Study: Sires' Genes Form the Equine Placenta

The term "do-it-all-dad" just took on a whole new meaning: Cornell University researchers have recently determined that, in equids at least, it’s the father’s genes that take the lead in developing the mare's placenta.

“We discovered more paternally (than maternally) expressed imprinted genes in the equid placenta, suggesting a greater contribution of placental development from the paternal genome,” said Xu Wang, PhD, research associate in the Department of Molecular Biology and Genetics at Cornell University, in Ithaca, N.Y. “And this runs counter to your first guess that the female ought to be the one controlling the placenta in her own body.”

But there are complex evolutionary advantages to this design, Wang explained, such as better management of genetic conflict between a male fetus and its mother. And from a breeding point of view, it could help explain the “maternal grandsire” phenomenon—the fact that high-performance foals often skip a generation, coming from the daughter of a champion sire.

While the majority of an organism's expressed genes come from both the mother and the father, a small number of genes are what scientists call “imprinted,” meaning they are expressed only from the mother or from the father. Technically speaking, this means that one parental copy (the mother’s or the father’s) of that gene is “silenced”--it’s there, but it isn’t expressed.

In humans, scientists have identified 60 such genes. In mice, 100—but only 40 of them are the same as in humans, implying that evolution “turns imprinting on and off quite a lot,” said study co-author Andrew Clark, PhD, a professor of population genetics at Cornell. In equids, the Cornell group has identified 93 imprinted genes—including 78 that appear to be unique to the species.

To do this, the researchers sequenced RNA from the trophoblasts (principal type of the cells that will develop into the placenta) of mules and hinnies. The fact that mules (which have a donkey father and horse mother) and hinnies (which have a horse father and donkey mother) differ in appearance and temperament made them great candidates for a study on paternal influence, Wang said.

They found that the majority of the equid-specific imprinted genes they discovered were paternally expressed and imprinted only in the placenta. And this, they said, is unique (so far) to equine species.

“We were somewhat surprised to see the degree of evolutionary fluidity of genomic imprinting, with so many genes varying in imprinting status across species,” Wang said.

This discovery could have important impact for breeders: If a stallion has a mutation in a paternally expressed imprinted gene this could lead to defects in placenta development in mares bred to him, Wang said. Such defects could lead to early fetal loss or complications in late pregnancy from poor placentation, which can result in premature birth. Although such genetic defects are not common, it could be reasonable to check with the breeder to see if the stallion’s other mares have had difficulties with their placentas.

This research could also relate to the phenomenon of the “maternal grandsire effect,” said study co-author Doug Antczak, VMD, PhD, a professor of equine medicine at Cornell's Baker Institute for Animal Health. Some Thoroughbred stallions become known as “broodmare sires” because of their tendency to sire female offspring that produce outstanding racehorses, he said. While many noted broodmare sires also produce first-generation offspring that perform well, others do not; this suggests that at least two types of inheritance could be at work in the transmission of athletic ability.

Secretariat is one example of a successful maternal grandsire that did not produce many successful direct offspring, he added. However, many of Secretariat’s daughters became leading broodmares, producing several excellent racehorses. Consistent with this hypothesis is the observation that Princequillo, the sire of Secretariat’s dam, was the leading sire of broodmares of his generation, he said.

There are several genetic mechanisms that could account for this skipping of generations in the transmission of athletic performance, and genomic imprinting is one of them, Antczak said. Research in this area by the Cornell team is ongoing.

The study, "Paternally expressed genes predominate in the placenta," was published in June in the Proceedings of the National Academy of Sciences of the United States of America

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