Why be completely outcrossing? Evolutionarily stable outcrossing strategies in an environment where outcross-pollen availability is unpredictable

We have developed a game theoretic model to analyse the conditions which favour complete outcrossing in environments where outcross-pollen availability is unpredictable, assuming that reproductive assurance when outcrossing is incomplete is possible. We assumed that a plant produces ovules before the availability of outcross-pollen for that year is known. We also assumed that there is a size-number trade-off of seeds, and the plant is not allowed to allocate its resources differently to its outcrossed and selfed seeds. These assumptions result in the reduction of the size of outcrossed seeds if the plant accepts selfed seeds as well. We found that complete outcrossing is likely to evolve if the degree of inbreeding depression is large, if the optimal seed size which maximizes the reproductive success (seed number x establishment probability of individual seeds) when there is no outcross-pollen limitation and no constraint on seed size is large, if the mean number of ovules potentially fertilized with outcross-pollen is large, or if the number of ovules potentially fertilized with outcross-pollen is not extremely variable. Thus, the best-of-both-worlds mechanism (maximizing both outcrossing and seed set with reproductive assurance by selfing) is not always advantageous even if outcross-pollen availability is unpredictable.