Data from: Heterozygosity is linked to the costs of immunity in nestling great tits (Parus major)

There is growing evidence that heterozygosity–fitness correlations (HFCs) are more pronounced under harsh conditions. Empirical evidence suggests a mediating effect of parasite infestation on the occurrence of HFCs. Parasites have the potential to mediate HFCs not only by generally causing high stress levels but also by inducing resource allocation tradeoffs between the necessary investments in immunity and other costly functions. To investigate the relative importance of these two mechanisms, we manipulated growth conditions of great tit nestlings by brood size manipulation, which modifies nestling competition, and simultaneously infested broods with ectoparasites. We investigated under which treatment conditions HFCs arise and, second, whether heterozygosity is linked to tradeoff decisions between immunity and growth. We classified microsatellites as neutral or presumed functional and analyzed these effects separately. Neutral heterozygosity was positively related to the immune response to a novel antigen in parasite-free nests, but not in infested nests. For nestlings with lower heterozygosity levels, the investments in immunity under parasite pressure came at the expenses of reduced feather growth, survival, and female body condition. Functional heterozygosity was negatively related to nestling immune response regardless of the growth conditions. These contrasting effects of functional and neutral markers might indicate different underlying mechanisms causing the HFCs. Our results confirm the importance of considering marker functionality in HFC studies and indicate that parasites mediate HFCs by influencing the costs of immune defense rather than by a general increase in environmental harshness levels.

越来越多的证据表明，在严苛环境条件下，杂合度-适合度相关性（heterozygosity–fitness correlations, HFCs）的表现会更为显著。现有实证研究显示，寄生虫侵染在HFCs的发生过程中发挥着中介效应。
寄生虫介导HFCs的途径主要有二：一是普遍引发机体高水平应激，二是诱导免疫必要投入与其他耗能功能之间的资源分配权衡。
为探究这两种机制的相对重要性，我们通过调控窝雏数来改变大山雀雏鸟的生长环境（此举可改变雏鸟间的竞争强度），同时对雏鸟窝群进行体外寄生虫侵染处理。本研究首先探究了何种实验处理条件下会出现HFCs，其次验证了杂合度是否与免疫和生长之间的权衡决策存在关联。我们将微卫星标记（microsatellites）划分为中性标记与假定功能标记两类，并分别对相关效应进行分析。
实验结果显示：在无寄生虫侵染的巢箱中，中性杂合度与雏鸟对新型抗原的免疫应答呈正相关，但在被寄生虫侵染的巢箱中则无此关联；对于杂合度较低的雏鸟，在寄生虫压力下开展的免疫投资，会以羽毛生长减缓、存活率下降以及雌性雏鸟身体条件变差为代价。无论生长环境如何，功能杂合度均与雏鸟的免疫应答呈负相关。
功能标记与中性标记的这种截然相反的效应，或许表明驱动HFCs产生的潜在机制存在差异。本研究结果证实了在HFCs研究中考虑标记功能属性的重要性，并表明寄生虫是通过影响免疫防御的成本，而非通过普遍提升环境严苛程度来介导HFCs的产生。