1. Present address: Institute Pasteur Lille, Lille, France
2. Milner Centre for Evolution and Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath, UK
3. Present address: Quadram Institute, Norwich, UK
4. Wellcome Trust Sanger Institute, Hinxton, Cambridge, UK
5. Present address: Department of Veterinary Medicine, University of Cambridge, Cambridge, UK