Quantitative Genetics of Continuous Reaction Norms: Thermal Sensitivity of Caterpillar Growth Rates

Abstract

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 ...

Temperature‐driven selection on metabolic traits increases the strength of an algal–grazer interaction in naturally warmed streams

“…Instead, metabolic rates were higher for R. balthica from the warm stream across all measurement temperatures. These results are in broad agreement with the "hotter is better" hypothesis, which proposes that maximal perfor- Kingsolver, 1993;Kingsolver et al, 2004;Knies, Kingsolver, & Burch, 2009). Indeed, maximal respiration rates in the population from the warm stream were greater than those from the cool (ln(R) warm stream: 3.39 AE 0.14 lmol O 2 L À1 hr À1 , and cool stream:…”

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Adaptation, Plasticity, and Extinction in a Changing Environment: Towards a Predictive Theory

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Prinsimp

“…We start with a simple example that replicates some of the analysis from . We use the estimated genetic covariance matrix of the caterpillar growth rate data described in Kingsolver et al (2004). Growth rates (mg/hour) were recorded on 529 individuals from 35 families at six temperatures: 11, 17, 23, 29, 35, 40 degrees Centigrade.…”