Relationship between two serotypes of avian mycoplasmas by serological and DNA-DNA hybridization tests

Genetic relatedness among eight strains of avian mycoplasmas representing two serotypes, C-O and D-P, was determined using DNA-DNA hybridization tests. One strain from each serotype was found to be misclassified. The average percent homology between the two serotypes was 34.1% at 56 C and 18.6% at 66 C. The average homology among the four C-O organisms was 99.6% at 56 C and 93.5% at 66 C, while among the four D-P strains the average homology was 91.6% at 56 C and 92.2% at 66 C. These relationships correlated with antigenic cross-reactions observed in complement fixation (CF) and fluorescent antibody (FA) tests. Thermal denaturation of the pure DNA from each strain resulted in base compositions of 25.0 to 28.2 moles percent guanosine plus cytosine (GC). Thermal denaturation of selected hybrid DNA revealed a linear relationship between percent homology and delta melting temperature (1 C delta Tm for 15% difference in percent homology). Thirty-four percent homology between the organisms of C-O and D-P serotypes indicates a minor antigenic relationship; thus, it is recommended that these two serotypes be retained as separate groups.