Evaluation of Spectinomycin Resistance as a Marker for Ecological Studies with Rhizobium spp.

Resistance to the antibiotic spectinomycin has been examined as a possible marker to supplement streptomycin resistance in ecological or genetic studies with rhizobia. Single step spontaneous mutants resistant to high levels of spectinomycin were isolated from 8 effective strains representing 4 species of Rhizobium. There was no evidence of cross resistance to streptomycin, and streptomycin resistant mutants were not cross resistant to spectinomycin. Minor changes in antigenic characteristics examined by immunodiffusion in agar were detected for mutants from 2 strains but these variants were still identifiable with the parent strains. Partial or full loss of symbiotic effectiveness occurred in only c. 20% of the spectinomycin resistant mutants and the resistance marker was unchanged through one plant passage. It is concluded that spectinomycin resistant mutants properly evaluated for possible pleiolropic effects should provide a useful marker system for use alone or in combination with streptomycin resistance in Rhizobium.