Figure 7

Response of sexual populations to injection of clones. Sexual populations (N = 10² (i), 10³ (ii), 10⁴ (iii)) under lag load are allowed to stabilize at steady state rates of parasite and host mutation (μ _p /μ _h = 1, L = 10). A constant value for μ _p /μ _h is an attempt to generate similar lag loads at all data points. After stabilization, a sexual female of the highest adaptation score is converted to clonality and given a relative fecundity of 2. Because the highest adaptation scores are present only in new advantageous mutations that are still rare, the parasite population develops a negative-frequency dependent response as the frequency of the clonal allele rises. If the parasite mutates rapidly, and the sexual population is under significant lag load, the original host clone and her descendants are progressively rendered unfit and are extinguished by sex. As the parasite mutation rate, μ _p , is reduced in μ _p /μ _h (with concomitant reduction in μ _h ), the rate of loss of clonal fitness is lower during expansion of the clonal population, and clones outcompete sex. Whether sex wins or clones win depends on population size and μ _p , for constant μ _p /μ _h . The three lines in this Figure show that large populations are more effective at preventing clonal invasion than small ones, as expected. All three populations show thresholds of ~45% for μ _p below which sex only succeeds because some clonal mothers are eliminated by drift immediately after conversion of the sexual mother. Errors are Standard Errors of the Mean.