POLLEN DISCOUNTING IN ERYTHRONIUM-GRANDIFLORUM - MASS-ACTION ESTIMATES FROM POLLEN TRANSFER DYNAMICS

Pollen discounting, a reduction in success as an outcross pollen paren t as a result of selfing, can reduce or eliminate the reproductive adv antage commonly attributed to selfing. Previous estimates of pollen di scounting have been based on segregation analysis of progeny from open -pollinated plants. Using data from Erythronium grandiflorum, we illus trate how direct measures of pollen transfer can be used to estimate d iscounting rates, and we discuss the relationship between absolute dis counting rates measured in this way and relative discounting rates mea sured through segregation analysis. Only about 0.4% of the pollen remo ved from anthers in E. grandiflorum is used in selfing, and only a lit tle more (0.5%) is delivered to the stigmas of other plants. Using the se estimates in the framework of a mass-action model suggests that the success rate of self-pollen is almost 80 times that of outcross polle n. Thus, variants increasing the discounting rate would appear to have a substantial reproductive advantage in E. grandifloum. Pollen discou nting cannot explain the maintenance of an outcrossed mating system in E. grandiflorum, and it may also fail in other plants in which a larg e proportion of the pollen produced never reaches a receptive stigma.