Pollen limitation and the evolution of self-compatibility in Linanthus (Polemoniaceae)

The magnitude of pollen limitation of reproduction was compared in three annual species of Linanthus with contrasting mating systems. In a 2-yr study, pollen limitation of total female reproductive success was measured in three populations of each species by comparing seed and fruit set in paired pollen-supplemented and open-pollinated plants. Linanthus parviflorus, a self-incompatible species, was found to be pollen limited for some measures of reproductive success in all populations in 1 yr and in two populations in the other. In contrast, no significant pollen limitation was detected in Linanthus bicolor, a highly selfing species, and in two populations of Linanthus jepsonii, a partially outcrossing but self-compatible species, indicating that autogamous selfing is providing reproductive assurance in most populations of these species. Significant spatial and temporal variation in pollen limitation was found in L. parviflorus and L. jepsonii. Although variable pollinator abundance is likely to have been one cause of the observed variation, it appears that a number of other factors, including resource availability, wind pollination, and pollen inviability, also contributed to variation in the magnitude of pollen limitation. Spatial variation in pollen limitation may explain why some lineages maintain outcrossing mechanisms whereas others evolve toward self-fertilization.