Selection on pollen donors by Echium vulgare (Boraginaceae)

Fewer than 25% of the ovules of Echium vulgare in the field develop into viable seeds, even with no pollen limitation. The loss of ovules could enhance the fitness of the maternal parent, if less fit embryos are selectively aborted. Two pollination experiments were performed to examine the selectivity of maternal parents on self-pollen and different cross-pollen sources. Pollinated with one pollen genotype per flower, self-pollen was, on average, as successful in siring seeds as cross-pollen. However, the relative success of self-pollen compared to outcross-pollen differed significantly among the maternal parents. These results suggest that, under certain conditions, selfing can produce a greater number of seeds than cross-pollination. Pollen donors differed significantly in outcrossing success. The plants that were more successful in selfing were also more successful pollen donors in outcross-pollinations. No significant interaction between maternal parent and paternal genotype was found. Pollinations with a pollen mixture produced selfed and outcrossed seeds in the same ratios as the single-donor experiment. Overall, only slight differences were found between the single- and mixed-donor experiments. Pollen tube growth did not show a significant correlation with the success of the parental genotypes in the mixed-donor experiment, indicating that pollen tube growth is not the determining factor controlling the paternity of the seeds. These results are discussed with reference to possible mediating mechanisms.