Grouping of Frankia Strains on the Basis of DNA Relatedness

Abstract

Summary The diversity among 27 Frankia strains was estimated by DNA-DNA hybridization (membrane filter method) and by determining DNA base compositions. The guanine + cytosine contents of DNA ranged from 69 to 73 mol%. The levels of DNA relatedness ranged from 3 to 100%, which allowed us to differentiate nine genospecies. Five genospecies were delineated among 18 strains compatible with plants of the genus Alnus (isolates from Alnus and Comptonia) and four genospecies were delineated among 9 strains compatible with plants of the family Elaeagnaceae (isolates from Elaeagnus, Hippophae, Shepherdia, and Colletia). Two additional strains were assigned to certain genospecies, since their total DNA restriction banding patterns and restriction fragment length polymorphism profiles were identical to those of representatives of these genospecies. The levels of DNA relatedness between members of the same genospecies ranged from 64 to 100%. In the Alnus host compatibility group, three genospecies displayed relatedness at the level of about 30% but were only 7 to 17% related to two other genospecies represented by single strains. In the Elaeagnaceae host compatibility group, strains from three genospecies showed relatedness at the levels of 15 to 36% but were only about 5% related to the fourth genospecies represented by a single strain. Low degrees of relatedness (3 to 20%) were also found between representatives of the two different host compatibility groups. The genospecies delineated cannot receive the status of nomenspecies, since few phenotypic criteria are available for their differentiation.