Egg Size Plasticity in a Seed Beetle: An Adaptive Maternal Effect

In the seed beetle, Stator limbatus, the fitness consequences of egg size vary substantially among host plants. There is intense selection for laying large eggs when larvae will develop on seeds of Cercidium floridum (caused by high mortality penetrating the seed coat) but selection for laying small eggs when larvae will develop on seeds of Acacia greggii (caused by very low mortality penetrating the seed coat and an egg size/egg number trade-off). We test the hypothesis that host-associated variation in egg size within populations of S. limbatus represents an adaptive maternal effect in which females adjust egg size in response to host species. In laboratory experiments, S. limbatus females laid significantly larger and fewer eggs on C. floridum than on A. greggii. When switched between hosts, females readjusted egg size, producing progressively larger eggs on C. floridum and smaller eggs on A. greggii. When conditioned to lay either small eggs (on A. greggii) or large eggs (on C. floridum), and then forced to lay on C. floridum, females conditioned on C. floridum laid eggs that had substantially higher survivorship than eggs laid by females conditioned on A. greggii. These experiments demonstrate that egg size is an adaptively plastic character in S. limbatus.