Featured Research

from universities, journals, and other organizations

Loss Of Egg Yolk Genes In Mammals And The Origin Of Lactation And Placentation

Date:
March 19, 2008
Source:
PLoS Biology
Summary:
The major egg yolk genes, those that express vitellogenins, appear to have progressively lost their functionality during mammalian evolution, probably due to the emergence of the mammalian-specific developmental nourishment resources, 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: Photo by 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 article 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.

Journal reference: 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


Story Source:

The above story is based on materials provided by PLoS Biology. Note: Materials may be edited for content and length.


Cite This Page:

PLoS Biology. "Loss Of Egg Yolk Genes In Mammals And The Origin Of Lactation And Placentation." ScienceDaily. ScienceDaily, 19 March 2008. <www.sciencedaily.com/releases/2008/03/080318094610.htm>.
PLoS Biology. (2008, March 19). Loss Of Egg Yolk Genes In Mammals And The Origin Of Lactation And Placentation. ScienceDaily. Retrieved August 29, 2014 from www.sciencedaily.com/releases/2008/03/080318094610.htm
PLoS Biology. "Loss Of Egg Yolk Genes In Mammals And The Origin Of Lactation And Placentation." ScienceDaily. www.sciencedaily.com/releases/2008/03/080318094610.htm (accessed August 29, 2014).

Share This




More Plants & Animals News

Friday, August 29, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Killer Amoeba Found in Louisiana Water System

Killer Amoeba Found in Louisiana Water System

AP (Aug. 28, 2014) State health officials say testing has confirmed the presence of a killer amoeba in a water system serving three St. John the Baptist Parish towns. (Aug. 28) Video provided by AP
Powered by NewsLook.com
Raw: Australian Sheep Gets Long Overdue Haircut

Raw: Australian Sheep Gets Long Overdue Haircut

AP (Aug. 28, 2014) Hoping to break the record for world's wooliest, Shaun the sheep came up 10 pounds shy with his fleece weighing over 50 pounds after being shorn for the first time in years. (Aug. 28) Video provided by AP
Powered by NewsLook.com
Minds Blown: Scientists Develop Fish That Walk On Land

Minds Blown: Scientists Develop Fish That Walk On Land

Newsy (Aug. 28, 2014) Canadian scientists looking into the very first land animals took a fish out of water and forced it to walk. Video provided by Newsy
Powered by NewsLook.com
Huge Ancient Wine Cellar Found In Israel

Huge Ancient Wine Cellar Found In Israel

Newsy (Aug. 28, 2014) An international team uncovered a large ancient wine celler that likely belonged to a Cannonite ruler. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins