Placentation in lizards and a new syncytin

The South American skink Mabuya mabouya 

Mark Stevens from Warrington, UK CC BY 2.0

Viviparity is common in lizards and some have evolved quite complex placentas. One of the first to be studied was Chalcides chalcides. Daniel Blackburn, Luana Paulesu and others have just written an interesting historical account of the 1891 paper by Giacomini (here).  

Placentome and paraplacentomal region in Mabuya sp.

From Cornelis et al. PNAS 2017 (here)

An even more complex placenta is found in South American species of the genus Mabuya. Martha Ramirez-Pinilla, a reproductive biologist from Colombia, has authored several papers on Mabuya placenta (e.g. here). Now she has joined forces with the group at Gustave Roussy in Paris to look for syncytins (here).

As explained in previous posts (e.g. here), syncytins are the products of endogenous retroviral genes. The envelope (env) genes of retroviruses function to promote fusion of the viral membrane with the plasma membrane of a host cell. Syncytins are derived from env genes and are expressed in the placenta, where they promote fusion of cytotrophoblasts with the syncytiotrophoblast. Hitherto they have been identified in six orders of eutherian mammals and in one marsupial (previous post).

Cornelis et al. first determined the transcriptome of Mabuya placenta and identified four env genes. One of these (named Mab-Env1) was highly expressed in placenta and with the highest expression of RNA and protein occurring at the fetal-maternal interface including in a maternal syncytial layer. Importantly, Mab-Env1 was fusogenic in an ex vivo assay, which is an essential criterion for designating the protein as a syncytin. The receptor for Mab-Env1 was also identified in this study. 

Tree shrews move from branch to branch

Species tree by three coalescent-based approaches
from Esselstyn et al 2017 © The Author 2017

In an heroic effort to resolve some of the difficult nodes in the mammalian tree, Esselstyn et al. used data from published genomes and added new data for a total of 100 mammals. Focussing on ultraconserved elements, they analysed all this by a battery of techniques, These included the classical maximum likelihood (ML) on concatenated data and three coalescent-based approaches. How did they do?

The first problem was to resolve the root of the eutherian tree. Here they did rather well. Both the ML tree and two of the coalescent-based methods gave strong support to the Atlantogenata hypothesis, i.e. a sister relationship between Afrotheria (elephants, dugongs, tenrecs and hyraxes) and Xenarthra (sloths and armadillos) with a common ancestor basal to Boreoeutheria (all other eutherian mammals).

South American Tapir (Tapirus terrestris)

Photo by Bernard Dupont CC BY-SA 2.0

They did almost as well with the second problem: the sister group to horses, rhinos and tapirs (Perissodactyla). In most analyses the found Cetartiodactyla (cetaceans and even-toed ungulates) as sister to Perissodactyla. However, they could not rule out one of the alternative hypotheses, which had bats (Chiroptera) as the sister group.

Whereas horses and cetartiodactyls (including ruminants, pigs, dolphins) all have epitheliochorial placentation, no bat does, so I am happy with their majority finding!

Pen-tailed tree shrew (Ptilocercus lowii)

From Wolf 1848 via Wikipedia Commons

On the third thorny problem the exercise failed. Tree shrews used to be thought the closest relatives to primates but have been toppled from this position in favour of colugos (previous post). Because some previous studies did not even include colugos, the present authors sought a remedy in including two colugos and three tree shrews, including the pen-tailed tree shrew above. The hope was that high taxon coverage would give a sounder result. But they were forced to conclude that placement remains uncertain. Tree shrews may be sister to rodents and lagomorphs (Glires) or to colugos plus primates (Primatomorpha).

Fortunately they could confirm colugos as sister to primates, which was the basis for my recent paper with Andrea Mess on the evolution from labyrinthine to villous placentation (here).

Marsupial and eutherian placentation

Placenta of the tammar wallaby showing the bilaminar (BOM)
and trilaminar (TOM) omphalopleure. From Guernsey et al.
eLife 2017 CC The Authors

A brand new paper compares the transcriptomes of marsupial (tammar) and eutherian (mouse and human) placentas and mammary glands (here). It confirms that marsupials have fully functional placentas expressing many of the same genes as eutherian ones.

There is evidence for a division of function between the two parts of the yolk sac placenta, with the nonvascular part (BOM) being responsible for uptake and metabolism of nutrients and the vascular part (TOM) for respiration. I am not sure how much oxygen the tiny marsupial embryo needs. Perhaps the TOM is more important for removing CO₂ and regulating the acid-base balance of the embryo. The tammar has an embryonic-type hemoglobin more capable of sequestering oxygen (protecting the embryo from reactive oxygen species) than transporting it to tissues.

