Wood ontogeny as a model for studying heterochrony, with an example of paedomorphosis in Moringa (Moringaceae)

Modern studies of heterochrony in plants have focused mostly on determinate organs or annuals, leaving long-lived plants unstudied. Wood offers remarkable access to the issue, because ontogeny during the life of a woody plant is recorded by changes in cell attributes from the inner wood to the outermost wood. An example is provided, examining vessel element ontogeny in an explicitly phylogenetic context to infer paedomorphosis in the evolution of tuberous shrubs, represented by Moringa longituba (Moringaceae, the drumstick tree family), from an arborescent ancestral-type ontogeny represented by sister taxon M. ruspoliana. Two main difficulties with the use of wood in heterochrony studies are identified and their implications are discussed: (1) Some variable must be chosen against which to plot ontogenetic data; time, either observed directly or inferred from growth rings can be used, but stem diameter may be an acceptable or more informative alternative; (2) 'Onset' and 'terminal' ontogenetic reference points are required by the prevailing paradigm for studying heterochrony. However, in most cases it will be impossible to detect differences in the onset of wood ontogeny that lead to differences in mature morphologies. Likewise, it may often be impossible to identify a terminal point (e.g. sexual maturity, the traditional zoological reference point, is usually not associated with cessation of major ontogenetic changes in woody plants). As a result, distinguishing between subcategories of heterochrony (neoteny, progenesis, etc.) is usually impossible. Nevertheless, it should usually be possible to distinguish between the major categories (paedomorphosis and peramorphosis), or to reject an hypothesis of heterochrony. Finally, issues such as the sensitivity of ancestral character state reconstructions to branch lengths and coding of terminal taxa, and opportunities for incorporating intraspecific variability into studies of wood ontogenetic evolution are shown.