GIBSON, J. PHIL. Department of Biology, Agnes Scott College, Decatur, GA 30030. - Developmental differences between ray and disc achenes in heteromorphic species of Prionopsis and Heterotheca.
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