Serologic and Genomic Relatedness of Group VIII and Group XVII Spiroplasmas and Subdivision of Spiroplasma Group VIII into Subgroups

Abstract

Spiroplasmas are currently classified in a group system. Criteria for separation of the twenty-four currently designated groups include serologic relatedness, polyacrylamide gel electrophoretic patterns of proteins, guanine-plus-cytosine base ratios, and, in some cases, DNA-DNA homology. The analysis of DNA-DNA homology and serologic data from a large array of strains recently discovered in dipteran insects reveals that group VIII strain EA-1 from a syrphid fly, strain TAAS-1 from a horse fly, and group XVII strain DF-1 from a deer fly belong to a large complex (supergroup) of strains with various degrees of interrelatedness. Strains DF-1 and EA-1 share DNA-DNA homology of 33 to 48% (high-stringency conditions), while strain TAAS-1 shares 42 to 67% homology with DF-1 and EA-1. The strains had temperature optima of 30 to 37°C, but the temperature minima and maxima reflected the geographic region of strain origin. These three strains also share G+C values of about 30 mol%, utilize arginine, and tend to grow in culture to very high titers (1011 cells per ml). The helical cells of these strains are smaller than those of other spiroplasmas and readily pass through filter pores of 220 nm. These data support the taxonomic placement of the biotypes represented by strains EA-1, DF-1, and TAAS-1 into one supergroup, group VIII, with subgroups designated as VIII-1, VIII-2, and VIII-3, respectively. It is proposed that group XVII remain vacant.