Plant Size and Reproductive Success through Female and Male Function

Abstract

Size differences among individual plants are usually large, with a few individuals in the population producing most of the pollen and seeds (Weiner & Thomas 1986). In hermaphrodite plants, size may have different effects on the male (siring seeds) and on the female side (setting seeds) of reproduction. Both male and female fitness can be a decelerating, linear or accelerating function of plant size. In the literature on the subject all possibilities have been mentioned, but some views are more prevalent than others. With respect to male fitness, some authors emphasized the accelerating function (e.g. Willson & Rathcke 1974). More recently, this idea was clearly formulated by Broyles & Wyatt (1990), who refer to it as 'the pollen donation hypothesis'. The mechanism is that large plants produce more flowers, which attract more pollinators that, in turn, remove a larger fraction of the pollen. The arguments for the linear (Campbell 1989) or decelerating function (Devlin et al. 1992) will be detailed later. With respect to female fitness, the curve is often considered to be linear (Brunet 1992). However, Lloyd (1984) argued for decelerating fitness with size because of competition among seedlings from the same parent. The shape of the size-fitness relationship is important for sex-allocation theory as it is analogous to the socalled 'gain curve', which describes how fitness changes with resources invested (Charnov 1982). This theory addresses questions about the optimal allocation to male vs. female reproduction in hermaphrodites and the evolution of dioecy from hermaphroditism. The basic problem is how the organism should shift allocation of resources to each sexual function in order to maximize fitness (the sum of paternal and maternal offspring), at one (Charnov 1982) or at several resource levels or sizes (Charnov & Bull 1985). Fitness is maximized if the plant invests in that function that yields the highest returns per unit of investment. In this note we first consider female fitness and discuss how post-establishment success differs for small and large plants. The relation between male fitness and size is affected by flower visitation, pollen removal, export to other plants, the success of siring seeds in competition with other pollen and post-establishment success. We list how these factors may differ for small and large plants. The list should be helpful for understanding gain curves, planning experimental work and avoiding some caveats. Next, we briefly discuss the most likely shapes of the fitness curves in animaland wind-pollinated species and their consequence for optimal allocation to each sexual function.