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.

Emil Selenka

Emil Selenka (27 February 1842 - 21 January 1902

Emil Selenka died 113 years ago today, just before his 60th birthday. He was an eminent German zoologist, who spent much of his career studying the development of marine invertebrates, especially sea cucumbers. He turned to vertebrates rather late but made important contributions to the embryology and placentation of primates. His work on gibbons and orangutans is especially significant.

Pregnant uterus of Hylobates agilis (rafflei) showing the decidua

capsularis (d.c.) reproduced by Hill (here) from Selenka

I have shown this image before to document that gibbons resemble other great apes in having a decidua capsularis. Thus implantation is interstitial as it is in the human.

Fetus of Hylobates muelleri (Müller's Bornean Gibbon) and
uterus of Nomascus concolor (Black Crested Gibbon) from Selenka

In a moving tribute to Selenka, his pupil Hubrecht (previous post) wrote that his artistic talent was so great that it was almost a pity he became a professor of zoology rather than a painter.

Selenka amassed a large number of specimens and kept the skeletons as well as the reproductive tracts. He felt the maximum information should be gained from the animals that lost their lives. One recent study (here) estimates that Hubrecht may have bagged as many as 400 orangutans between 1892 and 1895.

Walking with whales

University of California Press Oakland CA 2014
ISBN 978-0-520-27706-9

This highly readable book by Hans Thewissen recounts a scientific odyssey in search of the ancestors of whales. Almost all the relevant fossils were found in Pakistan and western India. Dr. Thewissen worked there under trying conditions and in the shadow of political conflicts (his first field expedition was foreshortened by the 1991 Gulf War). He succeeded, however, in uncovering an evolutionary series of walking whales and eventually in demonstrating their affinity to even-toed ungulates (Artiodactyla).

The key discovery (here), made independently by Gingerich (here), was that ancestral whales possessed the double pulley astragalus (ankle bone) that is a defining feature of artiodactyls. The author does a very good job of explaining this and other anatomical details. Indeed, this book was written for the general reader and its author is at pains to explain the necessary terminology. He also gives fascinating insights into several aspects of cetacean biology such as the way in which aquatic mammals, including fossil and living whales, propel themselves through the water. 

The relation of whales to artiodactyls, with hippopotami as the closest living relatives, was first proposed on the basis of molecular phylogenetics (e.g. here). Because there is no fossil record for hippopotami, Dr. Thewissen does not find this analysis very useful. He prefers to keep Cetacea as a separate order even though this makes Artiodactyla paraphyletic.

Of particular interest to me are Dr. Thewissen's studies of a series of dolphin embryos and his proposed research on embryos of the bowhead whale.

This is a deeply personal book and no attempt is made to hide the author's prejudices. He is so concerned about Japanese whaling that he missed an opportunity to collaborate with Seiji Ohsumi, the grand old man of cetacean research who has done seminal work in reproductive biology.

Ohsumi S: Comparison of maturity and accumulation rate of corpora albicantia between the left and right ovaries in cetacea. Sci Rep Whales Res Inst 1964; 18:123-148. 

Ichthyostega and the first tetrapods

Reconstruction of Ichthyostega by Dr. Günter Bechly Stuttgart (CC)

Recently I visited the Zoological Museum in Copenhagen to see its new exhibition "Precious Things." On show is the skull of Ichthyostega, a fossil from Northeast Greenland that caused quite a stir when discovered in 1932. It was hailed as the missing link between fishes and land vertebrates, an early tetrapod.



Reconstruction of the Copenhagen skull of Ichthyostega
Wikimedia Commons uploaded by FunkMunk (CC)

Current research suggests Ichthyostega would have spent most of its time in the water, perhaps hauling itself up on to land to bask in the sun. It did not have the skeletal adaptations required for walking (here).

More recently discovered fossils such as Tiktaalik from Ellesmere Island offer better clues about the transition from water to land (here) but Ichthyostega has not yet lost its iconic status.