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Abstract
A molecular approach was used to examine the genetic relatedness of 19 Frankia isolates by measuring the extent of deoxyribonucleic acid-deoxyribonucleic acid homology and the fidelity of hybrid duplex molecules. The Frankia isolates examined were divided into two groups based on the results of hybridization tests. The members of genogroup 1, consisting of isolates from Alnus, Myrica, and Comptonia host plants, exhibited high levels of homology (67.4 to 94.1%) with strain ArI4, an isolate from Alnus rubra. Isolates from Elaeagnus, Ceanothus, Purshia, and Casuarina and isolate AirI2 from Alnus did not exhibit significant homology (less than 39%) with strain ArI4. None of the strains showed a high degree of homology with strain EuIla (less than 33%), an isolate from Elaeagnus umbellata. Among the isolates not belonging to genogroup 1, subgroupings seemed to exist, as evidenced by a very high level of homology (97%) between two isolates from Casuarina, strains D11 and the G2, but a low level of homology (27%) between other strains and strain G2. Thermal stability studies of the hybrid deoxyribonucleic acids which showed high levels of homology revealed an average mismatch of 3%, whereas the low-homology duplexes exhibited about 5% mismatch. The genome molecular weights of two probe Frankia strains, strains ArI4 and EuIla, were 8.3 × 109 and 6.0 × 109, respectively.
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