarily stable mixed mating system. Therefore, although inbre eding depression contributes to balancing the transmission advantage of sel fing, it is not likely to be the primary mechanism maintaining the mixed ma ting system of I. purpurea. The contribution of other mechanisms is discuss ed.