Pollen competition in silver birch (Betula pendula Roth) : an evolutionary perspective and implications for commercial seed production

Pollen competition arises from differences among pollen donors in pollen-tube growth rate. According to the pollen competition hypothesis, only the fastest-growing pollen tubes are assumed to achieve fertilization. By this thesis, I wanted to obtain new information about pollen competition and nonrandom fertilization and the relationship between pollen and progeny performance in Betula pendula Roth. I also considered the possible consequences of pollen competition for sexual selection, and for commercial production of genetically-improved B. pendula seed. The results of this thesis revealed that pollen competition is a real phenomenon in B. pendula seed orchards. There were significant differences among pollen donors in pollen-tube growth rate and a positive relationship between pollen-tube growth rate and seed siring success. These findings indicate that selection among pollen donors can occur on the basis of the differences in pollen performance. The role of pollen-tube growth rate as a predictor of progeny performance remained obscure. In natural birch stands, the relationship between pollen-tube growth rate and seed siring success might not be as straightforward as in controlled greenhouse conditions. Microclimatic variability and maternal environmental effects have random effects on pollen-tube growth rates and the outcome of pollen competition. Possibilities for pollen competition to lead to ongoing sexual selection and evolutionary consequences are likely to be diminished in nature. Microsite variability and genotype-environment interactions can partly explain the maintenance of variation in pollen-tube growth rates.