Figure 4.

Changes in direct impact on host fitness (virulence) in experimental evolution of gut symbionts. Effects of symbiont inoculation on growth and mortality were assessed in the absence of the Bt pathogen. Final larval mass correlates with mortality rates imposed directly by gut bacteria infection (p < 0.01). Labelled lineages are significantly different from the ancestor: lineages D5 and L4 impose significantly lower and higher mortality, respectively, than the ancestor (z = −0.22, p = 0.027, z = 3.36, p = 0.00079); while insects inoculated with lineages D5, D6 and L1 reached increased final weight relative to insects infected with ancestors (t = 4.085, p < 0.001; t = 3.51, p < 0.001; t = 2.19, p < 0.05, respectively, while L4 produced larvae with lower mass (t = −2.42, p < 0.05). Data on the y-axis are final weight after a defined period of growth since egg hatch and inoculation with symbiont, data on the x-axis are arcsine (sqrt) transformed larvae mortality observed over this period of growth. Data are means of 28 replicates per lineage in two independent experiments. (Online version in colour.)