Department of Mathematics, Imperial College London, London, SW7 2AZ, United Kingdom.|MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge, CB2 0XY, United Kingdom.|MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge, CB2 0XY, United Kingdom.|MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge, CB2 0XY, United Kingdom.|Faculty of Mathematics and Natural Sciences, University of Bergen, Bergen, Norway.|Department of Mathematics, Imperial College London, London, SW7 2AZ, United Kingdom.