Potential selection for female choice in Viola tricolor

That sexual selection can be an active force in plant evolution is still under debate. When the number of pollen grains deposited onto a stigma exceeds the number of available ovules, competition among pollen grains for fertilizations will result in selection on traits that increase siring ability (e.g. pollen tube growth rate). The pistil can be regarded as an arena for pollen competition, where pistil size and shape have the potential to intensify competition and thereby increase the possibility of sorting among pollen donors. Pollen tube growth rate can function as a cue for female choice if there is a positive relationship between pollen tube growth rate and sporophytic quality of individual donors when this quality is reflected in the offspring. In hermaphroditic violets, Viola tricolor, we found that pollen tube growth rate in vitro is an indicator of donor sporophytic quality expressed as seed production. In two-donor crosses, pollen tube growth rate had a strong influence on siring ability, which, in turn, was positively related to offspring seed production. Artificial selection of offspring shed by superior pollen donors also resulted in reduced variation in seed production and pollen tube growth rate. A father-offspring regression further indicated that pollen tube growth rate is heritable to a certain degree. There is thus a potential for selection on traits that increase the probability that ovules are fertilized by fast-growing pollen.