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58 F IVE Selection and Genetic Architecture of Plant Resistance MARY ELLEN CZESAK, ROBERT S. FRITZ, AND CRIS HOCHWENDER Basic and applied research programs can both benefit by approaching concerns regarding resistance to herbivores from a perspective centering on natural selection and genetic architecture of resistance. In natural systems, quantification of selection, determination of genetic correlations with other traits, and evaluation of genetic architecture (i.e., estimation of additive and nonadditive genetic effects) can enhance our ability to predict the evolutionary trajectory of plant resistance. Evaluation of genetic architecture has become increasingly emphasized because hybridization is widespread in plant species. Moreover , genetic architecture studies involving interpopulational or interspecific hybrids can give insight into the processes of population differentiation, speciation, and the formation and persistence of hybrid zones. From an applied perspective, programs proposing to use biological controls (e.g., insect herbivores and pathogens carried by insects) to regulate introduced weed species can benefit by considering the direct and indirect selective effects that those herbivores may have on the weedy species. Moreover , programs using integrated pest management strategies are increasingly utilizing plants bred or genetically engineered for increased resistance to pests to reduce pesticide use and its negative impact on beneficial organisms. Integrated pest management strategies require a fundamental understanding of selection pressure, correlational selection, and ecological costs in order to plan long-term management strategies. This chapter explores the selective agents that influence plant resistance to insect herbivores, the strength of selection , and the genetic architecture of resistance traits. Additionally , this chapter examines the relationship between plant hybrid resistance and the resistance of parental populations or species, which have implications for hybrid zone dynamics and for introgression of resistance traits between populations or species (see Scriber et al., this volume). Selection on Resistance within Populations Genetic Variation in Traits Although resistance traits can be controlled by one or a few loci (e.g., Daday 1954), it is more common for resistance traits to be controlled by many loci. Often, broad-sense heritability (h2 ) of resistance or resistance traits has been estimated in natural populations and agricultural species (e.g., significant effect of genotype in Piper arieianum C.D.C. [Marquis 1990]; cultivars of chrysanthemum Dendranthema grandi flora Tzvelev [deJager et al. 1995]; common milkweed Asclepias syriaca L. [Agarwal 2005]). Narrow-sense heritability is considered more valuable when quantifying genetic variation in resistance because a response to selection can occur only if significant additive genetic variance exists in a plant population. Additive genetic variation in resistance or resistance traits has been detected in a wide variety of plant species. These include leaf furanocoumarins in wild parsnip Pastinaca sativa L. (for most furanocoumarins, h2  0.33– 1.17) (Berenbaum et al. 1986); proportion of damaged leaves in common morning glory Ipomoea purpurea L. (Roth) (Simms and Rausher 1989); leaf trichome number in Brassica rapa L. (h2  0.56 0.14) (Ågren and Schemske 1994); glucosinolates in Brussels sprout hybrids B. oleracea L. (h2  0.72 or 1.09, sinigrin and progoitrin, respectively) (van Doorn et al. 1999); leaf trichome number in B. nigra L. (Koch) (h2  0.54 0.21) (Traw 2002); and phenolic glycoside concentration in Salix sericea Marshall leaves (h2  0.20–0.59) (Orians and Fritz 1995). For the wild radish, Raphanus raphanistrum L., additive genetic variance in induced glucosinolate production was detected in response to damage by caterpillars of the white cabbage butterfly (Pieris rapae L.). Induced responses varied among half-sib families, ranging from a 7% to 140% increase from constitutive levels (Agrawal et al. 2002b). Realized heritability of SELECTION AND GENETIC ARCHITECTURE OF PLANT RESISTANCE 59 resistance has been demonstrated through artificial selection experiments. For example, iridoid glycoside concentrations in leaves of the plantain Plantago lanceolata L. respond to selection and have a significant realized heritability estimate (0.23 0.07) (Marak et al. 2000). Natural selection within a population (or species) can give rise to epistatic genetic effects. Coadapted complexes arise when selection acts to favor the coevolution of alleles at different loci independent of the environment (Waser 1993), whereas local adaptation occurs when selection is contingent upon the environment (e.g., Sork et al. 1993). Epistatic genetic effects can create evolutionary trajectories that can be difficult to predict and can cause asymmetrical responses to selection (Merilä and Sheldon 1999; Galloway and Fenster 2001). For example, as the genetic composition of a population changes, nonadditivity can change the additive effects of alleles. Because additive...


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