A fascinating detail is that the yolk sac endoderm of the tammar has assumed functions, especially to do with trafficking of nutrients, that in eutherians are served by trophoblast.

Genes expressed in mammary gland and placenta
of marsupials and eutherians. From Guernsey et al.
eLife 2017 CC The Authors

Because much of development in the wallaby is supported by lactation, it is interesting to find considerable overlap in the transcriptomes of marsupial mammary gland and eutherian placenta.

My only criticism of this paper would be: the mouse has a yolk sac that supports early embryonic development and continues to function alongside the placenta right up to term. Perhaps the authors could not identify a data set on mouse yolk sac transcriptome, but they should have referenced the eutherian yolk sac in their discussion. An interesting theory by Claudia Freyer et al. (here) is that the stem species of therians (marsupials and eutherians) had both types of placentation.

For additional remarks on this paper see Nature News and Comments (here).

Hermann von Ihering and polyembryony in armadillos

Uterus of the mulita (Dasypus hybridus) with 9 identical
embryos. From Fernandez Morph Jahrb 1909; 39:302-333

In 1886 Hermann von Ihering opened the uteri of two pregnant armadillos. Both contained 9 fetuses of the same sex. Each fetus had its own amnion but all were enclosed in a common chorion (placenta). He was the first to propose that the embryos were derived from a single fertilized egg with splitting into separate embryos occurring early in development.

Arrangement of fetal membranes in the mulita (Dasypus hybridus).
From Fernandez Morph Jahrb 1909; 39:302-333

Ihering had studied the mulita or Southern Long-Nosed Armadillo (Dasypus hybridus). Later Fernández demonstrated that splitting occurred at the embryonic shield stage in the mulita. Newman & Patterson, working with the nine-banded armadillo (D. novemcinctus), came to a similar conclusion. Fernández, however, was the first to obtain early stages before splitting occurred. More recently, the nine-banded armadillo was the object of elegant studies by Allen Enders (summarized here). Specific polyembryony is known only from Dasypodinae and is thought not to occur in the two other subfamilies of armadillo.

Hermann von Ihering (1850-1930)
CC BY-SA 3.0 Wikimedia Commons

Hermann von Ihering was a German zoologist who relocated to Brazil in reaction to his family's disapproval of his marriage to a widow with a child. This was in 1880. His first years were spent as a collector in the southern state of Rio Grande do Sul, based on an Island known as Ilha do Doutor (Doctor's Island). In 1893 he became the first director of Museu Paulista (State Museum of Sao Paulo) and held this post for 23 years.

Hermann von Ihering´s principal area of expertise was mollusks. He also became an expert on the birds of the State of Sao Paulo, of which he observed 695 species and subspecies. For my Brazilian readers there is an excellent recent biography by Hitoshi Nomura (open access here). It lists 338 of his publications.

His son Rodolpho von Ihering (1883-1939) was also a zoologist. He was appointed vice director of Museu Paulista, which led to the accusation of nepotism that was to force Hermann's resignation. Rodolpho was an expert on fish and is credited with founding Brazilian pisciculture with stations at Porto Alegre in Rio Grande do Sul and Pirassununga, S.P.

References: Biol Zentralblatt 1886; 6:532-9 and Arch Physiol 1886; pp. 443-50

Foetus or fetus?

Human Fetus drawn by Leonardo da Vinci

There was a recent spate of tweeting about the correct spelling in British usage of fetus - or foetus. As the Oxford Dictionary makes clear, the spelling foetus has no etymological basis.

A similar debate 50 years ago was initiated by James Dixon Boyd and William James Hamilton in connection with the first edition of their influential textbook Human Embryology (previous post). This was in the BMJ. Coincidentally Bernard Towers (later Professor of Anatomy and Pediatrics at UCLA) raised the issue in Arch Dis Child. Earlier, Lionel Everard Napier had argued for "fetus" in The Lancet.

The thrust of their arguments was that "fetus" was the only spelling in use until 600 A.D., "foetus" being introduced by Isidorus of Seville on the basis of an erroneous etymology.

Statue of Isidorus of Seville in Madrid
Photo by Luis Garcia CC BY-SA 2.5

Boyd and Hamilton solicited opinion on the subject and the resulting letters fell out 5 to 1 in support of "fetus." Among the supporters was J.H.M. Pinkerton, later Professor of Midwifery and Gynaecology in Belfast. The counter argument, "Foetus is a word of respectable antiquity and lineage," was advanced by Hugh Gault Calwell who is known to have been skilled in Greek and Latin. Sadly, when Human Embryology appeared, it used "foetus" rather than "fetus."

