Placentation in the wildebeest

Blue Wildebeest (Connochaetes taurinus)

Photo by Muhammad Mahdi Karim (Wikimedia Commons)

In ruminants binucleate trophoblast cells (BNCs) migrate to and fuse with uterine epithelial cells to form a fetomaternal hybrid - either a syncytium or trinucleate cells. This remarkable mechanism was described some 40 years ago by Peter Wooding. The fusion is aided by a syncytin, the product of an endogenous retroviral gene (previous post).

Almost all the ruminants studied hitherto have trinucleate cells. A fetomaternal syncytium is formed in the basal tragulids (chevrotains), which have a diffuse placenta without cotyledons. The other exception hitherto is syncytium formation in sheep and goats.

Binucleate trophoblast cell of bovine placenta from Benirschke

Now Wooding et al. (here) have undertaken to survey a wide range of ruminants including a chevrotain (Tragulidae), 8 bovids (Bovidae), 8 deer(Cervidae), the pronghorn (Antilocapridae) and a giraffe (Giraffidae). Only the musk deer (Moschidae) are missing.

Almost all the pecoran ruminants studied had trinucleate cells. Exceptions were the sheep and the wildebeest (Connochaetes taurinus). This is a new and highly interesting observation.

Three groups of bovids, classified as Tribes by Groves and Grubb (previous post) and Subfamilies by Wilson and Reeder share a common ancester (here and here). These are Alcelaphini, Hippotragini and Caprini. The first includes the wildebeest and the last sheep and goat. So it is likely that the most recent common ancestor (MRCA) of sheep and goats and the wildebeest had a fetomaternal syncytrium.  

To summarize. The basal Tragulidae have fusion of BNCs and maternal epithelium to form a syncytium. The trinucleate cell replaced this in the MRCA of pecoran ruminants (those with cotyledons). Then a fetomaternal syncytium reappeared in the MRCA of wildebeest and sheep and goats. 

To test this hypothesis it would be useful to have studies of the third tribe Hippotragini, i.e. an oryx, the roan and sable antelopes or the bluebuck.

Placentation in lizards and a new syncytin

The South American skink Mabuya mabouya 

Mark Stevens from Warrington, UK CC BY 2.0

Viviparity is common in lizards and some have evolved quite complex placentas. One of the first to be studied was Chalcides chalcides. Daniel Blackburn, Luana Paulesu and others have just written an interesting historical account of the 1891 paper by Giacomini (here).  

Placentome and paraplacentomal region in Mabuya sp.

From Cornelis et al. PNAS 2017 (here)

An even more complex placenta is found in South American species of the genus Mabuya. Martha Ramirez-Pinilla, a reproductive biologist from Colombia, has authored several papers on Mabuya placenta (e.g. here). Now she has joined forces with the group at Gustave Roussy in Paris to look for syncytins (here).

As explained in previous posts (e.g. here), syncytins are the products of endogenous retroviral genes. The envelope (env) genes of retroviruses function to promote fusion of the viral membrane with the plasma membrane of a host cell. Syncytins are derived from env genes and are expressed in the placenta, where they promote fusion of cytotrophoblasts with the syncytiotrophoblast. Hitherto they have been identified in six orders of eutherian mammals and in one marsupial (previous post).

Cornelis et al. first determined the transcriptome of Mabuya placenta and identified four env genes. One of these (named Mab-Env1) was highly expressed in placenta and with the highest expression of RNA and protein occurring at the fetal-maternal interface including in a maternal syncytial layer. Importantly, Mab-Env1 was fusogenic in an ex vivo assay, which is an essential criterion for designating the protein as a syncytin. The receptor for Mab-Env1 was also identified in this study. 

Marsupial placentation - now with endogenous retroviral genes

Gray Short-tailed Opossum (Monodelphis domestica)
Wikipedia CC-BY-SA-2.5 (uploaded by Dawson)

In marsupials gestation is short and the embryo is supported by a yolk sac placenta (previous post). Just a few have an additional chorioallantoic placenta; they include wombats and the koala.

