Genes from Extinct Ancestors of Mammals from Elephants to Humans Shows Evolution of Pregnancy Linked to a Single Protein

In a discovery the Galaxy team finds as fascinating as discovery life on Mars, researchers at Yale led by Gunter Wagner, the Alison Richard Professor of Ecology & Evolutionary Biology found that a crucial regulatory link in the evolution of pregnancy involved the altered function of protein, HoxA-11. The finding was made by analyzing DNA from many species of mammals, including resurrecting genes from the extinct ancestors of mammals.

The specific change they found in HoxA-11 is present in all known placental mammals — from elephants, the most primitive lineage with a placenta, to humans — but does not exist in marsupials, like opossums or wallabies, where there is a brief and rudimentary pregnancy followed by development of the offspring outside the mother, or in egg-laying mammals like the platypus.

Pregnancy is a biologically unusual situation where one organism lives and develops inside another that is genetically different. Ordinarily, the immune system identifies and destroys the dissimilar tissue as if it were a parasite. But in some early mammals, changes 'turned down' the immune system, allowing the developing embryo to grow and thrive unchallenged by the maternal immune response.

With the evolution of the uterus and placenta, it became possible for mammals to protect their growing young and to ensure they were not exposed to an unpredictable environment, like their egg-laying relatives. This study identified one of the genetic switches that tempered the immune system and allowed formation of the placenta and internal development of young.

The textbook story is that regulatory proteins, like HoxA-11, are ancient, universal and unchanging tools, and that new functions arise by using an existing tool from the gene regulatory 'toolbox' in a new or different place.

According to Yale graduate student Vincent Lynch, lead author of the study, "We are writing a different chapter. In this case the function of a major regulatory tool was altered — it is like we found a redesigned hammer."

Posted by Jason McManus.