Lifetime fitness through female and male function: Influences of genetically effective population size and density

An individual’s lifetime fitness and patterns of mating between individuals are interdependent features of sexual organisms. Mating systems (outcrossing vs. selfing or mating between close rela-tives) can affect the distribution of offspring fitness, which generally declines with inbreeding, which in turn is related to a population’s genetically effective size (N_e). Fitness and mating patterns are also expected to vary with proximity of mates (i.e., population density). Consequently, density and N_e may influence demographic and genetic changes over generations and interact in their effects. Here, we report an experiment designed to assess the influence of these two population-level properties on mating system and lifetime fitness. In experimental arrays under quasi-natural conditions, we varied the density and N_e of the hermaphroditic annual legume Chamaecrista fasciculata. We recorded components of fitness for each individual and employed microsatellite markers to estimate outcrossing and assign paternity. We used aster analyses to estimate lifetime fitness for genetic families using female (seeds set) and male (seeds sired) reproduction as fitness measures. With estimates from these analyses, we assessed the evidence for a trade-off between fitness attained through female versus male function, but we found none. Lifetime fitness increased with density, especially under high N_e. Outcrossing rates increased with density under high N_e but declined modestly with density under low N_e. Our results show that density and N_e have strong direct effects on fitness and mating systems, with negative fitness effects of low N_e limiting the positive effects of increasing density. These findings highlight the importance of the interactive effects of density and N_e on lifetime fitness.