Manipulative approaches to testing adaptive plasticity: Phytochrome-mediated shade-avoidance responses in plants

Phenotypic plasticity is often assumed to be adaptive, but this hypothesis has rarely been tested. To support the hypothesis, it is necessary to demonstrate that the phenotype induced in each relevant environment confers high fitness in that environment, relative to alternative phenotypes. Unfortunately, such tests are difficult to perform because plasticity prevents the expression of "inappropriate" phenotypes within each environment. Genetic and physiological manipulation can be used very effectively to extend the range of phenotypes within environments and thus provide powerful tools for testing the adaptive plasticity hypothesis. The expression of specific genes involved in cue perception or signal transduction can be altered by mutation or the introduction of transgenes, thus altering the plastic response of an organism to environmental cues. It is also possible to alter the cue itself or to manipulate the developmental response physiologically so as to obtain alternative phenotypes. The relative fitness of these alternative phenotypes can then be measured in each relevant environment. However, these techniques will be most useful when combined with techniques such as phenotypic selection analysis to identify the specific traits under selection in natural populations. We illustrate these approaches using phytochrome-mediated "shade avoidance" responses in plants as a model system. We review the genetic and physiological mechanisms underlying these responses, illustrate how genetic manipulation can elucidate their adaptive value, and discuss the use of physiological manipulation to measure natural selection on plasticity in the wild.