The Evolution of Inflorescence Size and Number: A Gamete-Packaging Strategy in Plants

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Abstract
The evolution of inflorescence size and number is considered by analyzing the eovolutionarily stable proportion of resources invested per inflorescence, a parameter that accounts for both the size and the number of inflorescences per plant. For self-incompatible plants, the evolutionarily stable inflorescence size-number combination is determined by the functional relationship between the resources available for flower production, seed set per flower, pollen contribution per flower, and the inflorescence size-number combination. Factors promoting the evolution of many small inflorescences per plant include fixed costs per inflorecence that accelerate with inflorescence size, sectorial transport of resources within the shoot system, and seed set and/or pollen contribution per flower that decreases with infloresence size. Factors promoting the evolution of one large inflorescence per plant include free intraplant transport of resources, fixed costs per inflorescence that decelerate with inflorescence size, and seed set and/or pollen contribution per flower that increases with inflorescence size. Intermediate inflorescence size-number combinations can occur in a wide variety of situations in which resources available for flower production, seed set, and pollen contribution do not all change in the same direction with inflorescence size or in which one or more of these fitness components is maximized in intermediate-sized inflorescences. The local stability of many of these strategies is determined by the shapes of the relationships (i.e., accelerating or decelerating) between the fitness components and the inflorescence size-number combination. For self-compatible plants, the relationship between self-fertilization rate and inflorescence size and the magnitude of inbreeding depression shown to be additional factors that can influence the evolution of inflorescence size and number. In particular, it is shown that self-fertilization occurring between flowers on the same plant can set an upper limit to the evolutionary stable size of inflorescences when inbreeding depression is pronounced. It is suggested that extending the principles used to model the evolution of influorescence size and number will prove useful in analyzing other gamete-packaging problems such as the sizes and numbers of flowers per plant and the sizes and numbers of flower parts (stamens, carpels) per flower.