Clonal analysis of cell lineage patterns in plant development

Somatic sectors induced by ionizing radiation provide a great deal of information about cell lineage pattems in both plants and animals. Somatic sectors arise when the dominant allele of a mutation with a visible, cell-autonomous phenotype is lost as a result of a deletion or somatic recombination. In addition to marking the fate of cells in a primordium at different stages of development and in different tissues, this technique also provides information about the distribution, orientation, rate, and duration of cell division. The technology and underlying assumptions of this method, termed clonal analysis, are described in this paper. IN ORDER TO DEFINE the cellular processes that control morphogenesis it is important to have detailed information about the number of cells and their fate in a primordium and an accurate picture of the pattern of cell division, cell expansion, and cellular differentiation in the primordium at different stages in its development. This.descriptive information is crucial for subsequent experimental analyses of the morphogenetic process and provides important clues about the factors that govern morphogenesis. Unfortunately, this information is not easily obtained. Although a wide variety of methods can be used to observe cell behavior in animal systems in vivo, in plants many of these approaches are either impossible or difficult to exploit. Thus, much of what we know about the cellular dynamics of plant morphogenesis has come from histological analyses of specimens killed and fixed at different stages of development. Although this approach has proven extremely useful, it has some serious drawbacks, such as the tedium involved in conducting quantitative studies on sectioned ma