TABLE 3.

Results of the McDonald-Kreitman testa

ETEC fimbrial system component (protein)bNo. of polymorphic sitesNo. of fixed sitesNIc
SRSR
Chaperone (CfaA)12111083850.89
Major subunit (CfaB)12011127310.81
Outer membrane usher (CfaC)98973953451.13
Minor subunit (CfaE)53911701951.50d
  • a Numbers of synonymous (S) and replacement (R) sites that are polymorphic within alleles of a subclass or that represent fixed differences between subclasses for each fimbrial component are shown. Under the neutral mutation theory, the S/R ratio should be equivalent for polymorphic and fixed sites under strict neutrality.

  • b Protein function CFA/I homolog.

  • c NI, neutrality index.

  • d ΔG = 4.03; df = 1; P = 0.04 (two-tailed Fisher's exact test).