Loss of mammalian egg yolk gene and source of lactation and placental formation

If you are reading this article, you have not started life by hatching from an egg. This is one of the many things you have in common with our relatives, mammals.

An article on PLoS Biology this week explored the genetic changes that make mammals nourish their babies through the placenta and milk, rather than through eggs, and find out that these changes take place one Slowly in our evolutionary history. The paper shows that milk protein genes appear in the ancestors of all existing mammalian lineages along with the gradual decline of egg protein genes.

There are three types of mammals: placental animals (you, me, dogs, sheep, tigers, etc.), marsupials (found in Australia and South America including kangaroos, ops), and animals monogamous objects (platypus and two species of porcupine). The form of reproduction in the three animal groups mentioned above is very different. The placental animals have a longer gestational period and complex placenta provides nutrition to the embryo, followed by a relatively short period of lactation. The marsupial has a simpler placenta and much shorter gestation period, and the calf is then fed for a period of time, the composition of the milk will change to suit the nutritional needs of the child. young

The emergence of other forms of nourishment (lactation and ovule) in mammalian evolution creates a rapid reduction phase of nourishment with egg yolk (resulting in red yolk genes). eggs also decreased) (Photo: Rasmus Kaessmann)

Single-vented animals - which used to consist of many different species, but now have very few species and are restricted to distribution - have the same beginnings as reptiles, they lay eggs with yolk. They also breastfeed, but the milk is released from a skin area, not from the nipple. How did these reproductive and nurturing forms appear from our reptilian ancestors?

An article by authors David Brawand, Walter Wahli, and Henrik Kaessmann explored the transition in caring for young children by comparing genes representing three different species of mammals and chicken genes. - a species that lays eggs, and has no milk. The authors found that there are similar genetic regions between the three mammalian groups, indicating that casein (a protein found in milk) genes appear in mammalian ancestors between 200 and 310. million years ago, before the placenta's evolution.

Eggs contain a protein called vitellogenin which serves as the main nutrient source. The authors searched for genes associated with vitellogenin production, and found three genes in chickens. They also found that while monogamous animals still have a function gene for vitellogenin, in placental animals and marsupial animals, all three genes become pseudogenes (DNA regions similar to functional genes, but There are a few differences that make these genes go away. Genetic modification to pseudogene occurs consecutively for all three genes, the last one losing its basic function around 30-70 million years ago.

So mammals have milk before they stop laying eggs. The placenta reduces the dependence on eggs as a source of nutrition for developing young, and the eggs have been completely eliminated in placental animals and marsupial when the placenta is formed. This means that the genes involved in egg production have gradually changed, becoming pseudogenes, without affecting the state of mammals.

Quote: Brawand D, Wahli W, Kaessmann H (2008) The loss of mammalian egg yolk gene and the origin of lactation along with placental formation.PLoS Biol 6 (3): e63.doi: 10.1371 / journal.pbio.0060063