Turner Paul E, Cooper Vaughn S, Lenski Richard E
Center for Microbial Ecology, Michigan State University, East Lansing, Michigan, 48824.
Evolution. 1998 Apr;52(2):315-329. doi: 10.1111/j.1558-5646.1998.tb01634.x.
It has been hypothesized that there is a fundamental conflict between horizontal (infectious) and vertical (intergenerational) modes of parasite transmission. Activities of a parasite that increase its rate of infectious transmission are presumed to reduce its host's fitness. This reduction in host fitness impedes vertical transmission of the parasite and causes a tradeoff between horizontal and vertical transmission. Given this tradeoff, and assuming no multiple infections (no within-host competition among parasites), a simple model predicts that the density of uninfected hosts in the environment should determine the optimum balance between modes of parasite transmission. When susceptible hosts are abundant, selection should favor increased rates of horizontal transfer, even at the expense of reduced vertical transmission. Conversely, when hosts are rare, selection should favor increased vertical transmission even at the expense of lower horizontal transfer. We tested the tradeoff hypothesis and these evolutionary predictions using conjugative plasmids and the bacteria that they infect. Plasmids were allowed to evolve for 500 generations in environments with different densities of susceptible hosts. The plasmid's rate of horizontal transfer by conjugation increased at the expense of host fitness, indicating a tradeoff between horizontal and vertical transmission. Also, reductions in conjugation rate repeatedly coincided with the loss of a particular plasmid-encoded antibiotic resistance gene. However, susceptible host density had no significant effect on the evolution of horizontal versus vertical modes of plasmid transmission. We consider several possible explanations for the failure to observe such an effect.
有假说认为，寄生虫水平（感染性）传播模式与垂直（代际）传播模式之间存在根本冲突。寄生虫增加其感染性传播速率的活动被认为会降低其宿主的适应性。宿主适应性的这种降低会阻碍寄生虫的垂直传播，并导致水平传播与垂直传播之间的权衡。考虑到这种权衡，并假设不存在多重感染（寄生虫在宿主体内不存在竞争），一个简单的模型预测，环境中未感染宿主的密度应决定寄生虫传播模式之间的最佳平衡。当易感宿主数量丰富时，选择应有利于水平传播速率的提高，即使以垂直传播的降低为代价。相反，当宿主稀少时，选择应有利于垂直传播的增加，即使以水平传播的降低为代价。我们使用接合质粒及其感染的细菌对权衡假说和这些进化预测进行了测试。使质粒在具有不同密度易感宿主的环境中进化500代。质粒通过接合进行水平转移的速率以宿主适应性为代价而增加，这表明水平传播与垂直传播之间存在权衡。此外，接合速率的降低反复与特定质粒编码的抗生素抗性基因的丢失同时发生。然而，易感宿主密度对质粒水平传播与垂直传播模式的进化没有显著影响。我们考虑了几种可能的解释来说明未能观察到这种影响的原因。
