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 whether 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 genomewide 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, suggests 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实施交配后选择的能力，可遴选更为适配的精子以完成卵子受精。既往研究表明，主要组织相容性复合体（major histocompatibility complex, MHC）的等位基因差异，在决定伴侣间遗传适配性的过程中扮演着关键角色。选择MHC等位基因差异较大的伴侣，可使后代获得更高的MHC多样性，进而通过增强对病原体与寄生虫的抵抗力提升生存适配能力。MHC基因的高度多态性，还可使雌性进一步区分亲缘雄性的精子，从而降低后代的纯合度与近交风险。尽管学界在交配前选择领域已开展了大量研究，但针对伴侣间MHC相似性在交配后选择中的作用，相关探究仍极为匮乏。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）为实验材料，旨在探究当两份雄性精子竞争同一批卵子时，MHC相似性是否会对受精结果产生影响。此外，我们通过检测10个微卫星位点（microsatellite loci）的遗传相似性，对全基因组相似性进行了控制。与实验预期相悖的是，我们发现MHC相似性更高的雄性产生了更多的后代，而微卫星位点的相似性则无法预测精子竞争的结果。本研究结果提示，MHC介导的识别机制或许并非用于规避近交，而是参与避免杂交或远缘交配。这一结论结合鲑鱼中的同类研究成果，表明对MHC相异伴侣的偏好并非普遍存在，且交配前与交配后性选择过程确实可能朝着相反的方向发挥作用。