CONTENTS XI 



IX. Variation in Virulence in Relation to Adaptation to New Hosts by 



Frank Fenner and John Cairns 225 



I. Introduction 225 



II. Mechanisms of Virus Variation 227 



A. In Bacterial Viruses 228 



B. In Animal Viruses 228 



III. Animal Virus-Host Cell Systems 230 



A. Simple Cell Systems 230 



B. Structurally Complex Systems 234 



C. Systems Involving Sequence 240 



IV. Summary 246 



References 247 



X. Serological Variation by T. Francis, Jr 251 



I. Introduction 251 



II. Serological Reagents 252 



III. Serological Variation among Influenza Viruses 253 



A. Viral Structure 253 



B. Serological Types 254 



C. Antigenic Analysis of Type A Strains 254 



D. Variation in Type B Influenza Virus 265 



E. Serological Variation during Passage 265 



IV. Variation in Arthropod-Borne Viruses 269 



A. Demonstration of Strain Relationships 269 



B. Value of Successive Exposure in Demonstrating Relation- 

 ships in Virus Groups 269 



References 270 



XI. Genetic Interactions between Animal Viruses by F. M. Burnet . 275 



I. Introduction 275 



II. The Different Types of Genetic Interaction 276 



III. The Technical Requirements in Virus Genetics 277 



A. Pure Clone Isolations 277 



B. Marker Characteristics 279 



C. Principles of Recombination Experiments 280 



IV. Genetic Interactions within the Myxovirus Group 282 



A. Historical 282 



B. Available Markers in the Influenza Viruses 283 



C. Interchange of Linkage Groups 285 



D. Phenotypic Mixture and Heterozygosis 286 



E. Use of Nonviable Virus as an Effective "Parent" 288 



F. Redistribution of Virulence 289 



G. Interpretation of Influenza Virus Recombination 293 



