Parasite infectivity, virulence and host resistance have been in the centre of the scientific interest when it comes to host–parasite relationships. In addition to resistance, hosts may also vary in their tolerance against parasites. This is important to notice because resistance and tolerance have different consequences in host–parasite co‐evolution. Here, we show that families of farmed rainbow trout (Oncorhynchus mykiss) show both host defence strategies, resistance, and tolerance, against infectivity and virulence of Diplostomum sp. (Trematoda) parasites. Both strategies have moderate genetic variation and are genetically independent of each other. It is also shown that the families having the highest performance measured as higher weight, better condition factor and lower mortality in absence of the parasites suffer the most when parasitism increases. For practical breeding programmes, this means that both resistance and tolerance can be improved by selection without compromising one of the strategies. These results give new insight into defence strategies against parasites in fish and into processes of fish–parasite co‐evolution.

中文翻译：

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遗传确定的对Diplostomum sp。的抗性和耐受性。养殖虹鳟鱼中的寄生虫

当谈到寄主与寄生虫的关系时，寄生虫的传染性，毒力和寄主抵抗力一直是科学关注的焦点。除抗药性外，寄主对寄生虫的耐受性也可能有所不同。必须注意这一点，因为抗性和耐受性在宿主-寄生虫共同进化中具有不同的结果。在这里，我们证明了养殖虹鳟鱼（Oncorhynchus mykiss）的家庭表现出宿主防御策略，抵抗力和耐受性，可以抵御梁龙的传染性和毒力。sp。（Trematoda）寄生虫。两种策略都具有中等的遗传变异，并且在遗传上彼此独立。还表明，在寄生虫增多时，表现出最高性能的家庭表现为体重增加，条件因子更好，无寄生虫时死亡率降低。对于实际的育种计划，这意味着可以通过选择来提高抗性和耐受性，而不会影响任何一种策略。这些结果提供了对鱼类中寄生虫的防御策略以及鱼类与寄生虫共同进化过程的新见解。