Evolutionarily Stable Reproductive Strategies in Sexual Organisms: IV. Parent-Offspring Conflict and Selection of Seed Size in Perennial Plants

Abstract

The provisioning of offspring in sexually reproducing organisms provides an arena in which genetic conflict of interests between parents and their offspring may be expressed. While most existing models of parent-offspring-conflict consider the case of a parent that rears one offspring a year, this paper is concerned with perennial plants that produce many seeds at one time. Parent-offspring conflict is examined in the context of an integrated analysis of reproductive allocation, sex allocation, and the amount of resources invested in each offspring. I derive the evolutionarily stable strategy (ESS) results for the allocation of resources when the mother plant is in control as well as when the offspring are in control taking into account both density-independent and density-dependent population growth. To the extent that the relationships between gamete output and resource investment are linear for both sex functions, the separate treatment of reproductive effort, sex allocation, and offspring size-number compromise in modern life-history theories is justified, regardless of which side, parents or offspring, wins the conflict. In such cases, the ESS sex allocation is exactly what is found in traditional sex allocation theory, and the ESS reproductive effort maximizes the rate of population growth in density-independent populations, or the amount of resources allocated to reproduction during an average lifespan in density-dependent populations. In contrast to the previous theoretical conclusions based on the analyses of single-offspring cases, the ESS reproductive effort under the offspring's control of allocation to individual offspring is found to be lower than that when mothers are in control. This paradoxical result occurs because a mother producing fewer ovules fares better if she knows that each of her seed offspring will get more resources than the amount she is selected to give. The evolutionarily stable offspring size in both density-independent and density-dependent populations does not depend on mother's reproductive effort and investment-independent mortality, just as traditional models of offspring size-number trade-offs would predict.