Experimental Evolution: Concepts, Methods, and Applications of Selection Experiments

10. Laboratory Selection Studies of Life-History Physiology in Insects

217 10 LABORATORY SELECTION STUDIES OF LIFE-HISTORY PHYSIOLOGY IN INSECTS Anthony J. Zera and Lawrence G. Harshman OVERVIEW LABORATORY SELECTION: STRENGTHS, LIMITATIONS, CAVEATS MODEL SPECIES AND RESEARCH TOPICS Drosophila Gryllus Lepidoptera CONCLUSION Experimental Evolution: Concepts, Methods, and Applications of Selection Experiments, edited by Theodore Garland, Jr., and Michael R. Rose. Copyright © by the Regents of the University of California. All rights of reproduction in any form reserved. OVERVIEW The physiological basis of life-history traits and trade-offs has been a long-standing issue in the study of life-history evolution (Fisher 1930; Tinkle and Hadley 1975; Townsend and Calow 1981; Zera and Harshman 2001; Harshman and Zera 2007). Research on this topic has attempted to identify components of physiology (function at various levels of biological organization: Swallow et al. this volume) that contribute significantly to variation in individual life-history traits, such as early-age fecundity, and trade-offs between traits, such as the commonly observed negative correlation between early-age fecundity and longevity. The main goal of this research has been to illuminate the mechanisms of life-history evolution by identifying, for example, functional causes of trait interactions that explain why life-history traits often evolve in concert (e.g., trade-off). Early studies of life-history physiology mainly focused on energetic correlates of lifehistory traits, such as reproductive effort in lizards and birds at the level of variation among species or populations (Tinkle and Hadley 1975; Drent and Daan 1980). Starting in the 1980s, the introduction of quantitative genetics and laboratory selection revolutionized the study of life-history evolution by shifting the focus to within-species genetic variation, genetic covariation, and microevolution (Rose et al. 1996; Zera and Harshman 2001). Because of their short generation time and amenability to genetic analysis, insects have figured prominently in these studies. Physiological analysis of laboratoryselected lines now comprises a major focus of research in life-history evolution and evolutionary physiology (Service et al. 1985; Service 1987; Garland and Carter 1994; Rose and Bradley 1998; Gibbs 1999; Feder et al. 2000; Zera and Harshman 2001; Harshman and Zera 2007; Zera et al. 2007a; Gibbs and Gefen this volume; Huey and Rosenzweig this volume; Swallow et al. this volume). Because of the central role played by resource allocation in hypotheses concerning the physiological causes of life-history variation and trade-offs (figure 10.1), a primary focus of early functional characterizations of laboratory-selected lines was whole-organism energetics such as metabolic rate and energy reserves (Service 1987; Rose and Bradley 1998). However, the scope of physiological analysis has dramatically expanded in the past decade with studies investigating additional topics such as endocrine regulation , parasitism and infection, and resistance to oxidative damage (all discussed in this chapter). Furthermore, physiological studies of laboratory-selected lines have expanded beyond analysis of whole-organism physiology to include investigations of resource allocation and other topics at the level of flux through pathways of intermediary metabolism, enzyme activities, and gene expression. These physiological-genetic studies have contributed significantly to our understanding of the mechanisms underlying the generation of life-history variation, the functional causes of life-history trade-offs, and the evolution of life histories. We summarize the current status of major topics of research on the physiological causes of life-history traits and trade-offs conducted during the past twenty-five years on 218 • L E V E L S O F O B S E R V A T I O N laboratory-selected lines of insects. Most detailed studies have been undertaken using two experimental models: the fruit fly Drosophila melanogaster and crickets of the genus Gryllus. Hallmarks of the Drosophila work are the wide diversity of selection experiments and physiological traits investigated. The hallmark of the Gryllus work is the unusual depth of physiological analysis of two integrated laboratory selection studies. The third part of this chapter focuses on several notable physiological studies of laboratory selection on life-history traits in the Lepidoptera. LABORATORY SELECTION: STRENGTHS, LIMITATIONS, CAVEATS Detailed descriptions of the various types of laboratory selection can be found in other chapters of this volume (Gibbs and Gefen this volume; Huey and Rosenzweig this volume ) and earlier publications (e.g., Gibbs 1999; Harshman and Hoffmann 2000; Garland 2003; Swallow and Garland 2005). Briefly, there are two main types of laboratory selection: artificial selection and laboratory natural selection. In artificial selection, a specific phenotype is measured on each individual in...