SUMMARY: Molecular hybrids were prepared between unlabelled DNA from representative strains of eleven genetic races of and [C]DNA from typical strains of each of the three main races. The thermal stability of each hybrid was determined. The nature of the hybrids formed varied with the incubation temperature and the kind of DNA used. Hybridization in 2 × SSC-30% dimethylsulphoxide below 59°C yielded two kinds of hybrids: a labile one of unknown nature, denaturing below 59°C, and a more or less stable hybrid denaturing above that temperature. The latter was the only one formed in hybridizations at or above 59°C. There were three kinds of stable hybrids. Within each of the main Agrobacterium races thermal stability of the molecular hybrid was about the same (within 2°C) as for the homo-duplex. Between two races of 50% DNA relatedness, the duplexes were about 6°C less stable. Between races of 10 to 15% DNA relatedness, the duplexes were weak, and the stability was at least 13°C lower. The stability of the hybrids decreased concomitantly with the degree of DNA relatedness. The decreased hybrid denaturation curve is not due to AT-rich sequences. The less two races of agrobacteria appeared to be evolutionarily related, the more mutations occurred within the common part.


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