Diversifying and Stabilizing Selection of Sialidase and N -Acetylneuraminate Catabolism in Mycoplasma synoviae

Abstract

ABSTRACT Sialidase activity varies widely among strains and tends to correlate with strain virulence in the avian pathogen Mycoplasma synoviae . To characterize the forms of selection acting on enzymes required for sialic acid scavenging and catabolism, the ratios of nonsynonymous ( K a ) to synonymous ( K s ) mutation frequency were calculated for codons in the sialidase gene of 16 strains of M. synoviae and for its nearly identical homolog in four strains of Mycoplasma gallisepticum . The K a / K s (ω) values for the linked genes required for nutritive N -acetylneuraminate catabolism ( nanA , nagC , nanE , nagA , and nagB ) from nine strains of M. synoviae were also determined. To provide context, ω was determined for all corresponding genes of 26 strains of Clostridium perfringens and Streptococcus pneumoniae . Bayesian models of sequence evolution showed that only the sialidase of M. synoviae was under significant ( P < 0.001) diversifying selection, while the M. synoviae genes for N -acetylneuraminate catabolism and all genes examined from M. gallisepticum , C. perfringens , and S. pneumoniae were under neutral to stabilizing selection. Diversifying selection acting on the sialidase of M. synoviae , but not on the sialidase of M. gallisepticum or the sialidases or other enzymes essential for sialic acid scavenging in other Firmicutes , is evidence that variation in specific activity of the enzyme is perpetuated by a nonnutritive function in M. synoviae that is influenced by the genomic context of the organism.