Syncytins are endogenous retroviral proteins that promote fusion of trophoblast to form a syncytium. They have been identified in several orders of placental mammals including ruminants and rodents. One marsupial, the Gray Short-tailed Opossum, has syncytial trophoblast. This led Guillaume Cornelis and colleagues to ask if it also had a syncytin. Their findings just appeared in PNAS (here). In brief, they did find a syncytin, which was named syn-Opo1; it was expressed in the placenta and able to promote cell fusion. The gene was present in most but not all members of the genus Monodelphis and absent in other marsupials. Thus it represents a relatively recent capture of a retroviral envelope (env) gene.

Even more intriguing was the presence of an env gene that was expressed in the placenta but unable to promote cell fusion. This gene was present in the genome of the Tammar Wallaby (Macropus eugenii), the Tasmanian Devil (Sarcophilus harisii) and 23 other marsupials. The gene was named pan-Mar-env2. Because it has been conserved (and is under purifying selection) it likely plays an important role in placentation that as yet is undefined.

Reproduction in domestic ruminants

ISBN13: 9781899043637

Every four years there is a conference on reproduction in domestic ruminants. The latest was held this August at Obihiro, which is on Hokkaido, the North Island of Japan. The accompanying book (details here) is a useful collection of reviews covering most aspects of male and female reproductive biology.

The opening chapter Ruminant phylogenetics: a reproductive biological perspective by William J. Silvia is an excellent overview that integrates molecular phylogenetics with morphology including various aspects of placentation.

There are several chapters on placentation including Early placentation and local immune regulation by Kazuhiko Imakawa and colleagues. This deals with transcriptional regulation of IFNT the gene coding for interferon-tau, which is secreted by the trophectoderm at the blastocyst stage and results in maternal regulation of pregnancy. Another topic is syncytins (previous post) in ruminants. In addition, I have contributed a chapter on Evolution of placental structure and function in ruminants.

Previous volumes in this series have appeared under the imprint of the journal Reproduction. Although the current volume conforms to that journal's format, it has been published in book form by Context. As far as I can make out, it is not possible to purchase individual chapters.

Placental hormones and syncytins in the blind mole rat

A blind mole rat Spalax ehrenbergi
(Wikimedia Commons)

Mole rats (Spalacidae) occupy a basal position in the superfamily Muroidea. The blind mole rats, Spalax spp., are highly adapted to a subterranean lifestyle. They are resistant to both spontaneous cancer and carcinogens. For all these reasons they are widely studied and now we have a description of the genome of one species S. galili and the placental transcriptome both of that species and of S. carmeli (here).

Placental hormones

Placental-specific genes known from mouse and rat (Muridae) include cathepsins, placental lactogens (derived through duplication of the prolactin gene) and pregnancy-specific glycoproteins (reviewed here). Orthologues of many of these genes were present in Spalax and given the basal position of Spalacidae this indicates they had started to diversify in the last common ancestor of the muroid rodents.

Syncytins

Syncytins are endogenous retroviral envelope proteins thought to be essential for the formation of syncytiotrophoblast (previous post). Two such genes are known from mouse, rat (Muridae) and hamster (Cricetidae). Both are present in the Spalax genome although only Synb was confirmed in the placental transcriptome. This extends the timing of the env gene capture further back in time to an estimated 45 Mya.

Globin genes

Beta globin genes code for the two chains of haemoglobin molecules. One of them HBG is expressed in the embryo of most mammals but in the fetal haemoglobin of higher primates (discussed here). The HBG of Spalax shows evidence of rapid sequence evolution under positive selection. It is not clear to what extent this affects its affinity for oxygen.

Placentation in a marmot: the woodchuck

Woodchuck or groundhog (Marmota monax)
CC Wikimedia Commons

Not much has been written about placentation in squirrels and other sciuromorph rodents - at least in comparison to myomorphs (e.g. mouse, hamster) and hystricomorphs (e.g. guinea pig, capybara). Marmots belong with chipmunks and ground squirrels in the Tribe Marmotini. The most complete description is for the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) (Mossman & Weisfeldt Am J Anat 1939;64:59-109). It was based largely on specimens collected a century ago by Thomas George Lee (background here).

