The trouble with tree shrews

Pen-tailed tree shrew (Ptilocercus lowii)
By Joseph Wolf (1820 – 1899) [Public domain] via Wikimedia Commons

Tree shrews show so many resemblances to primates that the eminent anthropologist Wilfred Le Gros Clark included them in the same order. Molecular phylogenetics refuted this but placed them in the same clade or superorder (Euarchontoglires) as primates, colugos, rodents and lagomorphs. There has been much discussion about whether colugos (previous post) or tree shrews are the closest relatives to primates.

Just how troublesome tree shrews have become is highlighted by a new paper on mammalian evolution by Tarver et al. (here).  

Alternative roots to the mammalian tree from Mess & Carter (here)

In addition to Euarchontoglires, there are three major clades of placental mammals. But there has been considerable disagreement about how to root the tree with three hypotheses as shown above.

Tarver et al. attempted to resolve this using a huge amount of genomic data and more sophisticated modelling techniques. They argue convincingly for the hypothesis at the top of the diagram where Afrotheria and Xenarthra are sister groups in a clade called Atlantogenata.

But once again tree shrews caused trouble. In a consensus tree based on protein-coding genes, tree shrews were basal to Glires (rodents and lagomorphs). This is in agreement with another recent study (here). But in a separate data set based on genes for microRNA tree shrews were basal to all the other orders in Euarchontoglires. So much so that the clade itself collapsed as a valid taxon. Naughty tree shrews!

Placentation in several species of tree shrew was studied by Luckett (here) and later in Tupaia glis by Kaufmann (here and here). The placenta is labyrinthine and endotheliochorial. So far nobody has looked at a placenta from the pen-tailed tree shrew (pictured above). It occupies its own family and a new paper (here) characterizes it as a living fossil that has undergone little change since the Oligocene.

Incidentally, pen-tailed tree shrews have a large intake of fermented nectar from the bertram palm (described here); see this blog for "boozing tree shrews."

Larry Longo

Lawrence D. Longo M.D. 1926-2016

Larry Longo was among the most important perinatal physiologists of our time. He is perhaps best known for enhancing understanding of placental gas exchange by mathematical modelling. The work was conducted in close collaboration with Gordon G. Power and Esther P. Hill and summarized in a seminal review (here). Much of the input data was derived from experiments in his lab.

Together with Gordon Power and Raymond D. Gilbert, Larry founded the Center for Perinatal Biology at Loma Linda University. The research program was impressive and is still ongoing. It has been responsible for training a great number of good scientists.

Larry was not just interested in the history of science. He actively researched it. Of particular note is his 2013 book The Rise of Fetal and Neonatal Physiology (here). Larry's Collection of rare books is legendary and those priveleged to have entered his library are unlikely to forget the experience. Many images from these tomes will appear in a forthcoming book Wombs with a View co-authored with Lawrence P. Reynolds (here).

Larry will be remembered for his humility, courteousness and encouragement of all who came his way. He passed 5 January 2016.

A fuller account of Larry Longo's career can be found here and here.

Steller's sea cow

Reconstruction of Steller's sea cow (Hydrodamalis gigas)
By Emőke Dénes (Natural History Museum in London) (CC BY-SA 2.5)

When first described by Steller in 1741, the range of this sirenian was already restricted. Intensification of hunting led to extinction of the species within another 27 years.

A recent paper with a "stellar" cast of authors (here) uses molecular data to firmly establish the position of Steller's sea cow within crown Afrotheria; with additional morphological data it also shows shows how the species fits into the fossil record.

Steller's sea cow (Hydrodamalis gigas) belongs with the dugong (Dugong dugon) in Family Dugongidae whereas the three species of manatee belong to Trichechidae. Together they make up the Order Sirenia, which is sister to the elephant Order Proboscidea.

Endotheliochorial placenta of the Amazonian manatee

Fetal (fc) and maternal (mc) capillaries are present

Elephants have an endotheliochorial placenta. Therefore it perplexed us that Wislocki had described the placenta of the West Indian manatee (Trichechus manatus) as haemochorial. Therefore we reopened the matter by studying placentas from the Amazonian manatee (T. inunguis). We were able to demonstrate two sets of capillaries (shown above) and conclude this placenta to be endotheliochorial (here).

The allantoic sac of elephants and manatees has four chambers and we have shown this to be a shared, derived character of the superorder Afrotheria (here).

Placentation in salps

Placenta of a salp (Salpa fusiformis)
Reproduced from Bone, Pulsford and Amoroso (here)
(C) 1985 with permission from Elsevier

Sea squirts? asked a faithful reader of this blog. OK, sea squirts do not have placentas, but salps do! They, too, are tunicates, and thus included in the sister group to vertebrates (previous post). Whilst sea squirts are mainly sessile, salps are pelagic, living near the surface of oceans and at times more numerous than krill. They are tubular animals and swim by jet propulsion.

The life cycle involves alternation between asexual and sexual generations. The asexual phase (oozooid) develops within the jet chamber of the sexual phase (blastozooid). The image above is from Amoroso's last paper, published almost three years after his death (see previous post for Amo). The placenta consists of two layers: an outer cortex (co) and an inner central layer (c). These separate the embryonic (E) and maternal (M). circulations. Both layers are syncytial and both are maternal in origin. However, embryonic leucocytes pass into and add to the cortical layer.

Blastozooids are hermaphrodite, but the egg develops before the testis matures so is fertilized by sperm from a different blastozooid

HMS Rattlesnake on which Thomas Henry Huxley served as
Assistant Surgeon during the voyage to Australia and New Guinea 1846-50
National Maritime Museum (public domain)

The first English language description of placentation in the Family Salpida was given by T. H. Huxley R.N., "late of HMS Rattlesnake," in 1851 (here) although he cites even earlier work by Cuvier, Chamisso and Meyen.

We tend to think of placentation in terms of mammals, reptiles and fish, but a current paper in Biological Reviews (here) shows that maternal provision of nutrients (matrotrophy) and even placentation is not infrequent in invertebrate phyla.