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 demon strate that the phenotype induced in each relevant environment confers high fitness in that environment, relative to alternative phenotypes. Unfortuna tely, such tests are difficult to perform because plasticity prevents the e xpression of "inappropriate" phenotypes within each environment. Genetic an d physiological manipulation can be used very effectively to extend the ran ge of phenotypes within environments and thus provide powerful tools for te sting the adaptive plasticity hypothesis. The expression of specific genes involved in cue perception or signal transduction can be altered by mutatio n or the introduction of transgenes, thus altering the plastic response of an organism to environmental cues. It is also possible to alter the cue its elf or to manipulate the developmental response physiologically so as to ob tain alternative phenotypes. The relative fitness of these alternative phen otypes can then be measured in each relevant environment. However, these te chniques will be most useful when combined with techniques such as phenotyp ic selection analysis to identify the specific traits under selection in na tural populations. We illustrate these approaches using phytochrome-mediate d "shade avoidance" responses in plants as a model system. We review the ge netic and physiological mechanisms underlying these responses, illustrate h ow genetic manipulation can elucidate their adaptive value, and discuss the use of physiological manipulation to measure natural selection on plastici ty in the wild.