BACTERIOPHAGE GENETICS 339 



Some of these can be eliminated on the basis of available evi- 

 dence. If recombination occurred early as a necessary prelude 

 to multiplication, one would expect to find recombinants pri- 

 marily in large clones in single cell bursts. In actual experiments 

 with closely linked markers, Hershey and Rotman (1949) found 

 only a small proportion of bursts with more than 10 recombinant 

 particles, so recombination does not necessarily precede multi- 

 plication. If recombination were a terminal event followed 

 invariably by maturation, the distribution of recombinants in 

 single cell bursts should be Poissonian, since each act of recom- 

 bination would be independent of other similar acts. The 

 actual frequency distribution observed by Hershey and Rotman 

 (1949) lies between a purely Poissonian distribution and a purely 

 clonal distribution of the type observed for phage mutations by 

 Luria (1951). These results suggest that each act of recombina- 

 tion is an independent event but that the product or products of 

 the act have a definite probability of replication before being 

 withdrawn from the vegetative pool by the act of maturation. 

 Because the pool size remains constant during maturation, 

 replication and maturation must proceed at the same rate and a 

 newly formed recombinant must have about an even chance of 

 replicating or not before maturation providing that the act of 

 recombination does not alter the probability of subsequent 

 maturation. The latter is not likely to be an important con- 

 sideration because studies to be discussed later suggest that most 

 of the progeny particles of mixedly infected bacteria have under- 

 gone one or more acts of mating. The biochemical evidence 

 indicates clearly that maturation is an irreversible process so 

 that the mature phage particles do not reenter the vegetative 

 pool. One may conclude tentatively from the available evi- 

 dence that phage particles multiply in the vegetative pool after 

 recombination, but produce on the average only very small 

 clones, before maturation intervenes. Large clones of recom- 

 binants could seldom arise unless the recombination occurred 

 early in the development of a particular vegetative pool when the 

 rate of replication was much greater than the rate of maturation. 



