Correlation between enzyme activities and routine metabolic rate in Drosophila

Drosophila species show substantial variation in levels of enzyme activity and evidence for coregulation of some enzymes involved in intermediary metabolism (Clark & Keith, 1989; Clark & Wang, 1994). An important extension of studies of cellular processes is to determine how they in ̄uence properties of the whole organism. Links between organismal performance and enzyme activity are important for at least two reasons. First, correlations between enzyme activity and organismal performance are required to determine if there is an opportunity for selection on enzyme activity. Second, these correlations might provide evidence for tradeoffs between enzyme activity and other aspects of performance. Correlations have been found between the activities of several individual enzymes and organismal performance. For example, succinate dehydrogenase activity and metabolic rate are positively correlated in the brine shrimp Artemia salina (Packard & Taylor, 1968) and, in Colias butter ̄ies, ̄ight performance and ®tness are correlated with population differences in phosphoglucose isomerase (PGI) activity (Watt, 1992). In vertebrates, swimming endurance in Fundulus heteroclitus varies between genotypes differing at the LDH-b (lactate dehydrogenase) locus (DiMichele & Powers, 1982; Powers & Schulte, 1996); maximal levels of oxygen consumption and LDH activity are positively correlated in lizard hearts (Garland, 1984); and studies of ®sh and mammals have examined the relationship between the activities of enzymes involved in glycolytic and oxidative pathways and body size and metabolic rate (Childress & Somero, 1979; Emmett & Hochachka, 1981). The above studies analyse one or a few enzymes. However, populations and species usually display differences in the activities of many enzymes, and some theoretical work suggests that a number of enzymes can be important in controlling the dynamics of a metabolic pathway (Kacser & Burns, 1973, 1995). Thus, focusing on only a few enzymes could give misleading answers about the relationship between enzyme activity and performance. Few studies have yet addressed the connection between performance and activities of suites of interconnected enzymes involved in linked metabolic pathways (Clark & Wang, 1994). One exception is the work of Laurie-Ahlberg et al. (1985); they report scant evidence for correlations between activities of 15 enzymes and energy expenditures during ̄ight in Drosophila melanogaster. Interspeci®c comparisons could increase the power of tests for correlations between enzyme activity and organismal traits by increasing the amount of variation available. Clark & Wang (1994) analysed activities of 12 enzymes in males from nine species of Drosophila. To examine correlations between enzyme activity and organismal performance we measured routine metabolic rate, body mass and activity levels of individual males and females