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. 

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.