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.