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Phenotypic plasticity, where a single genotype expresses different phenotypes across microsites, is commonly viewed as an adaptive strategy in heterogeneous environments. Many studies have suggested that the evolutionary benefit of plasticity is reduced by its costs (reviewed in DeWitt et al., 1998; van Kleunen & Fischer, 2005), that is, the fitness of a plastic relative to a canalized genotype is lower even though the two genotypes express the same trait value in a given environment (Van Tienderen, 1991; Scheiner & Berrigan, 1998; Dorn et al., 2000). However, the evidence for costs is limited when compared with the number of traits measured (Van Tienderen, 1991; Scheiner & Berrigan, 1998; Donohue et al., 2000; Dorn et al., 2000; van Kleunen et al., 2000; Agrawal et al., 2002; Steinger et al., 2003; Stinchcombe et al., 2004; van Kleunen & Fischer, 2005; Weijschede et al., 2006). One obvious explanation for the empirical rarity of costs is that plasticity has no intrinsic costs, for example, if the same sensory equipment and response mechanisms are used in multiple environments. Alternatively, plasticity may only be costly in specific genetic settings, or in stressful (Stinchcombe et al., 2004) or resource-limited environments (van Kleunen et al., 2000; Steinger et al., 2003). Although plants in natural populations would likely experience stressful or resource-limited conditions during their lifetime, most cost studies have been carried out in controlled environments. Finally, genotypes with costly plasticity may have been culled from natural populations by selection (Agrawal et al., 2002; Weinig et al., 2006).
Most studies testing for plasticity costs have used arrays of genotypes from natural populations, from which selection has had time to decrease or eliminate genetic costs of plasticity (Donohue et al., 2000; Dorn et al., 2000; van Kleunen et al., 2000; Agrawal et al., 2002; Relyea, 2002; Steinger et al., 2003; Weijschede et al., 2006; but see Callahan et al., 2005; Weinig et al., 2006). In particular, selection in natural populations would be expected to favor genotypes in coupling phase between plasticity and vigor loci, that is, genotypes in which alleles for plasticity are associated with neutral or favorable alleles at vigor loci, and to favor the evolution of modifiers that reduce plasticity costs resulting from epistasis and/or pleiotropy (DeWitt et al., 1998; Agrawal, 2001). At least in selfing species, where selection effectively acts among clonal variants, genotypes with plasticity costs might well be purged from the population.
In contrast to natural populations, the creation of experimental segregating progenies, such as recombinant inbred lines (RILs), breaks up genetic associations and amplifies variation among RILs, therefore increasing the likelihood of detecting genetic costs (Callahan et al., 2005; Weinig et al., 2006). In one approach to creating RILs, two inbred homozygous parents are crossed to produce a heterozygous F1 individual, which is in turn mated to itself, and the resulting F2s are propagated by single seed descent for six or more generations (Poelman & Sleper, 1995). The generations of selfing (and attendant recombination) used to create RILs may regenerate genotypes with both coupling and repulsion phase associations between plasticity and vigor loci, therefore increasing the potential to detect costs resulting from linkage. The creation of RILs can also generate novel combinations of alleles at interacting loci, which increases the likelihood of detecting costs resulting from epistasis. In comparison to natural genotypes, plasticity costs resulting from pleiotropy are only more likely in RILs if a modifier that decreases pleiotropy has arisen in one but not both of the parental genotypes, or if different modifiers exist in each genotype. A recent study examining plasticity to competition in RILs of Arabidopsis thaliana found significant costs of putatively adaptive plasticity in three out of six measured traits (Weinig et al., 2006), even though a previous study in the same species detected very few costs of plasticity for similar traits in an array of wild genotypes grown under similar conditions (Dorn et al., 2000). These observations are consistent with the hypothesis that genetic costs of plasticity exist, but may be greatly reduced by selection in natural populations.
Competitive environments lend themselves to plasticity studies, because cues of neighbor proximity are well defined and perception of these cues induces multiple phenotypic responses. Low red to far-red light ratios (R:FR) are highly predictive of above-ground competition, because chlorophyll in neighboring plants selectively absorbs light in the red region of the spectrum while transmitting far-red light (Smith, 1982; Casal & Smith, 1989; Neff et al., 2000). Plants respond to low R:FR by elongating stems and petioles, reducing branching, and accelerating development (Smith, 1982). These responses can increase lifetime light interception in crowded settings, and thereby increase fitness (Schmitt et al., 1995; Dudley & Schmitt, 1996; Weinig, 2000a). In addition to adaptive elongation responses, plants may also exhibit maladaptive or nonadaptive reductions in overall growth in crowded stands, as a result of the lower resource levels and overall environmental quality of competitive relative to noncompetitive settings. Costs are more likely to exist when plasticity is adaptive, because genotypes expressing maladaptive plasticity should be removed by selection, as should genotypes with nonadaptive plasticity, when even small costs to plasticity exist.
In this study, we examine selection on architectural and life-history traits and their plastic responses to density in RILs of Brassica rapa to address the questions: is there selection for plasticity to density in B. rapa; and is there evidence for costs of plasticity?