Horses from the permafrost and beyond

Remarkably it has proven possible to sequence the genome of a Middle Pleistocene horse from a bone preserved for 700,000 years in the Canadian permafrost (summary and links here). Comparison with the genomes of modern horses and a more recent fossil yielded several interesting results. Among them, Przewalski's horse was shown to be a wild subspecies uncontaminated by domestic breeds.

Placenta and implantation site of Przewalski's horse

From the Benirschke web site (here)

Przewalski's horse was once listed as extinct in the wild but a successful captive breeding and release program has changed its status to endangered. There are several images of the placenta of Przewalski's horse on Benirschke's web site (see previous post). The equine placenta is epitheliochorial and has microcotyledons as pictured above.

Soft tissues usually are not preserved for posterity but a putative placenta accompanies a Middle Eocene fossil of the equid genus Propalaeotherium (cited here).

How wombats do it

Rossetti lamenting the death of his wombat

Wombat husbandry has improved since Rossetti's time, but wombats are still difficult to breed in captivity.The northern hairy-nosed wombat Lasiorhinus krefftii is among the rarest species of mammal and a captive breeding program might be needed to save it from extinction. A timely review (here) summarizes what we know and still need to learn about wombat reproduction.

Fetal membranes of the long-nosed bandicoot Perameles

nasuta from Hill (1897) redrawn by Amoroso

Wombats and their close relative the koala Phascolarctos cinereus have a chorioallantoic placenta as well as the yolk sac placenta characteristic of marsupials (previous post). In this respect they resemble the bandicoots. J. P. Hill described placentation in the Southern brown bandicoot in 1895 and two years later published a detailed description for the long-nosed bandicoot (here). As wombats and bandicots are not closely related, this might be an instance of convergent evolution. Although it has also been argued (here) that a chorioallantoic placenta was present in the common ancestor of marsupials and lost in the remaining lineages.

Our knowledge of placentation in wombats relies on specimens of Vombatus ursinus collected by Hill in 1899-1901 and conserved as part of The Hubrecht Collection at Museum für Naturkunde in Berlin. These specimens have been described by Hughes and Green (In: Wells RT, Pridmore PA Wombats Chipping Norton NSW: Surrey Beatty & Sons, 1998).

An aquatic past for elephants

Endotheliochorial placenta of the Amazonian manatee

Fetal (fc) and maternal (mc) capillaries are present

The closest relatives to elephants are the dugong and manatees. Placentation in the African elephant Loxodonta africana has been well studied (reviewed here). We were able to show strong resemblances in the placenta and fetal membranes between the elephant and the Amazonian manatee Trichecus inunguis (here).

This hoax photo of the Loch Ness monster

is thought to be an elephant swimming

It has long been suspected that the ancestor of proboscideans was aquatic or semi-aquatic (here). Clues include the presence of nephrostomes in the mesonephros of the fetus (here). Now support comes from an unexpected quarter: myoglobin.

Diving mammals such as whales and seals have high amounts of myoglobin in their muscles to act as a source of oxygen during prolonged submersion. A new study (here) shows that in such species there is an increase in surface charge of the molecule to prevent dimerization occurring at high concentrations of myoglobin. This molecular signature was also found in sirenians and proboscideans. The analysis included two extinct species: the woolly mammoth (previous post) and Steller's sea cow. A phylogenetic analysis suggested that substitutions contributing to increased surface charge on myoglobin occurred in the common ancestor of sea cows, elephants and hyraxes (paenungulates). The conclusion drawn is that this ancestor may have been semiaquatic.

How we do it

The subtitle of this new book is The Evolution and Future of Human Reproduction. Nobody is better qualified to discuss that than Bob Martin, an anthropologist based at Chicago's Field Museum. Although written for a general audience, there is no attempt to dumb down the material.

Human reproduction has many unique features even in relation to other primates and this book provides a comprehensive overview and the necessary evolutionary context. It is useful background for scientists who might be steeped in particular aspects (such as placentation) but not in others (menstruation or lactation). For readers stimulated to explore further there are excellent scientific reviews by the same author (e.g. here and here). 

Embryos and ancestors in deep time

I read Gavin de Beer's monograph while still at school. Scornful of Haeckel's idea that ontogeny recapitulates phylogeny, de Beer details the several ways in which embryonic development is modified during the evolution of new species. The book remains a useful reference source for concepts like neotony and paedomorphosis.

Fifty years on, I was delighted to read this new monograph by Marcelo Sánchez-Villagra. He is a paleontologist and gives fascinating examples from the fossil record. Developmental series of extinct species are relatively rare, but much can be induced from studies of bone and tooth structure. Modern and non-invasive imaging techniques are able to yield new information from rare specimens diligently guarded by museum curators. Phylogenetic bracketing (as of non-avian dinosaurs between crocodiles and birds) is another approach from which we can infer developmental processes in extinct lineages. It then becomes possible to speculate on the role of the Hox family of regulatory genes and draw on other data from molecular biology.

There is of course much here on the evolution of viviparity and placentation and on reproductive strategies. The fossil record has much to teach us here (see my paper here).

De Beer was scathing about Haeckel because his ideas had stood in the way of experimental embryology. Sánchez is kinder, recalling that more Europeans learned about evolution from the writings of Haeckel than of Darwin. His own view is summed up as "ontogeny does not usually recapitulate phylogeny."

The focus of this book is sharpest in the opening chapters and I would have liked to have seen the threads drawn together in a concluding chapter. It is nonetheless a good read and never gets bogged down in technicalities.

The primrose way

Space-filling model of the hydrogen sulfide molecule (Ben Mills)

Hydrogen sulfide (H₂S) is a signaling molecule in the brain and cardiovascular system. The antihypertensive effect of garlic has been attributed to the generation of H₂S from its contained polysulphides (reviewed here).

A recent study (here) indicates that H₂S may play a role in regulating blood flow on the fetal side of human placenta. One of the enzymes that generates H₂S (CSE) is found in the smooth muscle layer of stem villus arteries. Moreover it was shown using the H₂S donor NaHS that the molecule is able to dilate the fetal placental circulation. Because the placenta lacks innervation, signaling molecules like H₂S are likely to be important in the regulation of blood flow. The study found evidence that CSE expression is decreased in pathological pregnancies such as preeclampsia and fetal growth restriction.

H₂S is involved in oxygen sensing in the carotid body (here) and one wonders if it might perform a similar role in the placenta.