24 SEX IN MICROORGANISMS 



Studied in some detail, and it can be asserted that X is not related in 

 any way to genetic recombination or to the sexual compatibility 

 mechanism. Non-lysogenic, i.e., X-free cultures, are fully compatible 

 in sexual recombination, and the transfer of F+ is independent of the 

 transfer of \ which, unlike F+, can readily be obtained in cell-free 

 filtrates. Genetic studies of lysogenicity (Lederberg and Lederberg, 

 1953) have, however, demonstrated a close relationship between this 

 latent virus and the chromosomes of the bacterial host. 



The details of the compatibility system are being studied at the 

 present time. It has been noted that F-f- x F+ crosses tend to be con- 

 siderably less productive than comparable F-\- x F— . Many of the re- 

 sults are consistent with the concept of relative sexuality as noted in 

 many algae and fungi. That is to say, different cultures can be ar- 

 ranged in sequence of relative potencies, such that the productivity of 

 a cross will be related to the difference in potency of the two parents. 

 In E. coli K-12, the relative potency can be controlled both by envi- 

 ronmental variations, and by genotypic effects. Within K-12, differ- 

 ences in the F-\- agent itself have not been found. However, the F-h 

 state as conditioned by some other wild-type strains appears to be 

 unstable in K-12 cells, suggesting the possibility of genetic differences 

 in the presumed agent itself. 



The genetic basis of the observed "mutations" to F— in strain 

 K-12 is not known, and these have not been experimentally repro- 

 ducible. Many wild type strains are F— (i.e., non-infective) but 

 retain their compatibility status, so that it is impossible to generalize 

 on the causes of sterility or compatibility in the species E. coli taken 

 as a whole. 



In the absence of direct morphological evidence of sexual fu- 

 sions, the principal alternative explanation for genetic recombination 

 in E. coli has been "transformation" or "transduction." The biology 

 of bacterial transductions is not very well understood (Ephrussi- 

 Taylor, 1951; Austrian, 1952); in many ways it may be constructive 

 to regard them as a limited form of hybridization. The chief distinc- 

 tion between transduction and sexual recombination is that the former 

 seems to involve only a very small part of the whole genotype of the 

 bacterium at each transfer (as in the capsular transformations in the 

 pneumococcus), whereas sexual reproduction allows reassortment of 

 the entire genotype of each parent, as in E. coli. The former seems to 

 be correlated with an active unit that is morphologically and chemi- 



