The effects of pollen competition on progeny vigor in Fragaria virginiana (Rosaceae) depend on progeny growth environment

Competition among pollen grains to fertilize ovules is expected to lead to increased vigor of the resulting progeny. Tests of this "pollen-competitionhypothesis," however, have been equivocal, and this may, in part, result from the levels of pollen competition (pollen load sizes) and/or progeny growth environment chosen. Our study of Fragaria virginiana controlled for these variables by identifying seeds produced under low and high pollen loads from an empirically derived pollen load-seed set response curve. We assessed fitness of seeds at germination under uniform conditions but fitness of progeny at juvenile and adult life stages under two different resource levels in the greenhouse. We hypothesized that the effect of pollen load size on progeny vigor would be stronger when progeny were grown under low resources than when grown under high resources. We found that the effect of pollen load size was negligible at germination but was significant in later life when assessed under two resource levels. The direction of the pollen load effect, however, depended on the progeny growth environment. Under low resources, progeny from high pollen loads grew faster, attained greater biomass, and had a higher probability of flowering than progeny from low pollen loads, while the reverse was true under high resource availability. These results indicate that the effects of pollen competition on juvenile and adult characters depend on progeny growth environment.