Plants, animals, and fungi show a huge diversity regarding form and phenotype: the observable characteristics of an organism. The research presented in this video uses the model of a particular roundworm to investigate how the phenotype is changed by the environment; this is known as phenotypic plasticity. RALF SOMMER explains that Pristionchus pacificus develops alternative mouth forms depending on their environment, either feeding on bacteria or on other roundworms. By manipulating the worms’ genome, the researchers identified one gene which, if deactivated, results in animals that no longer develop the predatory mouth form. Theoretical biologists have been suggesting the existence of so-called developmental switch genes that determine which of several traits encoded in the genome of an individual is formed. Such genes had, however, not yet been identified. Further experiments revealed that this developmental switch is not constituted by one single gene but is part of a more complex genetic network.