Wednesday, 12 June 2019

Placentation in sloths

Placenta and fetal membranes of Hoffmann's Two-toed
Sloth (Choloepus hoffmanni) from Turner 1872
Two recent papers cast light on the evolution of sloths. Delsuc and colleagues obtained mitochondrial genomes from living and extinct sloths (here) while Presslee and colleagues used proteomics to describe Type I collagen - likewise from extant and extinct species (here). They agree that notions of sloth evolution based on morphology need revision. 

It was confirmed that two-toed and three-toed sloths are not closely related and group with different forms of ground sloths. A third group with a suspensory life style was found to have diverged even earlier. These sloths crossed a land bridge to the Greater Antilles and went extinct just a few thousand years ago.

Interhaemal barrier of the pale-throated three-toed sloth
(Bradypus tridactylus) Courtesy of Allen C. Enders
Together with anteaters and armadillos, sloths belong to the ancient superorder Xenarthra. Armadillo placenta has been studied in some detail. It is villous haemochorial (discussed here). Anteater placentation is rather similar (previous post). Sloths are different. As shown by Turner, the placenta is lobulated in appearance. It is labyrinthinre and endotheliochorial (here).

Given current opinion on the relations between sloths (Folivora), anteaters (Vermilingua) and armadillos (Cingulata), parsimony dictates that their common ancestor would have had a villous haemochorial placenta. Thus the sloth placenta represents a derived state. The most recent common ancestor of two-toed and three-toed sloths lived >25 million years ago whereas sloths diverged from anteaters >50 mya. 

Tuesday, 14 May 2019

Epiblast from the past

Mouse blastocyst with epiblast (EPI), trophoblast (TE)
and primitive endoderm (PE) or hypoblast from Selenka 1883
The three cell lineages of the blastocyst are trophoblast, hypoblast (primitive endoderm) and epiblast. The epiblast creates the embryo and contributes extra-embryonic mesoderm to the fetal membranes. An insightful review by Guojun Sheng (OA here) traces epiblast evolution across reptiles/birds, monotremes, marsupials and eutherians.

First he summarizes early development in the mouse as known since Selenka and Sobotta and now understood in terms of gene expression. Some of this translates to early human development (previous post). Yet the behaviour of the epiblast is very different in mammals as diverse as rabbit, pig, cattle and dog. As an example there is loss of the polar trophoblast to expose the epiblast. There are also important differences in gene expression as known from cattle (OA here). 

Meroblastic cleavage of the platypus egg
From Hughes 1993 (here)
Monotremes are more like birds and reptiles starting with the incomplete (meroblastic) cleavage of the yolky egg. Guojun Sheng suggests how the monotreme and reptile/bird patterns of epiblast epithelialization could each have evolved from a hypothetical amniote prototype.

Embryonic and trophoblastic areas of the marsupial
blastocyst as envisaged by Hartman 
Marsupials have a blastocyst-like stage with no inner cell mass (previous post) and it is not fully clear how the cell lineages segregate. Guojun Sheng thinks it can be seen as an intermediate in evolution from a hypothetical therian prototype. 
Serial sections through the blastula of the lowland streaked tenrec
(
Hemicentetes semispinosus) from Bluntschli 1937
Finally, he suggests that the blastula stages described in tenrecs by Bluntschli and elephant shrews by van der Horst (discussed here) may reflect the early eutherian prototype.

Altogether a very good read.






Friday, 18 January 2019

Lagoa Santa skulls still perplex Danes

Peter Wilhelm Lund (1801-1880)
Danish National Library (public domain)
When Peter Lund excavated human remains from Sumidouro Cave he was thoroughly perplexed. Not least because they were jumbled together with the bones of an extinct megafauna. This was in 1840 so well before Darwin had published On the Origin of Species (which cites Lund's work).

The skeletons were shipped to Copenhagen and examined there by several distinguished anthropologists (Lund never returned to Denmark). The consensus that arose was that "Lagoa Santa Man" was unrelated to present-day Native Americans. 


Springer 2017: ISBN 978-3-319-57465-3
Research and debate was renewed in the last century with new excavations in the caves near Lagoa Santa (now a suburb of Belo Horisonte). A key find was Luzia, a skull feared lost in the recent fire at the National Museum in Rio de Janeiro (previous post) (it survived but suffered heat damage). This skull was dated to 11,000 years ago.

Comparisons have been drawn between the features of the Lagoa Santa skulls and those of Australasian peoples such as Andaman Islanders. What can genomics tell us? A first set of clues hinted at an early wave of migrants designated "Population Y" who left a genetic signature that is strongest in some isolated Brazilian communities (see my review of David Reich)


Skull from Sumidouro Cave excavated by Peter Lund
Now a Dane has ventured into the Lund Collection and extracted ancient DNA from a 10,400 year-old skull. The results appeared in Science last December (here). Eske Willeslev and his group found a clear Australasian genomic signature that was absent in other ancient remains from the Americas. They concluded this signal, "implies that an early group possessing it had disappeared or that a later-arriving group passed through North America without leaving any genetic trace." A conclusion echoed by the lead author, José Victor Moreno-Mayar, who simply said (here), "How did it get there? We have no idea."

