354 



CHAPTER 38 



(segregation or integration) units from P T. 

 It is possible also to determine the linkage 

 relationships between auxotrophic markers in 

 the following way. The cross T+L+BrB' X 

 T-LBi^B^ is made on complete medium, 

 plated on complete medium, and replica- 

 plated four times on complete medium minus 

 T, or L, or Bi, or B. Prototrophic recombin- 

 ants grow on all four replicas, single auxo- 

 trophs grow on three, while double auxo- 

 trophs grow on two of the replicas. Since 

 prototrophs are found to be more frequent 

 than either T+L-B,+B+ or T'L+Bi+B+, Tand 

 L are linked. A further analysis of the results, 

 for other markers and from other experiments, 

 reveals that all the genetic markers tested in 

 E. coli are linked to each other, and can be 

 placed on a map arranged in a linear order 

 according to their recombination (segregation 

 or integration) distances. This means, in all 

 likelihood, that E. coli has a single chromo- 

 some. 



Can zygotes actually be demonstrated? 

 Since it would be almost impossible to find 

 these microscopically, we must look for them 

 genetically. Following mating, certain clones 

 behave as though they are mosaic for a num- 

 ber of markers. When single cell isolates, 

 made from such clones, are grown and tested, 

 it is found ^ that their progeny may possess 

 either of the parental genotypes of the origi- 

 nal cross, or they may be recombinants be- 

 tween the two. Clearly, the isolated cells 

 were derived from more or less persistent 

 heterozygotes, individuals that are diploid 

 for various markers. Thus, the segregants 

 within a clone offer unambiguous proof that 

 they were derived from a true zygote. We 

 conclude, therefore, that the zygote produced 

 following bacterial conjugation usually has 

 only a temporary existence which terminates, 

 after recombination, in the production of 

 haploid progeny. 

 3 By J. Lederberg and M. Zelle. 



SUMMARY AND CONCLUSIONS 



Genetic recombination occurs in Escherichia coli following the sexual process of conjugation. 

 This organism normally has a haploid nuclear body, in which all tested genes belong to a 

 single linear linkage group. 



REFERENCES 



Lederberg, J., "Gene Recombination and Linked Segregation in Escherichia Coli," Genetics, 

 32:505-525, 1947; reprinted in Papers on Bacterial Genetics, Adelberg, E. A. (Ed.), 

 Boston, Little, Brown, 1960, pp. 247-267. 



Lederberg, J., "Bacterial Reproduction," Harvey Lect., 53:69-82, 1959. 



Lederberg, J., and Tatum, E. L., "Gene Recombination in Escherichia Coli," Nature, London, 

 158:558, 1946; reprinted in Classic Papers in Genetics, Peters, J. A. (Ed.), Englewood 

 Cliffs, N.J., Prentice-Hall, 1959, pp. 192-194. 



QUESTIONS FOR DISCUSSION 



38.1. Criticize an attempt to prove the occurrence of genetic recombination by conjugation, 

 by simply mixing two bacterial strains which are singly auxotrophic for different 

 nutrients, and subsequently testing for prototrophs. 



38.2. Why is it futile to search cytologically for bacteria in the process of conjugation? 



38.3. Differentiate between, and give an example of: 



a. A selective and an unselected marker. 



b. A singly auxotrophic and a prototrophic bacterium. 



