Maternal adjustment of offspring provisioning and the consequences for dispersal
Phenotypic plasticity in seed provisioning is a widespread phenomenon in plant populations that is often manifested as environmentally induced maternal effects. Environmental maternal effects can be beneficial if they influence population dynamic functions of seeds in a way that increases fitness, such as escaping from crowding. Using the winter annual plant, Dithyrea californica, we studied the response of seed provisioning as a function of maternal competitive environment and the associated seed dispersal consequences in two different populations. We measured the average size of seeds produced by plants experiencing different competitive environments and the resultant plant mass in order to test the hypothesis that mother plants respond to crowding by providing fewer resources to each offspring. Plants from smaller seeds of this species are more prone to die and produce fewer seeds than plants from larger seeds, especially in crowded conditions. We also hypothesized that smaller seeds produced by crowded mothers would benefit from greater dispersal from their high density natal habitat. We painted seeds with fluorescent paint while still attached to the mother plant, recorded seed diameter, and followed them for nine months after dispersal, recording the distance they moved from the mother plant.
Plants that experienced more competition grew smaller such that for every unit of increase in the number of neighbors, ln plant mass decreased 0.084 ln mg and 0.047 ln mg in Rosario (P = 0.0001) and Coachella (P = 0.001) respectively; these plants also produced smaller seeds such that for every unit of increase in number of neighbors, average offspring seed diameter decreased 0.037 mm and 0.044 mm for Rosario (P = 0.0001) and Coachella (P = 0.0005) respectively; and dispersed farther from their mother plant such that for every mm increase in seed diameter, dispersal distance decreased by 19.74% (P=0.01). Heritability of seed diameter was not statistically significant (path direct effects, Rosario: P = 0.07, Coachella: P = 0.7). Instead, average offspring size was determined by plant mass (Rosario: P = 0.0009, Coachella: P = 0.04). Larger seed diameter is associated with greater competitive ability in D. californica, which, when combined with the phenomena measured here creates a potential tradeoff between competitive ability and dispersal, which arises from an environmentally driven aspect of phenotype. Fitness consequences on seed size evolution of this trade-off and of the year-to-year variation in rainfall and density are uncertain.