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... Click here for more information.

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"