Fate maps (embryology)
Diagrams of embryos showing the structures that the parts will become in the course of normal development. A series of fate maps for different developmental stages depicts the normal morphogenetic movements that the cells and tissues of an embryo undergo. However, a fate map for a given stage does not in itself include information about the developmental pathway to which different parts of the embryo are committed. States of commitment must be deduced by comparing the structures predicted by the fate map with those formed by tissue regions in isolation, or after grafting to unusual positions. An accurate fate map is thus a necessary tool for further studies in experimental embryology. See Embryogenesis, Transplantation biology
For embryos in which there is no increase in size during development, and no random mixing of cells, it is possible in principle to project the fate map back into the fertilized egg. For example, in the nematode Caenorhabditis elegans the exact lineage of every cell has been determined. Where there is some local cell mixing, as in amphibian embryos, the fate map cannot be so precise, and represents the average behavior of a population of embryos. See Cell lineage
Clonal analysis, a special form of fate mapping in which a single cell is labeled, can give two pieces of information that a fate map of extended multicellular regions cannot. It allows for the detection of stem cells, which produce an entire structure by a sequence of unequal cell divisions. It also can set a lower bound to the time of developmental commitment. If a single cell can populate two structures, then clearly it cannot have been irreversibly committed to become either structure at the time of labeling.
In clonal analysis of plants, cells are labeled by exposing the seed or developing plant to ionizing irradiation or a chemical mutagen to produce chromosome mutations that result in distinct phenotypic alterations, usually deficiencies in the pigments chlorophyll or anthocyanin. The low frequency of mutations produced indicates that these are single-cell events. Because all of the progeny of a labeled cell will carry the same chromosome mutation, a shoot meristem cell labeled with a chlorophyll deficiency mutation, for example, will produce a sector, or clone, of white tissue in the developing plant. It is possible to deduce the number and fate of meristem cells labeled at one stage of development by examining the size and position of sectors present in the plant at a later stage of development. For example, a sector extending the entire length of the shoot would be generated by a permanent initial cell at the center of the meristem; if the width of that sector occupied one-third of the circumference of the shoot, then the shoot must have been formed by three such initial cells. See Mutation, Plant growth