Haraguchi Y, Sasaki A
Department of Biology, Faculty of Science, Kyushu University, Fukuoka, Japan.
J Theor Biol. 1996 Nov 21;183(2):121-37. doi: 10.1006/jtbi.1996.9999.
If constantly changing genotypes are favorable in host and parasite coevolution, an indefinite escalation of mutation rates would result despite heavy mutational loads. We theoretically study this possibility by examining the mutation modifier dynamics of host and parasite that engage in genotype-specific epidemiological interaction. In the first model, we study the evolutionarily stable (ESS) mutation rate or switching rate if two alleles in a single locus are subjected to frequency-dependent selection favoring the rarer of the two. Mutation modifier locus is either tightly linked or unlinked to the selected locus. Sufficiently strong frequency-dependent selection may cause cycles in allele frequencies and a modifier with higher mutation rate enjoys the long-term advantage by randomizing the genotype of their offspring. Through the repeated events of invasion and replacement of mutation modifiers, the mutation rate continues to increase until the allele frequencies are stabilized. If some fraction of mutations are deleterious, there is no longer a pure ESS mutation rate: the evolutionarily stable population then consists of multiple strains concerning mutation modifier, typically one with a very high mutation rate and the other with a very low rate, stably coexisting and fighting off invasion by any other modifiers. These results are almost independent of the linkage between the selected and the modifier loci. In the second model, we consider the joint evolution of host and parasite mutation modifiers, assuming that a specific pair of host and parasite genotype densities change following the Nicholson-Bailey type model. If there is no cost of deleterious mutations, mutation rates of both species are escalated indefinitely by modifier evolution until they completely suppress the fluctuation of genotype densities. However, a small cost of deleterious mutation is enough to collapse this coevolutionary equilibrium of inflated mutations. Typical coevolutionary outcome is that the parasite mutation rate is accelerated to a high level; whereas the host mutation rate is driven to zero. Extension of our results to host-parasite coevolution of recombination modifier evolution is discussed.
如果不断变化的基因型在宿主与寄生虫的协同进化中是有利的，那么尽管存在巨大的突变负荷，突变率仍会无限上升。我们通过研究参与基因型特异性流行病学相互作用的宿主和寄生虫的突变修饰动态，从理论上探讨了这种可能性。在第一个模型中，如果单一位点上的两个等位基因受到频率依赖选择，且更青睐两者中较罕见的那个，我们研究了进化稳定（ESS）突变率或转换率。突变修饰位点与被选择位点紧密连锁或不连锁。足够强的频率依赖选择可能会导致等位基因频率出现循环，具有较高突变率的修饰基因通过使其后代基因型随机化而获得长期优势。通过突变修饰基因的反复入侵和替代事件，突变率持续增加，直到等位基因频率稳定下来。如果一部分突变是有害的，那么就不再存在纯的ESS突变率：进化稳定种群随后由涉及突变修饰基因的多个菌株组成，通常一个具有非常高的突变率，另一个具有非常低的突变率，它们稳定共存并抵御任何其他修饰基因的入侵。这些结果几乎与被选择位点和修饰位点之间的连锁关系无关。在第二个模型中，我们考虑宿主和寄生虫突变修饰基因的联合进化，假设一对特定的宿主和寄生虫基因型密度按照尼科尔森 - 贝利类型模型变化。如果有害突变没有代价，那么两个物种的突变率会通过修饰基因进化无限上升，直到它们完全抑制基因型密度的波动。然而，有害突变的微小代价就足以打破这种膨胀突变的协同进化平衡。典型的协同进化结果是，寄生虫的突变率加速到高水平；而宿主的突变率则被驱动至零。我们还讨论了将研究结果扩展到重组修饰基因进化的宿主 - 寄生虫协同进化情况。
