148 CELL HEREDITY 



S a b T 



FIGURE 5.18. Diagrammatic representation of two hypotheses of prophage recombi- 

 nation. In the upper diagram the prophage (white line) is shown completely inserted 

 in the bacterial chromosome (black line). In the lower diagram only a short segment 

 is inserted. In the former case, but not in the latter, an exchange between b and c 

 would recombine the outside markers S and T (after Jacob and Wollman, 1959, in: 

 Tunewoll, Recent Progress in Microbiology, p. 15, Springfield, III., Charles C. Thomas). 



prophage in lysogenic cells. Whether this is true of all reducible epi- 

 somes is yet to be seen. 



Only one prophage has been found in E. coli which does not segregate 

 in crosses as would be expected if it were located on the chromosome. 

 Since.it is inherited maternally, always from the F~, it seems to have 

 an extrachromosomal residence. It is interesting that this prophage 

 yields the only type of phages that will nonspecifically transduce many 

 different genes in strain K12. 



Although most work on bacterial mating has been done with one 

 strain of E. coli, K12, many other strains have been shown to undergo 

 the process. It has even been possible to cross E. coli with Shigella 

 dysenteriae and with Salmonella typhimurium, raising questions about 

 the meaning of genus and species in such organisms. Furthermore, other 

 bacteria, such as Pseudomonas and Bacillus, have been shown to mate, 

 and so have the commercially important bacterialike organisms, Strepto- 

 myces. In the last two cases, however, all strains do not show a pattern 

 of recombinant types among the offspring that allows an interpretation 

 of gene exchange in terms of crossing over or copy-choice mechanisms 

 in a linkage group with genes in a linear order. Indeed, out of such 

 matings come new genotypes that do not reflect the parental genes which 

 entered the cross. Whether such phenomena are to be explained on the 



