1. Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, UK
2. Institute of Infection and Immunity Research, University of Edinburgh, Edinburgh, UK
3. Centre for Immunity, Infection and Evolution, School of Biological Sciences, University of Edinburgh, Edinburgh, UK
4. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, USA
5. Moredun Research Institute, Pentlands Science Park, Bush Loan, Midlothian, UK