Mitochondrial DNA variants in Drosophila melanogaster are expressed at the level of the phenotype

Abstract

In metazoa, mitochondrial DNA (mtDNA) has been used extensively as a genetic marker due to its limited recombination, maternal mode of transmission and the perception that it evolves in a nearly neutral manner. A consequence of this perception is that a plethora of experimental studies use mtDNA as a marker of demographic processes without questioning the possibility that selection may bias their interpretations. In this thesis we critically examine whether the mtDNA genotype can influence the bioenergetics and organismal phenotype of the fly Drosophila melanogaster. We studied four lines of D. melanogaster that have a standardized nuclear DNA background but variable mtDNA. In Chapter 2, we completed DNA sequencing of the mitochondrial genomes (excluding the A+T rich region) of the four fly lines and compiled the differences. We then assayed fertility, starvation resistance, lipid proportion and physical activity. We show that mtDNA derived from flies collected from Japan exhibit differences in male fertility, starvation sensitivity and lipid proportion. Further, flies harboring mtDNA derived from the w1118 and Dahomey lines show the greatest decline in physical activity. We discuss these results in terms of the known differences between the lines and conclude that naturally occurring mtDNA variants in D. melanogaster are expressed at the level of the organismal phenotype. In Chapter 3, we tested the hypothesis that variation in mitochondrial bioenergetics in four lines of D. melanogaster is correlated with mitochondrial genotype. We assayed the rate of mitochondrial respiration between the four fly lines based on ATP synthesis, proton leak, uncoupling respiration and reactive oxygen species (ROS) production. We show that mtDNA derived from Japan flies has the highest rate of ATP synthesis and proton leak. Further, flies harboring mtDNA derived from Japan and w1118 have a higher rate of ROS production compared with other fly lines. We discuss these finding as a possible consequence of an amino acid change in complex I and conclude that naturally occurring mtDNA variants in D. melanogaster can cause variation in mitochondrial bioenergetics and these effects may contribute to differences in organismal phenotype expression. In Chapter 4, two major conclusions are drawn. Firstly, we conclude that mutations in mtDNA do affect the physiological traits of an organism. Secondly, we conclude that variation in naturally occurring mtDNA have significant effects on mitochondrial bioenergetics. We hypothesize that organismal phenotype are influenced by mitochondrial genotype and regulated through mitochondrial bioenergetics.