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潜力，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）的基因差异，在决定配偶间遗传适配性方面发挥着关键作用。选择MHC等位基因差异较大的配偶，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后代存活率。MHC基因的高度多态性还可使雌性辨别近亲雄性的精子，从而降低后代的纯合性与近交风险。尽管已有大量针对交配前阶段的研究，但关于配偶间MHC相似性在交配后阶段的作用，相关研究仍较为匮乏。本研究以具有高度多重交配特性的体内受精鱼类——孔雀鱼（Poecilia reticulata）为实验对象，探究当两份雄性精子竞争同一批卵子时，MHC相似性是否会影响受精结果。本研究同时通过检测10个微卫星位点（microsatellite loci）的相似性，对全基因组相似性进行了控制。与预期相悖的是，本研究发现与雌性MHC相似性更高的雄性，其后代数量更多；而微卫星位点的相似性则无法预测精子竞争的结果。本研究结果表明，MHC辨别作用可能参与了对杂交或远交的规避，而非近交规避。这一结果与鲑鱼中的同类研究发现相呼应，表明对MHC差异配偶的偏好并非普适性结论，且交配前与交配后的性选择过程确实可能朝着相反方向发挥作用。