The evolution of plant development

The last decade has witnessed a resurgence in the study of the evolution of plant development, combining investigations in systematics, developmental morphology, molecular developmental genetics, and molecular evolution. The integration of phylogenetic studies, structural analyses of fossil and extant taxa, and molecular developmental genetic information allows the formulation of explicit and testable hypotheses for the evolution of morphological characters. These comprehensive approaches provide opportunities to dissect the evolution of major developmental transitions among land plants, including those associated with apical meristems, the origins of the root/shoot dichotomy, diversification of leaves, and origin and subsequent modification of flower structure. The evolution of these major developmental innovations is discussed within both phylogenetic and molecular genetic contexts. We conclude that it is the combination of these approaches that will lead to the greatest understanding of the evolution of plant development. Evolutionary developmental biology, or the study of the underlying developmental basis for the origin and diversification of organismic structure, has matured into a vigorous discipline in the last 20 years. Beginning in the 1970s, with such seminal works as those by Eldredge and Gould (on punctuated equilibrium; 1972), Gould (Ontogeny and Phylogeny ; 1977), Alberch et al. (on the formalization of heterochronic models of developmental evolution; 1979), and McKinney and McNamara (Heterochrony: The Evolution of Ontogeny ; 1991, as well as earlier papers), attention was focused on the role of development during the evolutionary diversification of metazoan morphology. Within only a few years, modification of development with respect to timing (heterochrony), ontogenetic sequence (addition or deletion of specific developmental events), and positional status (heterotopy), as well as analysis of the rate of evolutionary change (i.e., gradual vs. saltational or punctuated), had become central themes in the search for explanation of the historical pattern of metazoan diversity. At approximately the same time, the century-and-a-half-old discipline of animal embryology, which had been excluded from the evolutionary ‘‘modern synthesis’’ of the 1940s and 1950s (Gilbert et al., 1996), began to undergo a resurgence. Embryologists began to incorporate molecular and genetic techniques into their studies of early developmental events. Discovery of the homeobox genes in Drosophila (Scott and Weiner, 1984; McGinnis et al., 1984) and elucidation of the ubiquitous roles of homeobox-containing genes in the establishment of developmental pattern in phylogenetically diverse metazoans (Caenorhabditis elegans, Drosophila, Xenopus, and Mus; Wilkins, 2002; Arthur, 2002) led to a revival of interest in the mechanistic basis of evolutionary diversification. By the mid-1980s, molecular biologists, embryologists, comparative morphologists, and paleontologists shared a common vision