Post-pollination mechanisms promoting outcrossing in a self-fertile conifer, Thuja plicata (Cupressaceae)

In conifers, polyembryony (multiple embryos within an ovule) may decrease the number of seeds lost to random embryo abortion, and (or) increase the proportion of outcrossed seeds if outcrossed embryos out-compete selfed ones. Western redcedar (Thuja plicata Don ex D. Donn, Cupressaceae) is a conifer with a mixed (selfed and outcrossed) mating system and high self-fertility with simple (archegonial) polyembryony. To test whether polyembryony can decrease seed abortion or the proportion of selfed seedlings, we conducted controlled pollinations in a seed orchard in southwestern British Columbia. Four trees received a total of 48 pollination treatments consisting of self, outcross, or mixtures of self and outcross pollen. Enzyme electrophoresis identified selfed seeds in the mixed pollen treatments. Reduction in the proportion of filled seeds (seed set) owing to selfing was approximately 30% for three of the trees and 93% for the fourth. Following mixed pollinations we did not observe an increase in seed set attributable to polyembryony. However, when trees received high ratios of self-pollen they produced fewer selfed seedlings than expected, suggesting embryo competition. The consequences of these results on the mating system of western redcedar are discussed.