SEX IN BACH RIA—Cil,Nl. TIC SIUDIL.S 21 



unequivocal demonstration of mitotic figures, as claimed in the pio- 

 neering and provocative work of DeLamater and his associates 

 (1951). Although nuclear aggregates that are very suggestive of 

 mitotic metaphases and anaphases can be found with a brief search, 

 definitive interpretations of E. coli cytology depend for the most part 

 on the validity of the conclusions that have been drawn from techni- 

 cally superior material. It is difficult for a geneticist to imagine how 

 bacteria could get along without some sort of mitotic process, but its 

 details require critical and objective definition. The comparisons of 

 haploid and diploid E. coll have revealed consistent and unequivocal 

 differences, as shown in Figures 3 and 4 and elsewhere (Lederberg 

 et ah, 1951). The determination whether the diploids show a doubling 

 of the chromosome number is not yet subject to independent, objec- 

 tive verification. 



The correlation of genetic heterozygosity with nuclear complex- 

 ity is only a small step in the direction of a bacterial cytogenetics. 

 It has been furthered by Witkin's studies, in which the segregation 

 of mutant genes during fission has been correlated with the nuclear 

 plurality of the bacterial cells at the time the mutations were induced 

 (Witkin, 1951). These observations do accord, however, with a chro- 

 mosomal theory of inheritance and sexuality in E. coli. 



The work cited so far has been done with derivatives of a single 

 strain, K-12, of Escherichia coli. A few early attempts to duplicate 

 genetic recombination in other E. coli strains popular in genetic work 

 were quite unsuccessful. Cavalli and Heslot (1949) discovered a 

 culture in the British Type Culture Collection, NTCC 123, that was 

 fertile with K-12, but a special screening method had to be developed 

 before many new strains could be effectively studied (Lederberg, 

 1951a). Of nearly 2000 independent isolations of E. coli from various 

 sources, over fifty have proven to be cross-fertile with K-12, and so 

 far as has been tested, with each other. All of the new strains conform 

 to the type E. coli, except for an occasional minor deviation, but are 

 otherwise as heterogeneous as any sample of strains. They are serolog- 

 ically quite diverse: an immunogenetic study has been initiated which 

 has so far put the antigens of E. coli on the same basis as the mam- 

 malian blood groups. 



One important reason for undertaking this study of new strains 

 was to investigate the sexual compatibility relations of E. coli. Until 

 recently, several lines of evidence conspired to substantiate the idea 



