8 SEX IN MICROORGANISMS 



1. In triparental crosses Hershey has shown that some prog- 

 eny particles contain markers derived from all three parental particles. 



2, In a two-factor cross with unequal multiplicity of infection 

 (20 particles of parental type AB and one particle of parental type 

 ab) the progeny contains more recombinants of one type, say aB, 

 than of minority parents ab (Doermann, 1952). 



As has already been mentioned, the progeny of a cross can be 

 examined at different times, either by inducing premature lysis or 

 by delaying the lysis. With closely linked markers, the fraction of 

 recombinants increases linearly with time between the moment of 

 appearance of the first mature phage and time 60 minutes after in- 

 fection. These two lines of evidence derived from premature lysis 

 and delayed lysis indicate that in the single growth cycle there is a 

 drift in the course of time toward genetical equilibrium. 



Two different kinds of hypothesis can be considered at this 

 stage: (a) either the new phage is formed out of some kind of pool 

 of the parental characters, or (b) matings occur between vegetative 

 phage particles in such a way that the product of one segregation can 

 mate again with some other particle. 



What we are actually considering is a choice between some com- 

 pletely novel type of recombination occurring during reproduction 

 of the phage and the classical mechanism of recombination by mating 

 between pairs as it might take place in clonal reproduction of vege- 

 tative phage. The first hypothesis was strongly influenced by the 

 phenomenon of multiplicity reactivation discovered by Luria in 

 1947. Bacteria infected with phage particles inactivated by ultraviolet 

 light multiply and produce viable phage offspring in a large propor- 

 tion of cases. Luria interpreted these results to mean that viable phage 

 particles could be formed by recombination even when all the paren- 

 tal particles were unviable as a result of one or more lethal mutations. 

 It is known now after more thorough investigation of this phenome- 

 non that reactivation is not due to a genetic mechanism. The second 

 hypothesis of the mating has been analyzed in great detail by Visconti 

 and Delbriick (1953). Their theory is based on the following assump- 

 tions: (1) prophage particles multiply and mate pairwise at random 

 inside the bacterial cell; (2) vegetative phage particles go on mating 

 and multiplying up to the time of lysis; (3) mature phage particles 

 which are accumulated in the bacterial cell do not mate. On the basis 