References: BMJ 1967 (5337): 425, (5539): 568, (5540): 631, Arch Dis Child 1967; 42:224, Lancet 1952; 260: 885-6.

From antelope placenta to the chi square distribution

The Four-horned Antelope (Tetracerus quadricornis)
Philip Sclater The Book of Antelopes 1894

Despite its appearance, this Indian species is not a true antelope, but belongs to the subfamily Bovinae. Its placenta was described in 1884 by Raphael Weldon then a Scholar of St John's College, Cambridge.

Gravid uterus of the Four-horned Antelope
Weldon Proc Zool Soc London 1884

There was a fetus in each horn and Weldon was struck by the relative paucity of placentomes (30 and 22, respectively).

One extremity of the chorion of the Four-horned Antelope
Weldon Proc Zool Soc London 1884 

Weldon thought the interplacentomal regions resembled the diffuse placenta of the pig. This may have been overinterpretation. There seem to be no subsequent descriptions of placentation in this species.

The Nilgai (Boselaphus tragocamelus)
Rufus46 (Wikimedia Commons) CC BY-SA 3.0

Together with the Nilgai, the Four-horned Antelope forms its own tribe. Benirschki examined a couple of Nilgai placentas (here). He did not find an unusual number of cotyledons but remarked they were not as neatly arranged in rows as in other species. So perhaps Weldon was on to something.

Raphael Weldon is not remembered for his placental research. He became a marine biologist and was professor of Zoology first at University College London then at Oxford. At UCL he collaborated with the mathematician Karl Pearson and founded the science of biometrics. Famously, he rolled a set of 12 dice no fewer than 26,306 times. The results showed a bias towards fives and sixes (more here). These data were used by Karl Pearson in the latter's seminal paper on the chi-square statistic.

Placentation in the pronghorn (Antilocapra americana)

Pregnant uterus of the pronghorn. Note the fused amnions in the corpus uteri.
From Wislocki and Fawcett Bull Museum Comp Zool Harvard 1949; 101: 545-559.

The pronghorn is the sole survivor of a North American lineage of ruminants. (Antilocapridae). Its placenta was described by Wislocki and Fawcett (full text available at Biodiversity Heritage Library). It is polycotyledonary and epitheliochorial. The chorionic villi show a pattern of branching distinct from that of other ruminants. Interestingly, Wislocki and Fawcett described and illustrated binucleate giant cells. The amnion is larger than the allantoic sac. There is a fetus in each horn of the bicornuate uterus and the two amnions fuse back to back in the region of the uterine corpus.

Female pronghorn in Wyoming.
Photo by Yathin S Krishnappa Wikimedia Commons CC BY-SA 3.0

The pronghorn bears a superficial resemblance to an antelope, but this is the result of convergent evolution. Pronghorns (Antilocapridae) share a common ancestor with the giraffe and okapi (Giraffidae) whereas Bovidae (including antelopes) is a distinct lineage. The evolution of placentation in even-toed ungulates has been traced by Andrea Mess and Karl Klisch (here).

Given the taxonomic position of the pronghorn, a recent study has examined glycosylation at the fetal maternal interface and compared it with the giraffe, okapi and various bovids (here). The expression of pregnancy-associated glycoproteins (PAGs) in binucleate trophoblast cells was also examined.

Embryo competition in the pronghorn: penetration of the membranes
of a distal embryo by the necrotic tip of a proximal embryo.
From O'Gara Amer J Anat 1969; 125: 217-232. 

Several authors have noted that the number of corpora lutea exceeds the number of fetuses (usually twins). O'Gara (reference here) found that some reduction occurred during the phase of blastocyst elongation. Often two blastocysts managed to implant in the same horn. However, the membranes of the proximal embryo (nearest the uterine body) formed a necrotic tip. As the conceptus grew, this tip pierced the chorion and allantois of the distal embryo, resulting in its death. As O'Gara wrote, "The phenomenom of the necrotic tip acting as a lethal weapon is apparently unique to the pronghorn."

Physiology of dinosaurs

Barosauras lentus rearing: mounted skeleton at AMNH
Photo by Greg CC BY 2.0 

In a previous post I discussed the increase in brain metabolic rate during hominin evolution. Roger Seymour, who co-authored that study, has used similar insights to analyse the cardiovascular physiology of sauropod dinosaurs (open access). 

One of his conclusions is that if Barosaurus reared its head (as in the exhibit above) its mean arterial blood pressure would have been 700 mmHg and the left ventricle of the heart would have weighed a metric ton. The heart would have filled the chest and its thick wall been so stiff as to consume a huge amount of energy.

