SFX IN BACTFRIA— GENKTIC SI UOIFS 19 



cle controlling each observed trait has been segregated, and is rep- 

 resented only once in the genotype of the recombinant cell. 



By compounding elementary principles, genetic maps of E. coli 

 can be constructed from segregation data involving numerous unse- 

 lected markers (Cavalli, 1950; Newcombe and Nyholm, 1950; Roth- 

 fels, 1952; Lederberg et j/., 1951). By using other methods, the auxo- 

 troph mutations can be relieved of their burden of the selection of 

 recombinants, and thus handled as unselected markers also. About 

 half of the known markers of E. coli K-12 have been satisfactorily 

 located in a single linear linkage group. Other markers have displayed 

 a more confusing behavior which does not fit any scheme very 

 satisfactorily, but is probably a result of rather complex chromosomal 

 aberrations, for w^hich there is independent evidence from the study 

 of exceptional diploids (v. infra). It was long thought that E. coli had 

 onh' one chromosome, but more recent evidence points to at least 

 two, the segregation of which is not, however, entirely independent 

 for secondary reasons (Fried and Lederberg, 1952). Cytological 

 observations on haploid K-12 cultures have been interpreted by De 

 Lamater (1952) as signifying three chromosomes, but further work 

 is needed for the detailed concordance of genetic with cytological 

 findings. Cytological study of E. coli has so far been confined to 

 vegetative cells, whereas the genetic studies deal principally with 

 segregation at meiosis. 



The difficulties, briefly mentioned, in the segregation of cer- 

 tain misbehaving markers might appear to be fatal to a straightforward 

 sexual interpretation of recombination except for the confirmatory 

 evidence provided by exceptional diploid cultures. In ordinary cross- 

 ings, the diploid condition has been inferred from its consequences of 

 recombination and segregation, but is not directly observed. In 1946- 

 47, many unsuccessful attempts were made to secure artificial diploids 

 with agents such as camphor, acenaphthene, colchicine, and heat 

 shocks, which have been used for other organisms (cf. Roper, 1952). 

 More recently, however, a mutation, Het, occurred in one of our 

 stocks which serves the same purpose (Lederberg et al., 1951). Little 

 is known of the action or generic transmission of Het, but when it is 

 present in one or both parents of a cross, several per cent of the 

 prototrophs prove to be persistent heterozygotes. These heterozygous 

 cultures continually segregate the alternative markers brought in 



