Summary

Phenotypic Plasticity during Development

• Phenotypic plasticity is the capacity for an individual of fixed genotype to exhibit two or more genetically controlled phenotypes. Because the phenotype expressed is often dependent on the prevailing environment, phenotypic plasticity is a process by which genotype and environment interact to determine the phenotypic characteristics of an individual.
• Phenotypic plasticity is often programmed by a genetically coded, physiological control system that determines which specific phenotypes are expressed under which specific environmental conditions. Such control systems are subject to natural selection over evolutionary time and may, therefore, in themselves represent adaptations. At the opposite extreme, changes of phenotype in different environments may simply be forced by chemical or physical necessity. A major challenge for biologists is to develop empirical ways to determine whether instances of phenotypic plasticity represent adaptations or products of chemical or physical forcing.
• Polyphenic development in insects is perhaps the most striking form of phenotypic plasticity. In a species with polyphenic development, an individual with a fixed genotype can express two or more highly distinct phenotypes. Solitary and gregarious forms of migratory locusts provide an example.

Epigenetics

• Epigenetics refers to changes in gene expression that are transmitted during gene replication despite the lack of any alteration in the DNA sequence. The mechanism of epigenetic control is that genes are marked in ways (e.g., DNA methylation or histone modification) that modify their expression and the marks are replicated when the genes are replicated.
• Epigenetic marking is often initiated by environmental conditions because of the action of programmed control systems or of chemical or physical forcing. Accordingly, epigenetic control is a specific mechanism by which genotype and environment interact to determine phenotype.
• Because epigenetic marks are replicated from cell to cell as cells divide in an individual, epigenetic marks induced by the environment early in development can be perpetuated throughout life and affect the adult phenotype.
• Increasing evidence indicates that polyphenic development, such as differentiation into castes in social insects, is controlled by epigenetic marking.