Interhaemal region in the labyrinth of a woodchuck placenta
Marmota monax Courtesy of Dr. Allen C. Enders

The woodchuck has a labyrinthine, haemochorial placenta. In the above section the large channels with maternal red cells are lined by syncytiotrophoblast. A fetal capillary is seen to the right.

Early Development of the placenta in the Colorado chipmunk
Tamias quadrivittatus Courtesy of Dr. Allen C. Enders

There are several ways to make a haemochorial placenta (discussed here). Squirrels go through a transient endotheliochorial phase. The figure is from a chipmunk and shows a maternal capillary in which part of the endothelium has been replaced by syncytiotrophoblast whilst two endothelial cells remain intact (further figures here). In the first instance this creates the equivalent of the spongy zone found in other rodent placentas.

Later the fetal mesoderm grows into the trophoblast bringing with it the fetal capillaries. To start with the outgrowths are fingerlike (villi) as can be seen in a recent publication on the woodchuck (here). Nearer term, however, the labyrinth occupies most of the depth of the placenta. The spongy zone is then very thin and occupied by syncytiotrophoblast with clumps of nuclei as well as mononucleate giant cells (Dr. Allen C. Enders, personal communication).

Syncytins are endogenous retrovirus envelope genes (previous post). Two occur in murid rodents and another in South American hystricomorphs. Now a search of the genome of the thirteen-lined ground squirrel has turned up several candidates and further work in the woodchuck has shown one of them to be a bona fide syncytin. By in situ hybridization the gene was not expressed in the labyrinth but rather in the part of the spongy zone that had yet to be reached by the fetal vessels.

Endogenous retroviruses expressed by ovine trophoblast

Urial Sheep (Ovis vignei)
Wikimedia Commons

Jaagsiekte sheep retrovirus (JSRV) is a nasty pathogen causing lung tumours in sheep. No fewer than 27 JSRV-related endogenous retroviruses (enJSRVs) occur in domestic breeds. Gene capture is an ongoing process and the distribution of enJSRVs has been exploited to test hypotheses about the domestication of sheep (here). For example, the most common variant (enJSRV-18) is absent in ancestral breeds such as the Urial (above) and Mouflon. Although found in goats, enJSRVs are not present in cattle so are much younger than a syncytin recently described by Cornelis et al. for ruminants (here).

Five enJSRVs are capable of transcription and like syncytins (previous post) they have an env gene. Are they then "nascent syncytins" as suggested by Cornelis et al., or do they already subserve a function in the ovine placenta?

A period of blastocyst elongation precedes implantation in sheep and binucleated trophoblast cells first appear then. enJSRV mRNAs are detectable at this stage and later are expressed in binucleate trophoblast cells as well as in the syncytial plaques formed by fusion of those cells with uterine epithelial cells (reviewed here).

Moreover, when antisense oligonucleotides were used to block enJSRV protein production, differentiation of binucleate cells was inhibited (here). The more i read about endogenous retroviruses the more intrigued I become.

An ancient endogenous retrovirus

Classes of ERVs Wikimedia Commons

Endogenous retroviruses (ERVs) are genes of retroviral origin that have been incorporated into the genome. They include the syncytins expressed in the placentas of various mammals. Six are known so far and each represents a separate capture of a retroviral gene (see previous post). 

Now an ERV has been described that occurs in representatives of all four superordinal clades of placental mammal (here). In this case the gene is orthologous and thus represents a single integration event that must have predated the divergence of placental mammals in the Cretaceous (for possible dates see my recent post).

This work was published in a special issue of Phil Trans R Soc B on the theme "Paleovirology: insights from the genomic fossil record." Although sequence substitutions occur over time, an ERV is essentially a "fossil" record of a virus as it existed at the time of incorporation into the host genome. Thus the study of ERVs in various organisms (not just mammals) can provide insight into the long term history of viruses and virus-host interactions (see overview here).

In a historical overview of the field (here) Robin A. Weiss notes, "The advantage to the host of ERV protein expression is most dramatic in the evolution of the mammalian placenta." Incidentally, if you find the whole concept hard to swallow you are in good company as the early history of the field so clearly shows.