DALZIEL, A.C.*; MOYES, C.D.; Queen’s University, Kingston, ON: Evolution of aerobic energy metabolism in high performance fish

The scombroid fish (tunas, mackerels and billfish) display many examples of specializations in muscle energetics, including high activities of mitochondrial enzymes. We are investigating the origins of variation in the mitochondrial matrix enzyme citrate synthase (CS), a marker for mitochondrial content, between clades within this group and also between fiber-types within a species. Our approach includes i) comparative sequence analyses to determine if differences in activity can be attributed to structural differences in the enzyme ii) assessment of the relationship between CS activity and gene dosage and iii) an evaluation of the variation in transcriptional regulation of the CS gene. Preliminary analyses of the coding region of CS indicate that molecular evolution of the enzyme is not responsible for higher activity in the tunas. Within a species, differences in CS activity per gram tissue between fiber types are almost entirely accounted for by gene dosage. For example, swordfish white muscle and heater organ have a 30-fold difference in CS per gram tissue but only a 3-fold difference in CS per CS gene. Conversely, inter-species differences in CS activity (approximately 2-3 fold between homologous tissues) remain when gene dosage is accounted for. This indicates that inter-tissue differences in CS activity are mainly due to constitutive expression while clade-specific differences are the result of evolutionary variation in regulatory control. Between species, CS mRNA levels do not correlate with CS enzyme activity, suggesting little transcriptional regulation. The post-transcriptional mechanisms by which variations in CS content are mediated are currently under investigation. Funded by NSERC and NMFS.