Interspecific and extraspecific pregnancies in equids: anything goes.

The research is an extensive investigation into the unusual capability of equid species (like horses and donkeys) to interbreed and carry extraspecies pregnancies to term. It analyzes the genetic implications of such mixed breeding and how the genetic makeup of the fetus influences placental development. Additionally, the study looks into the role of maternal environment and immune responses in determining the success or failure of these interspecific pregnancies.

Understanding Interspecific and Extraspecific Pregnancies

• Researchers found that equids, member species of the horse family genus, are able to interbreed regardless of their phenotypical and karyotypical diversity, often resulting in viable but usually infertile offspring. An example of this kind of breeding is the mule, the product of a female horse and male donkey.
• This kind of genetic crossover has been of great significance in broadening our understanding of genetic inheritance.
• In certain comparable instances, mares and donkeys have successfully carried xenogeneic extraspecies pregnancies to term through embryo transfer; embryos from different equid species, such as Prezewalski’s horse and Grant’s zebra, were transferred to these species and brought to term.

The Influence of Fetal Genotypes

• The genotype of the fetus significantly influences how the placenta develops in equids. This is most evidently seen in the development of the annulate chorionic girdle, a tissue associated with the production of equine chorionic gonadotropin (eCG), a key hormone in equine pregnancies.
• However, when intact embryos or half embryos of mules were transferred to Jenny donkeys, it showed that the maternal uterine environment can also significantly influence the development of the chorionic girdle, especially in terms of its invasion of the maternal endometrium.

The Role of Maternal Factors

• When donkey embryos were transferred to horses, an exceptionally small chorionic girdle was observed that utterly failed to invade the endometrium which usually results in the formation of endometrial cups. This indicated the presence of maternal factors in determining the nature of the pregnancy.
• About 70% of these donkey-in-horse pregnancies were aborted between the 80th and 85th days of gestation due to abnormal placental attachment and a strong maternal immune reaction against the foreign donkey trophoblast (part of the placenta).
• In such cases of pregnancy loss, there were strong indications of immune memory, and the rate of fetal death was reduced by immunizing the surrogate mare against donkey lymphocytes. This pointed to the role of maternal immunity in interspecific pregnancies.

Conclusion

• These findings emphasize the crucial role of the invasive trophoblast cells of the equine placenta in ensuring proper attachment and fetal sustenance.
• These cells also play a role in moderating maternal immunity to allow the survival of the foreign fetus in the uterus, shedding light on the complex interplay of genetics, environment, and immunity in interspecific equine pregnancies.