Diplodocus carnegii in the hall of the Natural History Museum in
South Kensington (now removed). Photo by Drow male CC BY-SA 4.0

Seymour goes so far to suggest that long-necked sauropods like Diplodocus were aquatic, floating on the surface of the water and lowering their heads to browse on vegetation. Abdominal air sacs would have helped it to float.

Sauropods are generally regarded as terrestial because of skeletal features, but perhaps they retained these because they needed to go on land to lay their eggs!

Dinocephalosaurus a live-bearing archosauromorph
From Liu et al. 2017

Large reptiles that became fully aquatic were viviparous as shown in a recent paper by Liu et al. (open access). This particular example was an archosauromorph and thus in the lineage that gave rise to dinosaurs (including birds) and crocodilians.

Prothero's Guide to Prehistoric Mammals

Princeton University Press ISBN: 9780691156828

This is an admirable effort to teach extinct mammals by showing them in  context with extant species. I found it helpful to view fossil skulls and skeletons alongside photos of familiar animals. As an additional aid, most chapters have one or more phylogenetic trees culled from the recent literature. Of course, there are extinct orders with a tenuous connection to living ones; inevitably a long chapter deals with orders that do not fit any scheme (e.g. Dinocerata).

Skeleton of Eobasileus - from the Order Dinocerata,
which is difficult to place in the mammalian tree.
Galerie de Paléontologie et d'Anatomie Comparée, Paris.

Donald R. Prothero is an experienced author and has penned a well written and informative text. The illustrations by Mary Persis Williams reconstruct the body forms of prehistoric mammals, usually with several in the same figure and with a human silhouette to give a sense of scale. These drawings are quite restrained compared to those of Velizar Simeonovski in Horned Armadillos and Extinct Madagascar (the cover is an exception).

Skeleton of the huge hedgehog Deinogalerix koenigswaldi.
National Museum of Natural History, Leiden, the Netherlands
Photo by Peter Maas (CC BY-SA 3.0) 

The book is aimed at a broad readership as "Princeton Field Guide" might suggest. The publishers have encouraged depiction of large and spectacular mammals at the expense of smaller ones. There is poor coverage of rodents. On the other hand it is the large fossils that are on display in museums of natural history.

Reconstruction of the ground sloth Eremotherium.
Exhibit in the Fernbank Museum of Natural History,
Atlanta, Georgia. Photo by Daderot (CC0)

Indeed, a field guide to fossils might be useful on museum visits. Yet though there are many excellent photos of exhibits, the museums are not identified. The two pages of Illustration Credits are of little help. The credit line to the above Eremotherium figure is merely "Daderot/Wikimedia Commons."

Figures showing phylogenetic trees suffer from a similar deficiency. One from the rodent chapter is credited to E.T. Prothero. It is redrawn from a paper on the Laotian rock rat (Laonastes) by Huchon et al. (here). The reader is ill served by not being guided to the source papers.

Caecilian offspring feed on the skin or oviduct lining of the mother

A caecilian (Dermophis mexicanus)
Franco Andreone CC BY-SA 2.5

Caecilians are limbless amphibians with a burrowing lifestyle. There are oviparous and viviparous species. In the former there is an extended period of parental care. The hatchlings have sharp teeth which they use to feed on the outer layer of the parent's skin. Because skin feeding occurs in both African and South American species, it probably was evolved more than 100 million years ago (argued here).

Viviparous species take it one step further. The fetuses have teeth and feed on the hypertrophied lining of the oviduct (described here). This is just one of several forms of matrotrophy practiced by invertebrates (here) and vertebrates (here).

Slow incubation of dinosaur eggs: why only birds survived

Cast of Mossospondylus eggs and embryo
Photo from Royal Ontario Museum by Daderot (CC)

Most people have heard that birds are dinosaurs. But that raises an obvious question: why did birds survive the end-Cretaceous mass extinction event that did for their non-avian kin? A paper just published in PNAS suggests this might be explained by different reproductive strategies.

Reconstruction of Protoceratops andrewsi
AntoninJury (Wikimedia Commons) CC BY-SA 4.0

In brief, the authors made CT scans of teeth from fossilized dinosaur embryos. They then counted the von Ebner lines, which reflect the incremental pattern of dentine formation. Applying some quite reasonable assumptions to the data, they estimated the incubation time of the dinosaur eggs as minimum 2.8 months for Protoceratops andrewsi and 5.8 months for Hypachrosaurus stebingeri. The incubation times of modern birds tend to be much shorter though the upper end of the range (11-85 days) overlaps with P. andrewsi.

It is suggested that the relatively long generation times of non-avian dinosaurs put them at a disadvantage in competing with birds, reptiles and mammals during the Cretaceous-Palaeogene transition.