Fitness of reciprocal hemizygous (RH) strains. (A) Fitness in vivo. RH strains and the wild-type Y/S hybrid were competed for 5 d in immunocompromised mice. Twenty out of 21 strains with fitness values significantly different from the wild type were fitter than the wild type (P = 10−5), despite showing no measurable growth differences at 30°C in vitro (Supplemental Figs. 1, 2). All but the two SLA2 RH strains differ significantly from the wild type. Error bars, ±1 SE. (B) High-temperature (37°C) growth in vitro predicts fitness in vivo. Each strain's relative fitness rank is plotted both in vivo and in vitro; a significant (P = 0.002) correlation is observed. (C) Fitness in vitro. Y-alleles yield higher fitness than S-alleles at 37°C in vitro for 11 of 11 RH strain pairs. Genes are ordered by decreasing significance of the Y-allele advantage. Error bars, ±1 SE.
