Phenotypic plasticity in plants: a case study in ecological development.

Ecological development has been described as “the meeting of developmental biology with the real world” (Gilbert 2001); in other words, the study of development as it occurs in nature and its ecological consequences. One key area in this field is phenotypic plasticity : environment-dependent phenotypic expression (Bradshaw 1965; Schlichting 1986; Sultan 1987, 1995, 2000; Scheiner 1993; Travis 1994; Schlichting and Pigliucci 1998; Pigliucci 2001). To determine patterns of individual plasticity, genotypes are cloned or inbred and the genetic replicates raised in a set of controlled environments. Traits of interest can then be measured in each environment to characterize patterns of phenotypic response (termed norms of reaction ) for each genetic individual. Ecologically meaningful plasticity studies are designed to test genotypes in a range of environments based on naturally occurring variation and to focus on phenotypic traits important to function and therefore fitness in those environments. The greatest wealth of plasticity data is available for plants, which are ideally suited for such studies because they readily produce genotypic replicates and can be grown in diverse experimental environments. However, all organisms express some degree of phenotypic response to environment. Recent studies have documented developmental as well as physiological and behavioral plasticity in amphibians, reptiles, birds, marine and freshwater invertebrates, insects, mammals, and even lichens (references in Sultan 2000; Gilbert 2001; see also Barata et al. 2001; Griffith-Simon and Sheldon 2001; Hammond et al. 2001; Negovetic and Jokela 2001; Jordan and Snell 2002; Relyea 2002). Although biologists have always been aware that organisms develop differently in different conditions, environmental effects on phenotype were formerly regarded as uninformative “noise” obscuring the “true” expression of the genotype (Allen 1979; Sultan 1992; Schlichting and Pigliucci 1998). In plants, for instance, individuals that encounter low resource levels inevitably grow less—in fact, the effects of resource availability on plant phenotypes are so profound that neo-Darwinian botanists were often quite frustrated in their attempts to discern genetically based local adaptations through this “environmental noise” (Stebbins 1980; Pianka 1988). This led them to overlook the much more interesting aspect of plastic response to environmental variation: The fact that phenotypic responses to different environments may also include highly specific developmental, physiological, and reproductive adjustments that enhance function in those environments (Bradshaw 1965; Travis 1994; Schmitt et al. 1999; Sultan 2000; and references therein). This capacity for specific functionally appropriate environmental response is called adaptive plasticity , as distinct from the inevitable effects of resource limits and other suboptimal environments on phenotypic expression (Sultan 1995). Both inevitable and adaptive aspects of developmental plasticity are fundamental to ecological development, because they influence the success of organisms in their natural contexts. However, functionally adaptive plasticity is of particular interest because it permits individual genotypes to successfully grow and reproduce in several different environments. Consequently, such plasticity can play a major role in both the ecological distribution of organisms and their patterns of evolutionary diversification. Taxa consisting of adaptively plastic genotypes may inhabit a broad range of environmental conditions; many widespread generalist species may upon examination show this property (Baker 1974; Oliva et al. 1993). Adaptive plasticity may also contribute specifically to species invasiveness by allowing rapid colonization of diverse new habitats without the need to undergo local selection (Williams et al. 1995). Finally, individual plasticity may influence patterns of evolutionary diversification at the population (and ultimately species) level by precluding selective divergence in environmentally distinct sites (Sultan and Spencer 2002).