GENETIC INTERACTIONS BETWEEN ANIMAL VIRUSES 299 



than one type of viral protein is being synthesized, the units will be distributed 

 at random in the surface membrane in proportion to the rates at which the 

 different types are being produced. 



There is much more doubt about how the genetic material (assumed to be 

 the central core of KNA nucleoprotein) is assembled. One way of looking at 

 the process is to think of each genome as comprising two nucleoprotein units 

 corresponding to ABDF and CEG genetic units. It may be that an aggrega- 

 tion of nucleoprotein, including appropriate numbers of both genetic units 

 to give n genomes, forms just under the cell surface. We have been inclined 

 to think of this as a random aggregation of genie material but the third way 

 of deriving homozygotes from a heterozygous input would be to assume that 

 only homologous genomes can be included together if a satisfactory viable 

 "nucleus" is to be produced. On the whole, however, we should still prefer the 

 view that the nuclear material of the virus particle is a loose aggregation of 

 approximately constant genetic composition built up by random selection of 

 what is available in the replicating pool. 



V. Genetic Interaction in other Virus Groups 



A. Recombination ivith Vaccinia and Rabbitpox Strains 



Typical recombination of genetic characters can be observed in the 

 poxvirus group. Work by Fenner (1958), and Fenner and Comben (1958) 

 has been presented at laboratory meetings in Australia and I am indebted 

 to Professor Fenner for permission to quote some unpublished work from 

 his group. 



Strains of variola, vaccinia, rabbitpox, cowpox, and mousepox show close 

 serological interrelationships (Downie and Dumbell, 1956) and can be 

 regarded as a natural group (Feimer and Burnet, 1957). Omitting mousepox 

 and variola strains, Fenner (1958) has made a detailed study of 24 strains 

 from the point of view of obtaining strains carrying suitable markers for 

 genetic experiments. These could be placed in four categories: 



(1) Standard dermal strains of vaccinia virus. 



(2) Neurovaccinia adapted to intracerebral passage in the rabbit and the 

 very similar strains isolated from rabbitpox epizootics. 



(3) Jennerian cowpox strains. 



(4) "White" variants especially characteristic of cowpox but also obtainable 

 from neurovaccinia and rabbitpox strains. These give opaque white pocks on 

 the chorioallantois (Downie and Haddock, 1952; van Tongeren, 1952). 



The following characters were adopted as markers after experience had 

 shown that they gave reproducible results with parallel clones and could be 

 readily used for screening tests: 



