Biological complexity: evolution of mammalian pregnancy (Introduction)

by David Turell , Saturday, October 15, 2016, 01:47 (2962 days ago) @ David Turell

This article discusses how placental pregnancy developed. it is complex story featuring a reto virus who entered the animals germ line:

http://www.pbs.org/wgbh/nova/next/evolution/endogenous-retroviruses/

"The critter—already the product of some 100 million years of evolution—looked like a modern mammal, with body hair and mammary glands, except for one tiny detail: according to a recent genetic study, it didn’t have a placenta. And its kind might never have evolved one if not for a chance encounter with a retrovirus.

Unlike most viruses, which infect, replicate, and then leave their host, retroviruses elbow their way into their host’s genome where they are copied and passed on to daughter cells for the life of the host. This retrovirus, however, managed to sneak its way into one of our ancestor’s sperm or egg cells, able to be passed on to every cell in every subsequent generation. Virus and host had become one.

***

"Early mammals used the spare viral parts left in the junk drawers of the genome to use a viral gene to help create the placenta, and other symbiotic viruses help turn us from a ball of cells into a fully-formed squalling infant and protect us from pathogens.

"Scientists are discovering that the so-called “junk DNA”—a significant portion of which is from symbiotic viruses—is actually a potent force in the evolution of new species. Although the evolution of pregnancy via the placenta might be some of the most persuasive evidence that viruses stashed deep within the genome can help give rise to new species, it’s not the only proof. New studies revealing the role of endogenous retroviruses in the more recent evolution of humans show that these snippets of DNA are helping to blur the boundary between human and virus. Humans are, in a very real sense, part virus. (my bold)

***

"Although most symbiosis research has focused on the role of the microbiome, the viruses tucked into our DNA can play a similar role in splitting apart two populations, turning one species into two. The first wedge scientists discovered was a protein called syncytin.

***

"Syncytin is produced only by certain cells in the placenta, and it directs the formation of the cellular boundary between the placenta and maternal tissue. Approximately one week after fertilization, the egg, now a hollow ball of cells called a blastocyst, implants itself into the uterus, stimulating the formation of the placenta, which provides the fetus with oxygen and nutrients while removing carbon dioxide and other wastes. .. The cells in the outer layer of the blastocyst form the outer layer of the placenta, and those in direct contact with the uterus are the only ones that made syncytin.

"When the scientists looked closer at the DNA sequence of syncytin, they found that it was nearly identical to a viral protein called env that caused the virus to fuse with its host cell. In the placenta, syncytin performed helped the fetus fuse with its mother. At last McCoy, Howe, and Mi knew what syncytin did.

“'This was a bona fide retroviral envelope protein that had somehow been captured during evolution and been trained to operate in human biology,” McCoy says.
The two other retroviral genes next to syncytin, gag and pol, were completely non-functional, McCoy says. Only env remained intact. “Everything else about that retrovirus had been trashed,” he says.

***

"An important step in mammalian evolution was accomplished by capturing this viral envelope gene,” McCoy says. “There’s plenty of examples of viruses picking up human genes, but this is one of the first examples of the reverse.”

"Humans aren’t the only species with a placenta, however. All mammals have placentas, including marsupials and egg-laying mammals. Although all of these mammals have a syncytin gene, they don’t all have the same syncytin gene. The syncytin produced by mice is completely different from the two syncytins found in humans and other primates. At numerous points in mammalian evolution, symbiotic retroviruses entered the genome and steered different groups of mammals along different evolutionary paths,

"The team identified genes derived from the human endogenous retrovirus HERV-K that were active around the time when the embryo was just eight cells. Of the many known edogenous retroviruses in humans, HERV-K is the newest—it inserted itself as recently as 200,000 years ago. ...But far from being detrimental, HERV-K activates key genes that help transform a single cell into a fully-formed infant.

***

"HERV-K may also have played an important role in separating some of the first humans from their primate ancestors by making small adjustments in when certain genes were switched on or off, according to Reijo Pera’s research."

Comment: Perhaps God invented and used retroviruses to drive evolution. Note my bold. Giant article worth reading for full details of research.