Thursday, 21 January 2016

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."

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