Patterns of Phenotypic Plasticity and Their Fitness Consequences in Wild Radish (Raphanus sativus: Brassicaceae)

Abstract

Phenotypic plasticity, whereby the phenotype expressed by a genotype depends on the environment in which it is raised, may represent two distinct phenomena. First, plastic responses to the environment may be the adaptive outcome of natural selection having favored genotypes that increase their individual fitness by altering their phenotype in response to one or more environmental cues. Second, phenotypic plasticity may simply represent a genotype’s inability to maintain a high‐fitness phenotype under certain environmental conditions (e.g., abiotic stress or limited resources). To determine which of these phenomena characterize the annual plant Raphanus sativus L. (wild radish: Brassicaceae), we conducted a common garden experiment. We examined 19 greenhouse‐bred paternal sibships raised at three planting densities to detect genetic sources of phenotypic variation and to quantify each sibship’s phenotypic plasticity with respect to life‐history traits, floral traits, and fitness components. To determine whether plasticity affected fitness under high‐ and low‐density conditions, we examined the correlation among sibship means between phenotypic plasticity in each measured trait (estimated as the difference between the phenotypic values of the trait in each density) and estimates of fitness in each density. At low density, sibships with the highest plasticity exhibited the highest fitness. In high‐density conditions, no correlation was detected. This indicates that, in wild radish populations where density varies in time or space, highly canalized genotypes are simply those that cannot respond opportunistically to conditions of high resource availability. By contrast, plastic genotypes responded to low density by growing larger and exhibiting higher fitness. We also estimated the “environmental sensitivity” of each paternal sibship and trait as the coefficient of variation of trait means across densities. Traits expressed by individual flowers and fruits (petal size, gynoecium length, pollen and ovule production, pollen size, mean individual seed mass, fruit set, and seed number per fruit) were less sensitive to density than traits more closely related to reproductive output (e.g., lifetime flower production, maternal fecundity), indicating that the former had experienced selection favoring canalized genotypes as phenotypic plasticity in the latter evolved.