Genetic relatedness and competition in Triplasis purpurea (Poaceae):: Resource partitioning or kin selection?

In annual plants where dispersal is limited, the nearest neighbors of an individual are often genetic relatives. The negative effects on growth and reproduction that result from intraspecific competition can be greatest when genetic relatives, rather than unrelated individuals, are competing. This is because resource partitioning among genetically different, unrelated individuals might result in improved growth and reproduction when they compete. Alternatively, kin selection is hypothesized to have occurred when growth and reproduction are greatest for an individual competing with relatives compared with one competing with unrelated plants. To determine whether resource partitioning or kin selection is more likely to describe the dynamics of local competition, a greenhouse experiment was conducted with Triplasis purpurea, a cleistogamous annual with restricted dispersal. Five pairs of families were subjected to intrafamily and interfamily competition and compared with noncompeting controls. There were highly significant effects of competition on shoot mass and number of seeds matured in cleistogamous (CL) spikelets. Families showed significant variation in shoot mass, number of seeds set in chasmogamous (CH) spikelets, and mean mass of CL and CH seeds. Shoot mass and mean CH seed mass were significantly greater for target plants in interfamily competition, while the mass of competitors was lower in intrafamily competition. There was no consistent tendency for target individuals competing with kin to outperform those competing with an unrelated family. Hence, there was no support for the kin selection hypothesis. Rather, the sibling competition that results from the restricted dispersibility of CL seeds reduces growth more than competition between unrelated plants, in accordance with the resource-partitioning hypothesis.