News: Loss Of Egg Yolk Genes In Mammals And The Origin Of Lactation And Placentation.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Thu, 20 Mar 2008 22:40:37 GMT
Loss Of Egg Yolk Genes In Mammals And The Origin Of Lactation And
Placentation.
http://www.sciencedaily.com/releases/2008/03/080318094610.htm
ScienceDaily (Mar. 19, 2008) ? 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
--
Bob.
.
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