Abstract
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.
Highlights
The major histocompatibility complex (MHC) influences sexual selection in various vertebrates
Each stallion was in contact with 9 to 15 mares, with rates of MHC-dissimilar mares ranging from 25–100%
Embryo recovery rate was 77.1% when mares were in contact with an MHC-dissimilar stallion and 63.3% when they were exposed to an MHC-similar stallion (Table 1; Fig. 1)
Summary
The major histocompatibility complex (MHC) influences sexual selection in various vertebrates. MHC-linked social signaling may influence kin recognition and mate choice and maternal investment into a pregnancy[1,3,4], either to avoid male infanticide (“Bruce effect”11,12) or as a late form of sexual selection that favors certain MHC genotypes over others[13]. In the former case, MHC-linked signals that reveal the presence of a new dominant male can induce a pregnancy block[14,15] while MHC matching between male and female would not be expected to matter[16]. MHC + endometritis + stallion MHC + endometritis + mare MHC + endometritis + mare + stallion + MHC x mare MHC + endometritis + mare + stallion + MHC x stallion effect tested d.f
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