Data from: MHC-similarity and sexual selection: different doesn't always mean attractive

Females that mate multiply have the possibility to exert postcopulatory choice and select more compatible sperm to fertilize eggs. Prior work suggests that dissimilarity in Major Histocompatibility Complex (MHC) plays an important role in determining genetic compatibility between partners. Favouring a partner with dissimilar MHC alleles would result in offspring with high MHC diversity, and therefore with enhanced survival thanks to increased resistance to pathogens and parasites. The high variability of MHC genes may further allow discrimination against the sperm from related males, reducing offspring homozygosity and inbreeding risk. Despite the large body of work conducted at precopulatory level, the role of MHC similarity between partners at postcopulatory level has been rarely investigated. We used an internal fertilizing fish with high level of multiple matings (Poecilia reticulata) to study if MHC similarity plays a role in determining the outcome of fertilization when sperm from two males compete for the same set of eggs. We also controlled for genome-wide similarity by determining similarity at 10 microsatellite loci. Contrary to prediction, we found that the more MHC-similar male sired more offspring while similarity at the microsatellite loci did not predict the outcome of sperm competition. Our results suggest that MHC discrimination may be involved in avoidance of hybridization or outbreeding rather than inbreeding avoidance. This, coupled with similar findings in salmon, suggest that the preference for MHC-dissimilar mates is far from being unanimous, and that pre- and postcopulatory episodes of sexual selection can indeed act in opposite directions.

多次交配的雌性个体具备行使交配后选择的能力，可挑选更适配的精子以完成卵子受精。过往研究显示，主要组织相容性复合体（Major Histocompatibility Complex, MHC）的基因差异在伴侣间遗传适配性的决定中扮演重要角色。选择MHC等位基因差异更大的伴侣，可使后代拥有更高的MHC多样性，进而通过增强对病原体与寄生虫的抵抗力提升存活率。MHC基因的高度多态性还可帮助雌性区分近缘雄性的精子，降低后代的纯合性与近交风险。尽管针对交配前阶段已有大量研究，但伴侣间MHC相似度在交配后过程中的作用却极少被探讨。本研究以一种具有高度多次交配特性的体内受精鱼类——孔雀鱼（Poecilia reticulata）为研究对象，探究当两份雄性精子竞争同一批卵子时，MHC相似度是否会影响受精结果。同时，我们通过检测10个微卫星位点的相似度来控制全基因组相似度的影响。与研究预期相悖的是，我们发现MHC相似度更高的雄性成功繁育了更多后代，而微卫星位点的相似度则无法预测精子竞争的结果。本研究结果表明，基于MHC的识别机制可能用于规避杂交或远交，而非避免近交。这一结论与鲑鱼中的同类研究结果相吻合，说明对MHC差异化伴侣的偏好并非普遍共识，且性选择的交配前与交配后环节确实可能朝着相反方向发挥作用。