Genes of the major histocompatibility complex (MHC) play a pivotal role in parasite resistance, and their allelic diversity has been associated with fitness variations in several taxa. However, studies report inconsistencies in the direction of this association, with either positive, quadratic or no association being described. These discrepancies may arise because the fitness costs and benefits of MHC diversity differ among individuals depending on their exposure and immune responses to parasites. Here, we investigated in black‐legged kittiwake (Rissa tridactyla) chicks whether associations between MHC class‐II diversity and fitness vary with sex and hatching order. MHC‐II diversity was positively associated with growth and tick clearance in female chicks, but not in male chicks. Our data also revealed a positive association between MHC‐II diversity and survival in second‐hatched female chicks (two eggs being the typical clutch size). These findings may result from condition‐dependent parasite infections differentially impacting sexes in relation to hatching order. We thus suggest that it may be important to account for individual heterogeneities in traits that potentially exert selective pressures on MHC diversity in order to properly predict MHC–fitness associations.

中文翻译：

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性别和孵化顺序调节着野生海鸟生命早期MHC-II多样性与适应性之间的联系。

主要组织相容性复合体（MHC）的基因在寄生虫抗性中起关键作用，其等位基因多样性与几种分类群的适应性变异有关。但是，研究报告在这种关联的方向上存在不一致之处，其中描述为正，二次或无关联。之所以会出现这些差异，是因为个体之间MHC多样性的适应成本和收益各不相同，这取决于他们对寄生虫的接触和免疫反应。在这里，我们以黑脚基蒂瓦克（Rissa tridactyla）雏鸡是否MHC II类多样性与适应性之间的关联随性别和孵化顺序而变化。MHC-II多样性与雌性雏鸡的生长和壁虱清除呈正相关，而雄性雏鸡则没有。我们的数据还揭示了MHC-II多样性与第二种孵化母鸡的存活率之间呈正相关关系（两个卵是典型的离体大小）。这些发现可能是由于条件依赖性寄生虫感染与孵化顺序有关而对性别的影响不同。因此，我们建议考虑可能对MHC多样性施加选择性压力的性状中的个体异质性很重要，以便正确预测MHC与健康的关联。