Abstract

ENETIC recombination in bacteria was first successfully studied in strain G K-12 of Esclzericlzia coli Since the nutritional mutants used in the crosses were derived directly from this strain under clonal propagation, their conipatibility implied a homothallic or self-compatible sexual system (cf. The inference that crossing was genetically unrestricted was supported by the absence of marked hereditary mating preferences among the segregants of a variety of crosses LEUPOLD 1950). More recently, evidence has been secured for a system of sexual compatibility which was previously obscured by its unique inheritance via an infective agent. METHODS AND CULTURES Experimental techniques differed slightly in Madison and Milan, but the concordance of the results obviates a detailed comparison. Crossing procedures, genetic symbols and the detailed pedigrees of many stocks are given elsewhere ( CAVALLI 1950 ; LEDERBERC 1947 ; LEDERBERG et al. 1951). Most of the experiments have involved crosses of the original K-12, of stocks clonally related to 59-161, a double mutant requiring biotin and methionine ( B -M -) , and stocks related to Y-10, a triple mutant requiring threonine, leucine, and thiamin ( T -L -T h -) . (Since the original isolation of 58-161 and early crossing esperiments (TATUM 1945 ; The methionineless stock is, fortunately, so stable that back mutations to prototrophy are undetectable under the conditions of crossing experiments.) Stocks carrying similar markers which have been extracted from crosses