The interactions between pollen and resource limitations and their influence on plant sexual reproduction

Abstract

While Bateman's principle established that male and female fitness have different limiting factors, subsequent work on hermaphrodites and sessile organisms has shown that these limiting factors often interact in wild populations and field studies can not differentiated the two. For example, in hermaphrodite plants there is competition between male and female sexual functions for a limited pool of resources that limits their expression. Sexual function within plants is limited by the number visits the plant receives. I formulated a model that examined the effect of dividing pollinator 'attraction' into two different types: 'attraction' and 'reward'. I found that the selection for allocation to attraction is always favoured over allocation to reward, and also over allocation to male and female functions when pollinators are rare and inbreeding depression is high. However the attraction of pollinators leads to a reduction of allocation to pollen or ovules, so there is an attraction maxima after which the additionally attracted pollinators add little fitness. I carried out a greenhouse experiment to quantify the interaction between ultimate and proximal resource limitation on female fitness and the intergenerational effect of resource differences. There was generally significant reduction in seed number produced by the whole plant as more leaves were removed, however overcompensation was never observed. As the flowers became more distal seed number and average weight declined, while offspring fitness, despite being positively correlated with seed weight, was highest in the distal flowers. The results of this thesis imply that the trade-off between male and female function and attraction within a flower is the product of the floral architecture of the plant, which governs the resources it receives, and the number, and length, of pollinator visits. Sexual and ontological differentiation between flowers likely leads to resource and attraction advantages dependent on flower position. Ultimate resource limitation influences the resources the plant can allocate, and the underlying trade-off between sexual function and attraction it experiences.