Monday, 25 November 2013

Placental gas exchange in ground squirrels

Golden-mantled ground squirrel (Callospermophilus lateralis)
(Wikipedia Commons)

Fetal blood tends to have a higher oxygen affinity than maternal blood. This aids oxygen transfer across the placenta. Some mammals (human, sheep) have a special fetal hemoglobin. In many others, hemoglobin is the same in fetus and mother, but in fetal blood it has increased affinity for oxygen because the red blood cells have a low content of DPG (here). This is the case in the few rodents studied so far.

Ground squirrels cannot use this trick. A new study (here) found that even adults have hemoglobin of high oxygen affinity and there is no effect of raising or lowering DPG.
Placenta of the golden-mantled ground squirrel from Carter and Enders (here)
© Carter and Enders 2004; Licensee Biomed Central Ltd.

So is oxygen transfer across the placenta of ground squirrels likely to be less efficient than, say, in a rat? Probably not. The same study established that an unusually large Bohr effect (defined here) aids unloading of oxygen to the tissues of ground squirrels. In any placenta there is a double Bohr effect because, as fetal blood takes up oxygen, it unloads carbon dioxide, while the opposite occurs in the maternal blood (discussed in detail here).

The new study concerns ground squirrels living at low and high altitude. The authors speculate that hemoglobin evolved to meet the demands of their fossorial life style. This in turn made it easier to expand into mountainous habitats. They did not consider the possible impact on reproduction - but perhaps they should.  


Saturday, 9 November 2013

Transgenic monkeys

Common marmoset (Callithrix jacchus) Wikipedia Commons

A news article in Nature (here) tells how new gene-editing techniques offer the opportunity to create transgenic primates. The emphasis is on neuroscience and models of brain disease. But if transgenic lines are developed there should be placentas available for study.

Although work on rhesus macaques is mentioned, more progress has been made using marmosets. One advantage might be that marmosets usually carry twins so a colony could be built up quicker. These Neotropical primates have placentas that differ in several ways from human placenta.

Haematopoietic foci in the placenta of a marmoset
(Callithrix jacchus) from Carter and Mess (here)
© Museum für Naturkunde Berlin
As an example, the placenta is an important site of haematopoiesis. The conventional wisdom is that there is little or no trophoblast invasion, but this is one aspect that calls for reexamination. If marmosets begin to emerge as important disease models there will be a need to look more closely at their placentation.

Sunday, 3 November 2013

Denisovans crossed the Wallace line

Wallacea is bordered to the west by the Wallace Line
and to the east by the Lydekker Line (Wikimedia Commons)

Denisovans were archaic hominins distinct from Neanderthals and modern humans (previous post). There is evidence for gene flow between all three populations and putatively a fourth. The greatest amount of introgressed Denisovan DNA is found in the aboriginal peoples of New Guinea and Australia (here).

This is remarkable for several reasons. Firstly, it is a long way from Denisova in Siberia to New Guinea. Secondly, there is no trace of Denisovan DNA in modern humans from mainland Asia or from Southeast Asia west of the Wallace Line. As pointed out in a recent essay (here), the most likely explanation is that Denisovans crossed the Wallace Line into Wallacea (see map) and that exchange of genes with modern humans occurred there. Traces of Denisovan DNA do occur in modern populations from Wallacea as well as further east to Polynesia and Fiji.

Does that mean there was no contact between Denisovans and the ancestors of present day Asian populations? Not necessarily. Some Denisovan genes may have been retained east of the Wallace Line because they conferred resistance to disease (purifying selection). Further west Denisovan DNA may have been "overwritten" in the course of time.