| Cell Lineage Mapping and C. Elegans |
Drosophila melanogaster is not the only model organism for developmental genetic studies. Starting in the 1960s geneticists interested in developmental questions turned to a free-living soil nematode, Caenorhabditis elegans. This species, usually referred to as just C. elegans, has several features that Drosophila and most other organisms don't have, which makes it attractive for developmental studies. Because embryos of this nematode are transparent, their cells can be observed easily and without much manipulation. The species also has a low number of cells. In fact, all normal individuals have the same number of cells: 959 somatic cells in the hermaphrodite and 1,031 in the male. Unlike Drosophila and mammals, which have extensive cell movement during development, the cells of C. elegans do not move very much during development. All of these features made C. elegans an ideal organism to study cell lineage history, the ancestral-descendant relationship of cells.
John Sulston and colleagues worked out the entire cell lineage history of C. elegans by 1983. Some cell lineage mutations alter the rate and/or timing of cell division. Others affect differentiation. One remarkable feature of C. elegans development is that seventeen percent of the cells generated during embryogenesis undergo programmed cell death, also called apoptosis. Normal development requires that certain cells die. There are several mutants in which the exact failure of cells to die has been tied to a phenotypic change. Many of the genes involved in programmed cell death in nematodes have counterparts in vertebrates that are also responsible for programmed cell death. Moreover, absence of proper cell death is a key feature of many cancers. (See the Cell Biology and Cancer unit.)