Heterochrony in plant evolutionary studies through the twentieth century

The evolution of plant morphology is the result of changes in developmental processes. Heterochrony, the evolutionary change in developmental rate or timing, is a major cause of ontogenetic modification during evolution. It i s responsible for both interspecific and intraspecific morphological differ ences. Other causes include heterotopy, the change of structural position, and homeosis, the replacement of a structure by another. This paper discuss es and reviews the role of heterochrony in plant evolution at the organisma l, organ, tissue, cellular, and molecular levels, as well as the relationsh ips among heterochrony, heterotopy, and homeosis. An attempt has been made to include all published studies through late 1999. It is likely that most heterochronic change involves more than one of the six classic pure heteroc hronic processes. Of these processes, we found neoteny (decreased developme ntal rate in descendant), progenesis (earlier offset), and acceleration (in creased rate) to be more commonly reported than hypermorphosis (delayed off set) or predisplacement (earlier onset). We found no reports of postdisplac ement (delayed onset). Therefore, although rate changes are common (both ne oteny and acceleration), shifts in timing most commonly involve earlier ter mination in the descendant (progenesis). These relative frequencies may cha nge as more kinds of structures are analyzed. Phenotypic effects of evoluti onary changes in onset or offset timing can be exaggerated, suppressed, or reversed by changes in rate. Because not all developmental changes responsi ble for evolution result from heterochrony, however, we propose that plant evolution be studied from a viewpoint that integrates these different devel opmental mechanisms.