It seems the Lagoa Santa remains are just as perplexing to Willerslev as they were to Lund and for a similar reason: they do not neatly fit into the scheme of things.

Postscript

The Willerslev paper is really worth reading. It builds on earlier work to confirm that Native Americans (other than Inuits) derived from a group that split from East Asians and resided in Beringia (Ancient Beringians). As they advanced into the Americas at the end of the Ice Age there was an early split into Northern (NNA) and Southern Native Americans (SNA). The SNA dispersed rapidly south of the remaining ice some 14,000 years ago. In a later phase there was admixture from a population in Mesoamerica that migrated both north and south. Another paper on ancient DNA from the Reich group also found evidence for rapid expansion into South America (here), but did not have Lagoa Santa in the data set.

Monday, 14 January 2019

Darwin's hunch

ISBN 978-1-4314-2425-2
Darwin's hunch was that humans evolved in Africa. As Christa Kuljian shows, for most of the twentieth century, this was not the prevailing view among palaeoanthropologists, who felt sure humans had emerged in Europe or Asia. Indeed, the ready acceptance of the Piltdown forgery reflected both this view and the importance attached to brain size. In contrast, the Taung child, an australopithecine discovered in 1924 by Raymond Dart, was disregarded because of its location and small brain size. 


The Taung child (Australopithecus africanus) discovered in 1924
Ditsong National Museum of Natural History CC BY-SA 4.0
Indeed this highly entertaining account of the South African contribution to anthropology by Dart, Robert Broom, Phillip Tobias and others exposes how misconceptions both within South Africa and without shaped interpretation of the fossil record.

Many of the same scientists were involved in biometric studies of living people. These were troublesome because of implicit racial bias. Support for such studies was forthcoming from Jan Smuts and other politicians with a racial agenda. Particularly disturbing was the treatment afforded to the "bushmen" or San people. Anthropologists such as Tobias and Hertha De Villiers were much interested in such female characteristics as steatopygia (increased fat in the region of the buttocks) and elongated labia minora. 

This book was published over a year ago and I was alerted to it by a lengthy review (and opinion piece) by Rebecca Rogers Ackermann (here). It certainly provides food for thought.

The book also covers more recent conflicts between South African palaeoanthropologists such as Ron Clarke and Lee Berger. This has again come to the fore with publication by Clarke of the first description of the australopithecine known as Little Foot (e.g. here) and the hasty reaction from Berger (here). 

The book does not go into detail about phylogenomics and studies of ancient and modern DNA. These provide robust support for the Recent African Origin hypothesis of human origins argued since 1988 by Chris Stringer (here). And thus for Darwin's hunch. Genomics is set to cast new light on the history of the San as recently reported in Nature (here).




Wednesday, 31 October 2018

Placenta Atlas Tool (PAT)

NICHD have just launched this tool to aid pregnancy research. It promises to be of great value (found here).

Those interested in comparative placentation should go to Explore Images and use the Species filter. This brings up a list that may include your favourite mammal. Click on that and you will get a menu of large icons, although only after a further click can you be sure what they represent (room for improvement here).

The images are fully annotated with original figure legends plus additional context from the source paper.

Open Access appears to be a prerequisite for images to be selected as they link to the Open-i resource of the U. S. National Library of Medicine. 

Monday, 22 October 2018

Pregnant tenrecs run hot and cold

Tailless tenrec (Tenrec ecaudatus)
Photo by Markus Fink (Wikimedia Commons CC)
Tenrecs do not maintain a constant body temperature. It waxes and wanes throughout the day. A recent study found this applies even to pregnant tenrecs. Yet after they give birth their body temperature becomes stable and warm.

As mentioned in a recent post, this tenrec (Tenrec ecaudatushas large litters. In the present study up to 19 young. They grow quickly from 12 g at birth to 400 g at 5 weeks (weaning). Perhaps thermoregulation becomes more important when the mother is lactating.

In the closely related species Echinops telfairi, gestation length varies in the range 50-79 days (here). Just speculating, but if body temperature varies could this affect fetal growth rate and thereby the timing of delivery?

Reproduction in tenrecs is quite interesting as we discuss for the ovary here and placenta here.

Monday, 8 October 2018

Congratulations Hiroaki Soma

Professor Hiroaki Soma (at left) recipient of the 2018
IFPA Senior Award
It is a pleasure to record that my good friend and colleague Hiroaki Soma received the Senior Award of the International Federation of Placenta Associations at their recent meeting in Tokyo.

In his address, Professor Soma looked back over 60 years of research including important clinical work on gestational trophoblastic disease in Japan and placental lesions associated with pregnancy at high altitude in Nepal.

In addition, he gave valuable insights about comparative placentation. The range of mammals included chinchilla, giant panda. Japanese serow, sloth, chimpanzee, elephant, manatee and hyrax. As if this were not enough, Professor Soma also presented his research on pregnancy in sharks and rays conducted at the Churaumi Aquarium in Okinawa.