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: Five things dung beetles do with a piece of poo

Related Stories

Five things dung beetles do with a piece of poo

September 18, 2015

Dung beetle behaviour has fascinated humans for thousands of years – including the ancient Egyptians, who incorrectly believed the beetles reproduced only from males. But Egyptian observations that the beetles' ball rolling ...

Male seahorse and human pregnancies remarkably alike

September 1, 2015

Their pregnancies are carried by the males but, when it comes to breeding, seahorses have more in common with humans than previously thought, new research from the University of Sydney reveals.

How a female X chromosome is inactivated

August 10, 2015

In female mammals, one of the two X chromosomes is inactivated. Thanks to research using special stem cells, geneticists at ETH Zurich have been able to provide detailed insight into the molecular mechanism behind this inactivation ...

Expanding theory of evolution

August 5, 2015

An Indiana University professor is part of an international team of biologists working to expand Darwin's theory of evolution to encompass factors that influence a species' growth and development beyond genetics—as well ...

Rejuvenating the comparative approach in modern neuroscience

July 20, 2015

65 years ago, the famed behavioral endocrinologist Frank Beach wrote an article in The American Psychologist entitled 'The Snark was a Boojum'. The title refers to Lewis Carroll's poem 'The Hunting of the Snark', in which ...

Express yourself: How zygotes sort out imprinted genes

February 16, 2012

Writing in the February 17, 2012 issue of the journal Cell, researchers at the Ludwig Institute for Cancer Research, the University of California, San Diego School of Medicine and the Toronto Western Research Institute peel ...

Recommended for you

Most EU nations seek to bar GM crops

October 4, 2015

Nineteen of the 28 EU member states have applied to keep genetically modified crops out of all or part of their territory, the bloc's executive arm said Sunday, the deadline for opting out of new European legislation on GM ...

The dark side of Nobel prizewinning research

October 4, 2015

Think of the Nobel prizes and you think of groundbreaking research bettering mankind, but the awards have also honoured some quite unhumanitarian inventions such as chemical weapons, DDT and lobotomies.

Internet giants race to faster mobile news apps

October 4, 2015

US tech giants are turning to the news in their competition for mobile users, developing new, faster ways to deliver content, but the benefits for struggling media outlets remain unclear.

Fusion reactors 'economically viable' say experts

October 2, 2015

Fusion reactors could become an economically viable means of generating electricity within a few decades, and policy makers should start planning to build them as a replacement for conventional nuclear power stations, according ...


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.