MIXED MATING IN A RECENTLY DERIVED SELF-COMPATIBLE POPULATION OF LEAVENWORTHIA ALABAMICA (BRASSICACEAE)

Premise of the study : A mixture of outcrossing and selfing is often observed in plant populations. Although mixed mating is ubiquitous, it has several potential evolutionary explanations. Mixed mating may be actively maintained by selection, passively determined by the pollination environment, or a transitional stage during the evolution of self-fertilization. Methods : We studied patterns of self-compatibility and selfing rates in a population of Leavenworthia alabamica that recently lost self-incompatibility. We also experimentally tested whether natural selection against selfing at the pre- or postzygotic stage is sufficient to explain mixed mating in this population. Key results : Visualizing pollen tube growth following self-pollination, we found that nearly all plants were fully self-compatible. Progeny array analysis revealed that the average selfing rate of the population was s = 0.523. The inbreeding coefficient in the parents (F = 0.539) exceeded the amount expected if the selfing rate (s) were constant [F-eq = s/(2 - s)], indicating either population subdivision or higher selfing rates in the past. Inference of family-level selfing rates revealed substantial variation. Experiments found that self and outcross pollen fertilized nearly equal numbers of ovules in competition. Comparison of seed production following self- or cross-pollination failed to implicate early acting inbreeding depression as a factor maintaining mixed mating. Conclusions : The results of our experiments suggest that mixed mating is not maintained by selection against self- pollen or zygotes in this population. Mixed mating is most likely a byproduct of the pollination process but may also be a transitional stage during the evolution of higher selfing rates.