Fitness distributions of WNV CP40 with or without co-infection on mosquito cell culture. Individual data points represent relative fitness values of 40 clones isolated following 24 h growth at MOI 10 (with co-infection) or 0.01 (without co-infection). Solid lines represent means +/- standard deviations and dashed lines represent relative fitness values of combined populations. The inset graph depicts individual viral titers of clones following 72 h growth on mosquito cell culture. Significantly higher relative fitness values and significantly lower viral titers were measured in the MOI 10 group (t-tests, p < 0.05), as indicated by the *.