Functional Conservatism in Mitochondrial Evolution: Insight from Hybridization of Arctic and Brook Charrs PIERRE U. BLIER Ã , SOPHIE BRETON, VE ´ RONIQUE DESROSIERS, AND HE ´ LE ` NE LEMIEUX Laboratoire de Biologie E ´ volutive, Universite´du Que´bec a`Rimouski, 300 Alle´e des Ursulines, Rimouski, Que., Canada G5L 3A1 ABSTRACT To assess the potential adaptive value of mtDNA, we evaluated functional properties and thermal sensitivity of key mitochondrial enzymes in two species that have originally evolved in different thermal environments (arctic charr, Salvelinus alpinus, and brook charr, S. fontinalis), as well as in their hybrids. We measured the activity of two enzymes of the electron transport system (cytochrome c oxidase and NADH-ubiquinone oxidoreductase), one enzyme of the mitochondrial matrix (citrate synthase), and one enzyme of the anaerobic glycolysis (lactate dehydrogenase) in the red muscle at three temperatures (61C, 121C and 181C). Surprisingly, the species presented no significant differences in enzyme activity, thermal sensitivity or thermostability of key metabolic enzymes even though they evolved in different thermal environments and present important differences in amino acid sequences. It seems that amino acid substitutions between those species have minor impact on the functional properties of mitochondrial enzymes studied. The thermal sensitivity results (Q 10 ) obtained for inner-membrane mitochondrial enzymes differed somewhat from those of mitochondrial matrix or cytosolic enzymes. This result indicates the modulation of thermal sensitivity of all mitochondrial inner-membrane processes by a common parameter, which could be the structural and functional properties of membrane phospholipids. J. Exp. Zool. (Mol. Dev. Evol.) 306B:425– 432, 2006. r 2006 Wiley-Liss, Inc. Biochemical and physiological processes are highly dependent on environmental temperature. In particular, temperature change is a major factor influencing metabolic rates in aquatic ectotherms, whose body temperature is regulated by their surroundings. Indeed, metabolic adjust- ments in response to thermal acclimation have been widely documented in fish species (Blier and Guderley, ’88; Guderley and Blier, ’88; Crockett and Sidell, ’90; Sephton and Driedzic, ’91; Guder- ley and St-Pierre, ’96, ’99; Blier and Lemieux, 2001). The most common response observed is improvement in aerobic capacity through increase in aerobic enzyme activities in cold-acclimated fishes, while the reverse is observed for glycolytic enzymes (Guderley and Blier, ’88; Guderley, 2004). These responses suggest metabolic limita- tion at the level of mitochondrial catalytic capacity at low temperature. Therefore, two fish species having evolved in different thermal environments could present specific adaptations, making them excellent organisms in which to examine evolu- tionary and phenotypic responses to temperature (Guderley, 2004). Enzymes involved in aerobic energy metabolism, and most specifically in the mitochondrial elec- tron transport system (ETS), are encoded by both mitochondrial (mtDNA) and nuclear (nDNA) genomes. They include the NADH-ubiquinone oxidoreductase (complex I) and cytochrome c oxidase (COX, complex IV). In these major ETS complexes, mtDNA-encoded peptides constitute the main catalytic centres whereas nDNA-encoded peptides have mostly structural functions (Schef- fler, ’99). The only exception is for the complex III (ubiquinone–cytochrome c oxidoreductase), in Published online 10 January 2006 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jez.b.21089. Received 23 August 2005; Accepted 3 November 2005 Grant sponsor: Natural Sciences and Engineering Research Council (NSERC) grants. Ã Correspondence to: P.U. Blier, Laboratoire de Biologie E ´ volutive, Universite´du Que ´bec a ` Rimouski, 300 Alle´e des Ursulines, Rimouski, Que´bec, Canada, G5L 3A1. E-mail: pierre_blier@uqar.qc.ca r 2006 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 306B:425–432 (2006)