Postpollination discrimination between self and outcross pollen covaries with the mating system of a self-compatible flowering plant.
OBJECTIVE
Variation in the mating system of hermaphroditic plant populations is determined by interactions between genetic and environmental factors operating via both pre- and postmating processes. Models predicting the maintenance of intermediate outcrossing rates in animal-pollinated plants often assume that the mating system is primarily controlled by floral morphology and pollinator availability, but rarely has the influence of postpollination processes on variation in outcrossing been examined.


METHODS
We investigated the influence of stylar discrimination between illegitimate and legitimate pollen-tube growth and the pollen-load capacity of stigmas on mating-system variation in the annual, tristylous species Eichhornia paniculata using controlled crosses and genetic markers. This species exhibits an exceptionally broad range of outcrossing rates in natural populations.


RESULTS
There was significant variation among populations in the pollen-load capacity of stigmas and the ability of styles to discriminate between illegitimate vs. legitimate pollen. There was strong correspondence between stylar-discrimination ability and variation in outcrossing rate among populations and style morphs. The combination of stigmatic pollen-load capacity and stylar discrimination explained more than 80% of the variation in outcrossing rates among populations.


CONCLUSIONS
The finding that stigmatic pollen-load capacity and stylar-discrimination ability contributed significantly to explaining the wide range of outcrossing rates in E. paniculata suggests that postpollination mechanisms play an important role in governing mating patterns in this species. The difference in levels of stylar discrimination between outcrossing and selfing populations may reflect a trade-off between selection for increased outcrossing and greater reproductive assurance.
Department of Biology, Portland State University, Portland, Oregon 97207 USA Cruzan@pdx.edu.
Mitchell B
MB
Cruzan
Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Ontario M5S 3B2 Canada.
Spencer C H
SC
Barrett
D003433:Crosses, Genetic; D031806:Eichhornia / Q000502:physiology*; D011058:Pollen / Q000502:physiology*; D054817:Pollination / Q000502:physiology*; D057945:Self-Fertilization / Q000502:physiology*
Eichhornia paniculata; Pontederiaceae; cryptic self-incompatibility; heterostyly; mixed mating; pollen competition; pollen-tube attrition; reproductive assurance; sexual interference; tristyly