Gametophytic Selection in Raphanus raphanistrum: A Test for Heritable Variation in Pollen Competitive Ability

Abstract

Competition among many microgametophytes for a limited number of ovules can lead to both nonrandom fertilization by pollen genotypes and selection for greater sporophytic vigor. The evolutionary implications of this process depend on the extent of heritable genetic variation for pollen competitive ability. Using flower color in wild radish as a genetic marker, we demonstrate differences among pollen donors in competitive ability. Significant differences were found in four out of five pairs of donors. For three pairs of donors, competitive differences were observed in certain maternal plants but not others. To test for heritability of pollen performance, we conducted a selection experiment. We manipulated the intensity of pollen competition for two generations and then tested for differences in the performance of pollen from two selected lines. Competitive ability of pollen derived from each line was assessed relative to a standard unrelated pollen donor, using pollen mixtures on six wild maternal plants. The intensity of previous pollen competition had no overall effect on the proportion of seeds sired by each selected line. In two maternal plants, pollen from intense previous competition was actually inferior, contrary to expectation. Thus, we found no evidence for heritable variation in this trait. Other factors, such as male-female interactions, may influence the outcome of pollen competition. Prevailing theory on the genetic basis of effects of pollen competition on subsequent generations is not supported by our results. Improved protocols for future experiments are discussed.