Untangling the relationship between pollination efficiency and pollen-ovule ratios

Intuition suggests that loss of pollen during dispersal will favour increased pollen production to counteract the loss. From the perspective of sex allocation theory, however, such loss constitutes mortality following parental investment, and theoretical models have consistently shown that sex differences in post-investment mortality have no selective effect on parental sex allocation, of which pollen and ovule investment is a conspicuous part. This conflict between intuition and theory can be ameliorated by comparison to animal reproductive ecology. Mating systems such as polygyny that leave many seemingly superfluous males do not favour compensatory adjustments in the sex ratio. Rampant egg mortality among broadcast-spawners does not create selection for greater egg production. These inefficient reproductive investments evolve because efficiency is not the basis of the rare-sex advantage governing sex allocation. Nonetheless, empirical associations of pollination efficiency with pollen-ovule ratios need explanation. A simple but unexplored possibility is that high pollination efficiency tends also to involve stigmatic deposition of related cohorts of pollen or repeated pollination of nearby recipient plants by a focal donor, thus creating local mating competition, a factor long known to affect sex allocation. This hypothesis predicts that low pollen-ovule ratios will be accompanied by high levels of correlated paternity.