Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK. john.parkinson@ed.ac.uk|Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK. john.parkinson@ed.ac.uk|Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK. john.parkinson@ed.ac.uk