The effect of pollen competition on seedling vigor in Fragaria virginiana

2000

Sara Kalla

Jeanne Thompson

Dr. Tia-Lynn Ashman

            Natural selection has long helped us to understand the presence of certain traits, but sometimes the mechanism by which it acts is not straightforward.  When an organism has a multiple stage life cycle, with an overlap in the genes expressed in each stage, selection for traits at any given stage can also affect traits at the other stages.  In flowering plants, selection acting on the gametophyte stage (pollen grains and ovules) can influence the sporophyte stage (seeds and plants).  If the same genes are expressed in both stages, then selection of genes in the gametophytic stage can drive the evolution of genetically correlated traits in the sporophytic stage.  In plants, many more pollen grains land on the stigma than can actually fertilize the ovule.  As the pollen grains compete to fertilize the ovule selection is acting on the gametophyte stage.  High gametophytic selection results from high pollen competition (i.e., many pollen grains per ovule), and low gametophytic selection results from low pollen competition (i.e., few pollen grains per ovule).

Our study focused on Fragaria virginiana (wild strawberry).  We took seeds that had been produced under varying levels of pollen competition.  We measured variables such as germination, size and growth rate to measure the fitness of the resulting seedlings, which should be effected by level of pollen competition.  We focused on the traits of the resulting plants (sporophyte generation) because while many studies explored pollen competition, few had measured its effects on the longer-term growth of the resulting plants. 

Our results suggest that pollen competition is of increasing importance throughout the plant’s life, e.g. the level of pollen competition does not effect seed germination, but does have an effect on the growth of a mature plant.  This lack of initial importance leaves room for ideas such as the maternal resource hypothesis, in which it is the mother plant's provisioning of her resources amongst her seeds that determines seed fitness. In reality, it is likely that both factors have an effect.  However, resource availability (such as fertilizer level) has proven to be a third factor which influences seedling vigor.  These factors interact and we have shown that the amount fertilizer determines which, maternal resources or pollen competition, has a greater effect.

By understanding the effects of pollen competition, light can be shed on how selection on one life stage influences another.  This can lead to a greater understanding of plant genetics as well as being a general model of how several complex factors combine to influence evolution in a natural setting. These ideas can also be applied to horticulture and crop improvement.