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Abstract. A continuous reaction norm or performance curve represents a phenotypic trait of an individual or genotype in which the trait value may vary with some continuous environmental variable. We explore patterns of genetic variation in thermal performance curves of short-term caterpillar growth rate in a population of Pieris rapae. We compare multivariate methods, which treat performance at each test temperature as a distinct trait, with functionvalued methods that treat a performance curve as a continuous function. Mean growth rate increased with increasing temperatures from 8 to 35ЊC, was highest at 35ЊC, and declined at 40ЊC. There was substantial and significant variation among full-sib families in their thermal performance curves. Estimates of broad-sense genetic variances and covariances showed that genetic variance in growth rate increased more than 30-fold from low (8-11ЊC) to high (35-40ЊC) temperatures, even after differences in mean growth rate across temperatures were removed. Growth rate at 35 and 40ЊC was negatively correlated genetically, suggesting a genetic trade-off in growth rate at these temperatures; this trade-off may represent either a generalist-specialist trade-off and/or variation in the optimal temperature for growth. The estimated genetic variance-covariance function (G function), the function-valued analog of the variance-covariance matrix (G matrix), was quite bumpy compared with the estimated G matrix; and results of principal component analyses of the G function were difficult to interpret. The use of orthogonal polynomials as the basis functions in current function-valued estimation methods may generate artifacts when the true G function has prominent local features, such as strong negative covariances at nearby temperatures (e.g. at 35 and 40ЊC); this may be a particular issue for thermal performance curves and other highly nonlinear reaction norms.Key words. Continuous reaction norms, genetic variation, insect growth, performance curves, temperature.Received September 29, 2003. Accepted March 19, 2004 The relationship between the phenotypic or genotypic value of an individual as a function of environmental conditions is called a reaction norm. If the environmental state varies continuously, as with temperature, light intensity, or soil moisture, then the phenotypic value may also vary continuously, yielding a continuous reaction norm. Understanding variation, selection, and evolution of continuous reaction norms has been a central challenge for evolutionary ecology for several decades (Scheiner 1993;Stearns 1989;Schlichting and Pigliucci 1998).One Fig. 1). Evolutionary physiologists have proposed three distinct patterns of variation in such performance curves: the hotter-colder, the faster-slower, and the generalist-specialist hypotheses (Huey and Kingsolver 1989; Fig. 1). First (hottercolder, Fig. 1, top panel), a set of individuals or genotypes might vary in the temperature at which performance is maximal, such that some individuals or genotypes have maximal performance ...

Cyanogenic glycosides (CNGs) act as feeding or oviposition deterrents and are toxic after enzymatic hydrolysis, thus negatively affecting herbivore performance. While most studies on CNGs focus on leaf herbivores, here we examined seeds from natural populations of Phaseolus lunatus in Mexico. The predominant CNGs, linamarin and lotaustralin, were quantified for each population by using ultra-high pressure liquid chromatography-mass spectrometry. We also examined whether there was a correlation between the concentration of CNGs and the performance of the Mexican bean beetle, Zabrotes subfasciatus, on seeds from each population(.) The concentrations of CNGs in the seeds were relatively high compared to the leaves and were significantly variable among populations. Surprisingly, this had little effect on the performance of the bruchid beetles. Zabrotes subfasciatus can tolerate high concentrations of CNGs, most likely because of the limited β-glucosidase activity in the seeds. Seed herbivory does not appear to liberate hydrogen cyanide due to the low water content in the seed. This study illustrates the importance of quantifying the natural variation and activity of toxic compounds in order to make relevant biological inferences about their role in defense against herbivores.

Theoretical models predict that selection on reaction norms should depend on the relative frequency of environmental states experienced by a population. We report a laboratory experimental test of this prediction for thermal performance curves of larval growth rate in Pieris rapae in relation to their thermal environment. We measured short-term relative growth rate (RGR) for each individual at a series of five temperatures, and then we assigned individuals randomly to warm or cool selection treatments, which differ in the frequency distributions of environmental temperatures. Selection gradient analyses of two independent experiments demonstrated significant positive selection for increasing RGR, primarily through its effects on survival to adulthood and on development rate. In both the warm and cool selection treatments, the magnitude of directional selection on RGR was consistently greater at lower (suboptimal) temperatures than at higher temperatures; differences in selection between the treatments did not match model predictions. The temporal order and duration of environmental conditions may affect patterns of selection on thermal performance curves and other continuous reaction norms, complicating the connections between variation in environment, phenotype, and fitness.