The effect of hybridization on metabolism and hypoxia tolerance in sunfish
Mathers, E. Katherine
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Hybridization between species has the potential to exert pleiotropic effects on metabolism. Reduced fitness in hybrids may arise through incompatibilities between nuclear- and mitochondrial-encoded subunits of the enzyme complexes of oxidative phosphorylation. In my thesis, I examined metabolic properties and hypoxia tolerance of bluegill (Lepomis macrochirus), pumpkinseed (L. gibbosus), and their unidirectional F1 hybrids (male bluegill x female pumpkinseed). Electron transport system (ETS) complex activities were examined in isolated mitochondria of bluegill, pumpkinseed and hybrids. The specific activities (units per mg mitochondrial protein) of complexes I, II, and V were indistinguishable between groups; however, both complex III and IV showed indications of depressed activities in hybrid mitochondria. The nature of sequence differences in complex IV catalytic subunits (CO1, CO2, CO3) were minor, however the mtDNA-encoded subunit of complex III (cytochrome b) showed 8 differences between bluegill and pumpkinseed, several of which could have structural consequences to the multimeric enzyme and contribute to the depressed complex III catalytic activity in hybrids. I next examined hypoxia tolerance in bluegill, pumpkinseed and hybrids to see if metabolic disruption in hybrids would lead to a reduced ability to cope with this stress. Though no difference in critical oxygen concentration (Pcrit) was noted, the time to loss of equilibrium (LOE) in 0.8 mg O2 /ml suggests that hybrids and bluegills have a lower hypoxia tolerance than pumpkinseeds. Hybrids showed a unique independence between size and LOE time. Analysis of tissue metabolite levels during normoxia, after LOE, and after a short-term hypoxia exposure suggest that differences in hypoxia tolerance are not due to differences in starting metabolite levels or differential metabolite use during hypoxia exposure.