Resource allocation and the selection of seed size and self-fertilization in hermaphroditic plants under pollen limitation

Background/Question/Methods:  Pollen limitation of plant reproduction is becoming increasingly common in nature because of factors such as human disturbance, habitat loss or fragmentation, and climate change, which may reduce plant and pollinator abundance. Plants may adopt a variety of ways to cope with pollen limitation. For example, plants may try to attract more pollinator visits by increasing resource allocation to pollinator attraction. Plants may overproduce ovules to capture the reproductive opportunities afforded by the rare flowers that receive unusually abundant pollen. Plants may also produce selfed ovules to provide reproductive assurance. However, the possibility that pollen limitation may change seed size has not been explored. Here, we examine the effect of pollen limitation (i.e., a reduction in ovule fertilization probability) on resource allocation and the selection of seed size and self-fertilization using an evolutionarily stable strategy resource allocation model in perennial iteroparous plants.

Results/Conclusions:  The model analysis indicates that if complete outcrossing is selected, pollen limitation increases seed size. The optimal seed size maximizes the ratio of juvenile survival rate to the resource investment needed to produce one seed (including both ovule production and seed provisioning), that is, the optimum maximizes the fitness effect per unit cost. Seed size determines resource allocation to post-breeding adult survival, and an increased seed size under pollen limitation increases post-breeding adult survival rate. Mixed mating is selected (i.e., producing selfed seeds to provide reproductive assurance) only when the size of outcrossed seeds and adult survival have increased to a certain extent such that resource investment in an outcrossed seed, adult survival, or a selfed seed produces equal marginal fitness returns. In such cases, outcrossed seeds should be larger than selfed seeds, and because pollen limitation does not affect the size of selfed seeds, pollen limitation should not affect the size of outcrossed seeds and adult survival. Moderate pollen limitation, strong inbreeding depression, and cheap cost of ovule production tend to select for complete outcrossing, while the reverse tend to select for mixed mating or complete selfing. The effect of pollen limitation on the selection of seed size and self-fertilization also applies to semelparous plants.