In most species, all fruits produced by a plant have uniform morphological and ecological characteristics that reflect a balance among conflicting fruit characteristics, such as seed number vs. seed size, and optimization of complementary characteristics such as seed size and seedling survival. Numerous species in the Asteraceae have evolved achene heteromorphism in which different ray and disc florets within the same capitulum produce morphologically and ecologically dissimilar achenes. One achene class is typically larger, have a thicker pericarp, lack dispersal structures, and require specific conditions to germinate. The alternative achene class tends to be lighter, have dispersal structures such as a pappus, and do not require specific stimulus to break dormancy. Ecological and population genetic studies have shown that achene heteromorphism is a bet-hedging strategy whereby plants offset the contrasting spatial and temporal risks of seed dispersal by partitioning reproductive output between two ecologically and genetically different seed pools. What is not known about achene heteromorphism is how development differs between floral meristems to produce morphologically distinct achenes. Furthermore, it is not known whether plants can adjust the proportions of distantly and locally dispersed achenes that are produced in response to different environmental conditions. Anatomical and developmental studies indicate that allometric differences produce dissimilar achene types, and that some species are able to alter the proportions of achene types produced.

Key words: achene development, Heterotheca Prionopsis fruit heteromorphism