Genetic Control of Development

A central question in development is whether a particular cell is predestined to become a specific cell type from the moment of its creation, or whether its fate is less determinate, depending on a variety of cues it receives from its local environment as development proceeds. The developmental geneticist Sydney Brenner dubbed these two alternatives the European way (what matters is who your ancestors are) and the American way (what matters is what your environment is and who your neighbors are). While no multicellular organism displays either alternative exclusively, the roundworm C. elegans operates primarily according to the European plan, with each of its exactly 959 cells largely following a set developmental path, and with few decisions made through interaction with neighboring cells. Drosophila, and mammals such as mice and humans, largely develop according to the American plan, with most cells only gradually taking on a final identity, through repeated communication and competition with neighboring cells.

The difference can be seen in transplant experiments, in which cells of the early embryo are moved from one region to another. In the roundworm, the transplanted cell generally follows its original developmental plan, regardless of the environment in which it finds itself. In fruit flies and mammals, the transplanted cell generally takes on the identity of the region into which it is transplanted, switching from one developmental pathway to another, such as from bone cell to gut cell. This change is not absolute and, most importantly, it is time-dependent. Cells transplanted later in development tend to remain committed to the pathway they were on, despite their new surroundings, leading to the aberrant development of bone cells in the gut, for instance.