132 BENZER ET AL. 



this virus. This has been interpreted as indicating that the excluded 

 virus remains at the outside of the infected bacterium. In view of later 

 findings, this interpretation seems forced and the case should be re- 

 studied. 



A question of great interest attaches to the possible similarity of 

 the effects here described with the so-called interference effects ob- 

 served in both animal and plant viruses. Are the interference effects 

 in animal and plant viruses to be put into analogy with genetic recom- 

 bination or with mutual exclusion or with the depressor effect? The 

 principal point to be noted for a discussion of this question is the fact 

 that in phages mutual exclusion and depressor effect are strongest for 

 pairs of unrelated viruses, and probably totally absent for very closely 

 related viruses, whereas in animal and plant viruses interference has 

 sometimes been taken as an index of close relationship. 



It should be noted that a still higher degree of unrelatedness than 

 that here discussed could be considered, namely, that in which the two 

 phages do not have the same host. If two such phages are added to a 

 culture containing a mixture of the two host bacteria, presumably no 

 interference would result. Such a case may be the analogue of the infec- 

 tion of a plant or an animal with two viruses attacking different tissues 

 or, within the same tissue, attacking different components of the cell. 



32. Mixed infections with related strains (T2, T4, T6). — Bacteria 

 mixedly infected with two related strains, like T2 and T4, yield in 

 the majority of cases both types of phage. If the particles of the two 

 strains are marked with genetic markers, recombinants are found in 

 the yield (Delbriick and Bailey, 1946), as in the cases described in 31. 

 Beside this, type-hybrids are found in the yield (Luria, 1949), i.e., 

 particles combining characters specific for one or the other of the pa- 

 rental strains, respectively. 



As mentioned in 8, the main characters differentiating strains are 

 the serological specificities and the host-range. Other strain-specific 

 characters are the level of sensitivity to ultraviolet light and the degree 

 of photo-reactivability. T2 and T4 are clearly separated by serological 

 and host- range specificity, by resistance to UV (T4 being twice as 

 resistant as T2), and by photoreactivability (T2 being considerably 

 more reactivable than T4). The type-hybrids observed in the crosses 

 between T2 and T4 always present the same combination of serological 

 and host-range specificity as one or the other of the parental strains. 

 On the other hand, their UV sensitivity may be either that of T2 or 

 that of T4, independent of their other specificities. No intermediate 

 levels of UV sensitivity have been found. 



