Tests of two hypotheses concerning pollen competition in a self-compatible,long-styled species (Lobelia cardinalis: Lobeliaceae)

Using the long-styled, self-compatible species Lobelia cardinalis, I examined the relative abilities of self pollen and two outcross pollen sources to fertilize ovules when these three kinds of pollen were applied simultaneously to stigmas. Paternity was determined electrophoretically for 712 progeny of 25 seed parents. Two hypotheses were tested. First, it was hypothesized that self pollen would be less effective than outcross pollen at fertilizing ovules, as a means of reducing the selfing rate. Outcross pollen outperformed self pollen in eight of the 25 seed parents, self outperformed outcross in five, and they were competitively indistinguishable in 12. The mean proportion of offspring produced by self pollen was not significantly different from the value expected under random tube growth and fertilization. The rate of self-fertilization is therefore determined by prepollination events. Second, to test one stage of the sexual-selection process, it was hypothesized that the pollen of some individuals would outcompete that of others, across a range of maternal plants. The 25 maternal parents were divided into five groups of approximately five individuals. Each member of a group received pollen from the same two outcross pollen sources (as well as from itself). With offspring pooled across seed parents, one outcross pollen parent outcompeted the other in two of the five groups. In two other groups, pollen sources were competitively superior or inferior depending on the maternal parent. Overall, pollen donors were competitively indistinguishable in 13 of 25 maternal parents. There is thus no evidence for strong postpollination sexual selection. If such selection is present, it is weak, and its importance to the evolution of style length in Lobelia cardinalis requires other kinds of study.