Loss of egg yolk genes in mammals and the origin of lactation and placentation

March 18, 2008
Loss of egg yolk genes in mammals and the origin of lactation and placentation
The emergence of alternative nourishment resources (lactation and placentation) during mammalian evolution set the stage for mammals' progressive loss of egg yolk nourishment (as a consequence of the loss of egg yolk genes). Credit: Image: Rasmus Kaessmann

If you are reading this, you did not start your life by hatching from an egg. This is one of the many traits that you share with our mammalian relatives. A new paper in this week’s PLoS Biology explores the genetic changes that led mammals to feed their young via the placenta and with milk, rather then via the egg, and finds that these changes occurred fairly gradually in our evolutionary history. The paper shows that milk-protein genes arose in a common ancestor of all existing mammalian lineages and preceded the loss of the genes that encoded egg proteins.

There are three living types of mammals: placental mammals (you, me, dogs, sheep, tigers, etc.), marsupial mammals (found in Australasia and South America, including kangaroos and possums), and monotremes (the duck-billed platypus and two species of Echidna).

The reproductive strategies of these three groups are very different. Placental mammals have long pregnancies and complicated placentas that provide nourishment to the embryo, followed by a relatively short period of lactation. Marsupials have a simpler form of placenta and much shorter pregnancies, followed by an extended period where the offspring is fed milk that changes in composition to meet the baby’s altering nutritional needs.

Monotremes—once a diverse group, but now restricted both in species number and distribution—have a much more reptilian beginning, as they lay eggs filled with yolk. While they do feed their young with milk, it is secreted onto a patch of skin rather then from a teat. How did these different strategies arise from our reptilian ancestors"

A new paper by David Brawand, Walter Wahli, and Henrik Kaessmann investigates the transition in offspring nutrition by comparing the genes of representatives of these three different mammalian lineages with those of the chicken—an egg-laying, milkless control. The authors found that there are similar genetic regions in all three mammalian lineages, suggesting that the genes for casein (a protein found in milk) arose in the mammalian common ancestor between 200 and 310 million years ago, prior to the evolution of the placenta.

Eggs contain a protein called vitellogenin as a major nutrient source. The authors looked for the genes associated with the production of vitellogenin, of which there are three in the chicken. They found that while monotremes still have one functional vitellogenin gene, in placental and marsupial mammals, all three have become pseudogenes (regions of the DNA that still closely resemble the functional gene, but which contain a few differences that have effectively turned the gene off). The gene-to-pseudogene transitions happened sequentially for the three genes, with the last one losing functionality 30-70 million years ago.

Therefore, mammals already had milk before they stopped laying eggs. Lactation reduced dependency on the egg as a source of nutrition for developing offspring, and the egg was abandoned completely in the marsupial and placental mammals in favor of the placenta. This meant that the genes associated with egg production gradually mutated, becoming pseudogenes, without affecting the fitness of the mammalian lineages.

Citation: Brawand D, Wahli W, Kaessmann H (2008) Loss of egg yolk genes in mammals and the origin of lactation and placentation. PLoS Biol 6(3): e63. doi:10.1371/journal.pbio.0060063

Source: Public Library of Science

Explore further: Behavioral biology: Ripeness is all

Related Stories

Behavioral biology: Ripeness is all

March 10, 2017

In contrast to other members of the Drosophila family, the spotted-wing fly D. suzukii deposits its eggs in ripe fruits. Biologists from Ludwig-Maximilians-Universitaet (LMU) in Munich have now elucidated the sensory basis ...

Using Big Data to understand immune system responses

January 25, 2017

An enzyme found in many bacteria, including the bacterium that gives us strep throat, has given mankind a cheap and effective tool with which to edit our own genes. This technology, called CRISPR, is also being used to understand ...

Recommended for you

Cities and monuments switch off for Earth Hour

March 26, 2017

The Empire State Building and United Nations headquarters in New York joined other iconic buildings and monuments around the world plunging into darkness for sixty minutes on Saturday to mark Earth Hour and draw attention ...

Study into who is least afraid of death

March 24, 2017

A new study examines all robust, available data on how fearful we are of what happens once we shuffle off this mortal coil. They find that atheists are among those least afraid of dying... and, perhaps not surprisingly, ...

Astronomers identify purest, most massive brown dwarf

March 24, 2017

An international team of astronomers has identified a record breaking brown dwarf (a star too small for nuclear fusion) with the 'purest' composition and the highest mass yet known. The object, known as SDSS J0104+1535, is ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.