Institute of Evolution, Centre for Ecological Research, Budapest H-1121, Hungary.
Centre for Eco-Epidemiology, National Laboratory for Health Security, Budapest, Hungary.
Parasitology. 2023 Aug;150(9):761-768. doi: 10.1017/S0031182023000598. Epub 2023 Jul 3.
An annotated synthesis of textbook definitions of parasitism is presented. Most definitions declare parasitism is a long-lasting relationship between individuals of different species harming the hosts. The infection-induced costs are interpreted as diseases in the medical-veterinary literature. Alternatively, evolutionary ecologists interpret it as a reduction of host's fitness (longevity, fertility or both). Authors often assume that such effects decrease host population growth and select for antiparasitic defences, which is not necessarily true because infections may simultaneously express opposite effects at different levels of biological organization. (i) At the cellular level, infection-induced cell growth, longevity and multiplication may yield tumours maladaptive at higher levels. (ii) At the individual level, reduced host longevity, fertility or both are interpreted as disease symptoms or reduced fitness. (iii) Contrary to common sense, the growth rate of infected host lineages may increase in parallel with the individuals' reduced survival and fertility. This is because selection favours not only the production of more offspring but also their faster production. (iv) Finally, infections that reduce host individuals' or lineages' fitness may still increase infected host populations' growth rate in the context of ecological competition. Therefore, differences between parasitism and mutualism may depend on which level of organization one focuses on.
现呈现出对教科书上寄生虫定义的注释性综合。大多数定义宣布寄生虫是指不同物种个体之间长期存在的、对宿主有害的关系。感染引起的代价在医学-兽医文献中被解释为疾病。或者，进化生态学家将其解释为宿主适应性降低（寿命、繁殖力或两者兼而有之）。作者通常假设这种影响会降低宿主种群的增长率，并选择抗寄生虫防御，但这不一定是正确的，因为感染可能在不同的生物组织层次上同时表现出相反的效果。(i) 在细胞水平上，感染诱导的细胞生长、寿命和增殖可能导致肿瘤在更高层次上不适应。(ii) 在个体水平上，宿主寿命、繁殖力或两者兼而有之的降低被解释为疾病症状或适应性降低。(iii) 与常识相反，受感染的宿主谱系的增长率可能会随着个体生存和繁殖力的降低而平行增加。这是因为选择不仅有利于产生更多的后代，而且有利于更快地产生后代。(iv) 最后，在生态竞争的背景下，降低宿主个体或谱系适应性的感染可能仍然会增加受感染宿主群体的增长率。因此，寄生虫和共生关系之间的区别可能取决于人们关注的组织层次。
