340 BACTERIOPHAGES 



It is probable that some kind of mating procedure is an un- 

 avoidable if not essential part of phage replication in the vegeta- 

 tive pool. Recombination would then be an incidental result 

 when the mating happens to occur between nonidentical phage 

 particles. 



/. Drift toward Genetic Equilibrium 



The proportion of recombinants among phage particles found 

 in premature lysates made at various times during the latent 

 period increases as a function of time (Doermann, 1953). With 

 unlinked markers the proportion increases from 0.32 at the start 

 of maturation to 0.42 at the time of lysis. With a given pair 

 of linked markers the proportion of recombinants increases from 

 0.06 at the start of maturation to 0.12 at the time of normal lysis. 

 Levinthal and Visconti (1 953), using the phenomenon of lysis inhi- 

 bition to prolong the latent period in crosses involving the closely- 

 linked markers h and rl3, found that the recombinant frequency 

 increased from 0.02 at 20 minutes to about 0.09 at 80 minutes. 

 The increase in recombination frequency paralleled the increase 

 in average burst size over this period of time. In a cross involv- 

 ing the less closely linked markers h and rl the recombinant 

 frequency increased from 0.25 at 30 minutes to 0.43 at 90 

 minutes. This effect had been originally reported by Hershey 

 and Chase (1951) who found that the fraction of recombinants 

 between h and rl was 0.17 at a burst size of 10 (premature 

 lysis), 0.29 at a burst size of 250 (normal lysis), and 0.42 at a 

 burst size of 1,710 (lysis inhibition). The recombinant fre- 

 quency for unlinked or distantly linked markers may approach 

 the equilibrium value of 0.5 but cannot reach this value for 

 several reasons. One obvious reason is that the final phage 

 yield contains phage particles that have matured at various 

 times during the latent period. Particles that have matured 

 early are taken from the vegetative phage pool long before 

 genetic equilibrium has been reached. Because maturation is 

 irreversible, the excess of parental genotypes in the early samples 



