Evolution of flowering plants

Amborella trichopoda, basal to all other flowering plants
(Wikimedia Commons)

Amborella is a sprawling shrub endemic to New Caledonia. This monotypic genus is basal to the angiosperms and sister to all other flowering plants. Now its mitochondrial and nuclear genomes have been sequenced and this is the subject of three papers in the current issue of Science (summarized here).

Amongst other things it addresses Darwin's "abominable mystery" - the question of why flowers suddenly proliferated on earth millions of years ago (discussed in this press release). It confirms, as had previously been suspected, that the evolution of flowering plants was preceded by a genome duplication.

Whole genome duplication has also played a significant role in the evolution of vertebrates (reviewed here). Tandem duplication of genes or groups of genes is common in the mammalian lineage and the basis for the evolution of placental hormones and fetal hemoglobins (my review here).

Footnote: In botany, placentation refers to the manner in which the ovules are attached within the ovary. It is an important taxonomic feature.

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.  

 

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.

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.   

Do not marsupials have trophoblast?

Embryonic and trophoblastic areas of the marsupial

blastocyst as envisaged by Hartman 

In the final chapter of her book (previous post), Zofia Kielan-Jaworowska writes on placental mammals, "Their enormous success depends in the first place on the acquisition of the trophoblast, on the basis of which the placenta is formed." In contrast, "Without the trophoblast, marsupials are unable to prolong their gestation period and are born at an extremely early level of anatomical development." This view or a version of it seems to be shared by other zoologists. 

The souce of the misunderstanding can be traced to influential papers by Jason A. Lillegraven (e.g. here). The marsupial blastocyst lacks the inner cell mass of a placental mammal blastocyst. Early on, however, there are two distinct areas that often have been referred to as the embryonic and trophoblastic areas. Lillegraven argues, convincingly in my view, that the "embryonic area" is not equivalent to the inner cell mass and that a considerable portion of the "trophoblastic area" forms part of the developing embryo. In an anatomical sense the trophoblastic area is not the trophoblast as defined by Hubrecht (see previous post).

Nonetheless, Lillegraven also says, "But from a functional point of view (as applied to extra-embryonic parts of the conceptus having direct nutritive significance), the term "trophoblast" may be used to good advantage throughout the Amniota, whether development occurs within a shelled or a shell-free setting."

Thus marsupials do have trophoblast and of course they all have at least a yolk sac placenta and sometimes a chorioallantoic placenta to boot. As shown by Ulrich Zeller and Claudia Freyer (here) there are marsupials where the trophoblast forms  a syncytium and exhibits invasive properties.

As noted in a previous post, there are sound reasons for distinguishing between Eutheria and Placentalia but it is little wonder that marsupial specialists bridle at the term "placental mammals."  

Karl Bogislaus Reichert

Karl Bogislaus Reichert (1811-1883)

Crucial to our understanding of the evolution of mammals is the origin of the bones of the inner ear from what were elements of the jaw articulation in reptiles. This was worked out in 1837 by the German embryologist Karl Bogislaus Reichert. Zofia Kielan-Jaworowska (previous post) notes this is all the more remarkable in view of the primitive tools available at the time. Later Ernst Gaupp built on this work and the supposed origin of the inner ear bones is known as the Reichert-Gaupp theory.



Reichert's membrane (Rm) in the placenta of the capybara

From Kanashiro et al. Reprod Biol Endocrinol 2009; 7: 57 (here)

Students of the placenta will be more familiar with the name of Reichert through the eponymous membrane that lies between the trophoblast and the endoderm of the parietal yolk sac in rodents and insectivores. It was first described by Reichert in the guinea pig (Abh Akad Wissensch Berlin 1862; pp. 97-216).

In pursuit of early mammals

A multituberculate (Catopsbaatar) about to fall prey

to a dinosaur (Saurornithoides)

This fascinating book is the autobiography of the eminent paleontologist Zofia Kielan-Jaworowska and an introduction to the early history of mammals.

Her descriptions of the Polish-Mongolian expeditions in 1963-71 recount the daunting logistics of working in the Gobi Desert 1000 km from Ulaanbaatar and transporting back fossils weighing several tons. The excitement of the fossil hunt is apparent: "I turned the block over and was left speechless! I had in my hands an almost complete, beautifully preserved skull of a small dinosaur." With this as a starting point we follow Zofia Kielan-Jaworowska's subsequent career, which included eight years on the faculty of Oslo University. In 2002 she returned to Gobi with her grandaughter Zosia; ten years on Zosia checked the references for the current volume. 

The second half of the book focuses on the evolution of mammals. It is restricted to the crown group that includes the living monotremes, marsupials and placentals and extinct groups with which they share a common ancestor. Arguably this includes the fossils of greatest interest to the general reader. For a more comprehensive treatment of the subject there is Mammals from the Age of Dinosaurs (2004) by Kielan-Jaworowska, Cifelli and Luo.

The new book contains potted biographies of many eminent vertebrate paleontologists, including some of the author's predecessors as well as recent stars such as Zhe-Xi Luo. It is richly illustrated with historical photos and line drawings of fossil mammals from the age of dinosaurs.