Figure 6

Amplification on selective hosts comparing separate and mixed competitions. (6A) Comparison of T7E₁ and T7₀ amplifications when grown in separate competitions. Five independent amplification measurements for the two phages are given; initial titers ranged 2−8×10⁴. These data are independent of those in Figure 5; the initial phage titers here fall between the rightmost two X-axis values in that figure. All 5 assays exhibit a (highly) statistically significantly greater amplification of T7E₁ than of T7₀. Bars span 2 standard errors. (6B) Changes in relative frequencies when grown in mixed competitions: a tragedy of the commons. The figure shows the proportions of T7E₁ before and after growth for 5 trials, the arrows indicating that the relative proportion of T7E₁ declined in all cases. The relative decline is interpreted as stemming from T7E₁ producing an enzyme that benefits itself and T7₀ equally but only T7E₁ experiencing the cost of providing the benefit. In all 5 cases, the proportional increase of T7₀ is statistically significant by a Fisher^′s exact test or χ²test; all are highly significant except the second, which is marginally significant. From left to right, each mixed assay trial in (6B) corresponds to the respective trial in (6A), done in different zones on the same plate. All assays used EV36 with M9 glucose media.