1. Departamento de Zoologia and Departamento de Bioquímica, Universidade de Coimbra, Coimbra, and Instituto de Biologia Molecular e Celular, Universidade do Porto, 4150 Porto, Portugal; Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803; and Centre for Applied Microbiology & Research, Porton Down, Salisbury, Wiltshire SP4 OJG, United Kingdom