Embryo survival in the oviduct not significantly influenced by major histocompatibility complex social signaling in the horse

The major histocompatibility complex (MHC) influences sexual selection in various vertebrates. Recently, MHC-linked social signaling was also shown to influence female fertility in horses (Equus caballus) diagnosed 17 days after fertilization. However, it remained unclear at which stage the pregnancy was terminated. Here we test if MHC-linked cryptic female choice in horses happens during the first days of pregnancy, i.e., until shortly after embryonic entrance into the uterus and before fixation in the endometrium. We exposed estrous mares to one of several unrelated stallions, instrumentally inseminated them with semen of another stallion, and flushed the uterus 8 days later to test for the presence of embryos. In total 68 embryos could be collected from 97 experimental trials. This success rate of 70.1% was significantly different from the mean pregnancy rate of 45.7% observed 17 days after fertilization using the same experimental protocol but without embryo flushing. Embryo recovery rate was not significantly dependent on whether the mares had been socially exposed to an MHC-dissimilar or an MHC-similar stallion. These observations suggest that MHC-linked maternal strategies affect embryo survival mainly (or only) during the time of fixation in the uterus.

Methods
See description in Jeannerat et al.

主要组织相容性复合体（major histocompatibility complex, MHC）可影响多种脊椎动物的性选择。近期有研究表明，马（Equus caballus）体内与MHC相关的社会信号也会影响受精后17天确诊的雌性生育能力。然而，目前尚不清楚妊娠终止发生于哪个发育阶段。本研究旨在探究马体内与MHC相关的隐秘雌性选择是否发生于妊娠早期，即胚胎进入子宫后不久、且定植于子宫内膜前的阶段。我们将发情母马暴露于多匹无亲缘关系种公马中的其中一匹，随后采用另一匹种公马的精液对其进行人工授精，并于8天后通过子宫冲洗术检测胚胎是否存在。本研究共完成97次实验，成功收集到68枚胚胎。该70.1%的胚胎回收率，与采用相同实验方案但未进行胚胎冲洗的对照组中，受精后17天观测到的45.7%平均妊娠率存在显著差异。胚胎回收率与母马此前是否接触过MHC相异或MHC相似的种公马并无显著关联。上述观测结果表明，与MHC相关的母体策略主要（或仅）在胚胎于子宫定植的阶段影响胚胎存活。

方法
详见Jeannerat等人的研究描述。