Placental mammals nurture their embryos within their uteri. This gives the offspring extra protection compared to animals that leave their eggs in nests. But how did this reproductive strategy arise in the first place? According to Yale University researchers led by Vincent Lynch, we can thank transposable elements for the ability to sustain a pregnancy.
A transposon is a snippet of DNA that can insert itself into the longer DNA sequence that makes up an organism’s genome. Once inserted, the cell’s DNA replication machinery can’t distinguish the transposon from any other part of the genome. Consequently, it gets copied and passed on to all subsequent daughter cells. Often the insertion is completely benign, but once in a while it either disrupts something important, or, as in this case, adds something important.
The researchers found that a group of transposable elements called MER20 were responsible for controlling the gene expression of more than an eighth of the genes involved in endometrial expression in placental mammals. In other words, once these transposons inserted into the DNA of some distant mammalian ancestor over 100 million years ago, they went about reorganizing the function of the endometrial cells so that the creatures could now maintain a prolonged pregnancy. Mammals without the coordinating efforts of MER20 can remain pregnant for only a fraction of the time their embryos require to fully develop. We know these animals as ‘marsupials’.
So, if you’re in the mood to thank your mother for giving you life, you can also thank her MER20 transposons for not requiring you to hatch out of an egg or a pouch.