Abstract
This research focuses on the mitochondrial Complex I (EC. 1.6.5.3), the genes that encode this enzyme complex, the pattern of enzyme activity within and among populations, and how mRNA expression covaries with Complex I activity. Complex I is found in nearly every organism and has diversified from a core complex of 14 subunits to the 45 subunit complex found in vertebrates. The appearance of subunits over time is characterized by a large gain of subunits by multicellular animals and only 5 vertebrate specific subunits. Subunit sequences are generally well conserved suggesting that these subunits are performing essential functions. Human disease has been associated with 21 Complex I subunits, though the function of most of the non-catalytic subunits is unclear. Correlation analysis of mRNA abundance in four taxa (humans, Mus musculus, Drosophila melanogaster and Fundulus heteroclitus) show that correlations among basal subunits (those that are found in bacteria) are shared more frequently among taxa, suggesting functional importance of these correlations. Variation in Complex I activity in Fundulus heteroclitus collected along the latitudinal thermocline of its range was measured. Individuals from southern populations—F. heteroclitus from Georgia and F. grandis from Louisiana had significantly higher Complex I activity per wet weight than F. heteroclitus individuals from Rhode Island or Maine. Individuals from northern populations had higher mitochondrial density, but this did not compensate entirely for the disparities in Complex I activity among populations. The increased similarity in Complex I activity between individuals from sister species relative to individuals from the same species suggests that Complex I activity is evolving by natural selection in Fundulus. The difference in environmental temperature between northern and southern populations, as well as the known importance of temperature on enzyme function makes it likely that temperature affects divergence among Fundulus populations. The effect of temperature on Complex I activity was investigated by acclimating individuals from two populations of F. heteroclitus (ME, GA) and one population of F. grandis to three temperatures for six weeks (12°, 20° and 28°) and then assaying Complex I activity at each temperature. Acclimation to higher temperatures caused Complex I to be insensitive to acute changes in temperature and increased mitochondrial specific activity at lower assay temperatures. Individuals acclimated to 12° show the opposite response, suggesting that Complex I is more sensitive to cold temperatures. Abundance of mRNA in five populations of F. heteroclitus and one population of F. grandis was measured using microarray methodology. One Complex I subunit, NDUFA5, showed patterns of expression consistent with directional selection. Complex I subunit mRNA abundance was regressed with Complex I activity to identify genes with expression patterns that may affect Complex I mitochondrial specific activity. Among F. heteroclitus populations, the genes that explained variance in Complex I activity were not the same. Each population had a different model for Complex I activity and mRNA abundance, suggesting that the effect of variation in mRNA expression on Complex I activity is population specific.