Mapping genes in microorganisms
When T.H. Morgan’s group developed the linkage analysis methods that enabled them to map genes on the chromosomes of Drosophila melanogaster, their methodology took advantage of the fact that the fruit fly is diploid and reproduces sexually. The recombination events that enabled clever scientists to locate genes were associated with meiosis; breeding was crucial to establishing linkage. Yet shortly after the Second World War, evidence surfaced for recombination in bacteria and viruses; a year later Joshua Lederberg published the first linkage map for Escherichia coli (Delbrück and Bailey 1946; Lederberg 1947). In 1949, Alfred Hershey and Raquel Rotman extended genetic mapping further down the microbial scale, producing the first linkage map of a virus, bacteriophage T2 (Hershey and Rotman 1949). What did it mean to extend the “mapping” of genes from sexually reproducing organisms such as flies and plants to viruses and bacteria, for whom even the existence of chromosomes was uncertain and disputed?1 And how did genetic mapping in the microbial world give new meanings to sex and mating, and even to the term gene itself ?