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Modern Genetic Analysis Chapter 9. The Genetics of Bacteria and Phages Bacterial Conjugation
Genetics In Process 9-1: Lederberg and Tatum discover genetic recombination in bacteria

In eukaryotes, the sexual cycle brings together genomes from two parents into one zygotic cell; then, in this cell or in descendent cells, meiosis takes place, which allows the recombination of parental alleles. This process has been known since the time of Mendel's paper in the 1860s. However, it was not until 1952 that a short paper by Joshua Lederberg and Edward Tatum appearing in Nature reported the discovery of "sex" in bacteria. The experiments that they used did not detect sexual union directly under the microscope, but indirectly, using a genetic method. They started with two strains of E. coli that had different nutritional deficiencies caused by mutations in genes that normally synthesize biotin, cysteine, leucine, phenylalanine, thiamine, and threonine. They wrote:

...single nutritional requirements were established as single mutational steps under the influence of X-ray or ultra-violet. By successive treatments, strains with several requirements have been obtained.

The strains that they used were both triple mutants, as shown below, where the genes are listed alphabetically:

Strains Y10 and Y24 were grown together in culture medium containing all six supplements. After a period of coincubation, cells were plated and recombinant genotypes were detected. Of the recombinants, the easiest to obtain and work with were wild types, which could be selected by plating cells on minimal medium. Wild types arose at a frequency of about one in a million cells. Wild types must have been of the following genotype:

As a control experiment, the parental strains were shown to never revert to wild type when grown individually; hence the wild types were the product of coculturing in some way.

Lederberg and Tatum concluded that there were only two likely explanations for these results:

1. The two cell types fused in some type of bacterial sexual process and engaged in recombination of alleles by "assortment in new combinations."
2. Something was passing through the medium from one cell type to another, transferring hereditary material. However, they found that sterile filtrates of one culture could not "transform" the other into the wild-type genotype, so they concluded that "these experiments imply the occurrence of a sexual process in the bacterium Escherichia coli."

© 1999 by W. H. Freeman and Company. All rights reserved.