Heteroblasty

Shifts in the morphology and physiology of leaf and stem units (metamers) within individuals are a normal part of plant development. Yet for many years, these developmentally determined shifts went largely unnoticed unless they resulted in dramatic changes in form. Recently shifts in metamer form have attracted attention because in at least some cases, they are associated with major life history stage transitions, for example, from non-reproductive to reproductive phases. I am interested the developmental basis of heteroblasty, in how heteroblasty evolves within lineages and in whether heteroblastic shifts in metamer form are functionally important. I have shown that shoot heteroblasty expressed in early stages of leaf development can be hidden by allometric adjustments in later stages of leaf expansion. I have also argued that marked heteroblastic changes in leaf form represent the most obvious expression of a continuum of morphological and functionally significant differences among metamers as a seedling develops.

Developmental phenology

In all organisms, the unfolding of an ontogeny is inextricably linked to the environment in which that organism grows. For spatially restricted, continuously developing organisms such as plants, temporal changes in environment are part and parcel of the ecological history, and thus evolutionary selective history, of that organism. I am interested in how temporal and morphological developmental patterns evolve in concert to result in adaptive phenotypes. In mayapple, a perennial herb of temperate forests of North America, my collaborators and I have found that shoot type determination at the apical meristem results from a combined sensitivity of the meristem to the growth rate of the shoot, to the reproductive status of the existing shoot, and to past history of the sympodial rhizome system.

Evolution of diversity

The origin and maintenance of species diversity is a cornerstone issue in evolutionary biology. The flora of South Africa offers many examples of speciose clades with large numbers of endemics. Pelargonium, known for its horticultural taxa, is the third largest genus in the Cape Floristic region. Using the phylogeny presented by Bakker et al., my collaborators and I are investigating questions focused around the evolution of leaf shape, size and growth form diversity in the genus. We are investigating evolutionary patterns in functional significance of shape variation in relation to leaf thermal balance and photosynthesis, the degree of plasticity in shape under a range of environmental conditions, and the developmental bases of leaf shape variation. Ultimately our goal is to link specific developmental shifts to functionally significant evolutionary divergence.