Caudwell A
Isr J Med Sci. 1984 Oct;20(10):1025-7.
There is no satisfactory theory to explain how parasites whose effect is to kill or to weaken their hosts have been able to survive evolution. A good explanation may be found in the study of mycoplasma-like organisms (MLO). We show here the existence of natural cycles of MLO between symptomless wild plants and unaffected vectors. Such natural cycles may be the basis of a coevolutionary process by which the prokaryotes and the whole cycle have persisted to our time. Disease outbreaks occur with intrusion of a cultivated plant (direct mode) or an imported insect (indirect mode). From an evolutionary point of view, the natural cycle constitutes a protection for the host plants and natural vectors against the introduction of foreign plants or vectors as competitors in their ecological niche. A role of MLO and other parasites in evolution could be to provide stability to ecosystems, which is necessary for new characters to emerge.
目前尚无令人满意的理论来解释那些会杀死或削弱宿主的寄生虫是如何在进化过程中存活下来的。在对类支原体生物（MLO）的研究中或许能找到一个很好的解释。我们在此展示了无症状野生植物和未受影响的传病媒介之间存在MLO的自然循环。这样的自然循环可能是一个共同进化过程的基础，通过这个过程，原核生物和整个循环一直持续到我们这个时代。疾病爆发是由于栽培植物的侵入（直接模式）或外来昆虫的引入（间接模式）。从进化的角度来看，自然循环为宿主植物和自然传病媒介提供了一种保护，使其免受外来植物或传病媒介作为其生态位竞争者的影响。MLO和其他寄生虫在进化中的作用可能是为生态系统提供稳定性，而这是新特征出现所必需的。
