ABERRANT HETEROZYGOTES IN ESCHERICHIA COLI* 



By Joshua Lederberg 



Department of Genetics, University of Wisconsin 



Communicated by R. A. Brink, February 9, 1949 



A mechanism of genetic recombination has been indicated in experiments 

 on Escherichia coli, strain K-12. 1 A synthetic agar medium was used as a 

 selective sieve to isolate occasional prototroph recombinants which appear 

 in mixed cultures of complementary biochemical mutants. Later, addi- 

 tional genetic factors were introduced, including fermentation and virus- 

 resistance mutations, and these factors were found to segregate in charac- 

 teristic ratios, suggesting linkage. All the cells in a given prototroph 

 colony showed the same combination of characters and were stable on 

 further cultivation. Therefore, it was inferred that segregation had 

 occurred before the initiation of the colony, and that the postulated zygote 

 had a very short life, probably a single cell generation. The life cycle 

 would resemble the ascomycete's, in which haploid nuclei fuse to form 

 a transient diploid zygote which undergoes meiosis without any intervening 

 mitoses. 



Exceptions to this rule have now been found in the form of unstable 

 prototroph cultures which continually segregate out various recombination 

 types so as to suggest that they are heterozygous and diploid. But 



[Reprinted from Proceedings of the National Academy of Sciences 35 : (4) 178-184, April, 

 1949] 



164 



