Recombination involving the ilv cluster of Salmonella typhimurium-Salmonella montevideo hybrids.

I N the original report on Salmonella typhimurium-Salmonella montevideo hybrids (TM hybrids) from this laboratory, it was noted that incorporation of montevideo genic material into the ilu (isoleucine-valine) region of the Salmonella typhimurium chromosome always involved the four known loci of the ilv cluster (GLATZER, LABRIE and ARMSTRONG 1966). The lack of evidence for “partial hybridity,” e.g., two loci of typhimurium origin and two of montevideo in the ilv region, was explained as a reflection of the close linkage of the ilv loci. Many more TM hybrids selected at the ilv loci have now been analyzed genetically; and the “all-or-none” incorporation of montevideo material into the ilv region has remained the dominant feature of these hybrids. To date, 150 such hybrids have been analyzed (GLATZER, LABRIE and ARMSTRONG 1966; ATKINS and ARMSTRONG 1969; unpublished results); and only nine (6%) display a different pattern of hybridity. Transductional analyses of these latter strains reveal that hybridity has resulted from a very limited incorporation of montevideo material within the locus of selection. Thus, the evidence suggests that, in the production of TM hybrids, incorporation of the entire ilv cluster is the favored mode of recombination and that other types of recombination involve insertion of small amounts of DNA. This report presents two additional lines of evidence that support the supposition that heterologous recombination involving the ilv cluster does not result in “partial hybridity” but, rather, in the incorporation of either the whole cluster or a very limited portion of it. These results were obtained from transductional analysis of: 1) recombinants derived from S. typhimurium x TM hybrid crosses; and 2 ) prototrophic hybrids produced by mating ilv auxotrophs of S. montevideo and S . typhimurium.