from Chance Event to Necessity, Genomic imprinting, Viviparity, Exapted genes

-What Makes a Mammal a Mammal?: An Approach to Mammalian-specific Traits from the Perspective of Genomic Imprinting and Genes exapted from LTR Retrotransposons/Retroviruses-


The consequences of a Fortuitous Encounter between a Common Mammalian Ancestor and LTR Retrotransposons/Retroviruses

We discovered the PEG10 gene as one of the paternally expressed imprinted genes in the course of research on genomic imprinting that is known to take place via a mammalian-specific epigenetic mechanism. PEG10 has some unique features, e.g. it: 1) resembles an LTR retrotransposon or retroviral DNA, 2) is present only in viviparous mammals, such as eutherians and marsupials, 3) plays an essential role as an endogenous gene required for placenta formation. Thus, it seems clear that PEG10 emerged by exaptation of an exogenous DNA acquired in a common ancestor of therian mammals. Moreover, we have accumulated evidence that approximately 10 such exapted SIRH/RTL genes play important roles in various aspects of eutherian development, growth and behavior.
Today, it is quite “natural” that a mammalian mother delivers one or more babies. However, it is highly likely that chance events, such as mutation and gene exaptation from exogenous DNA, underlay the emergence of mammals in the course of evolution, events that would result in viviparous reproduction as well as the highly sophisticated central nervous system (CNS). This was achieved by supplying essential placental as well as CNS genes in a mammalian-specific manner. It may be that only a very small number of external DNAs inserted into the ancestor mammalian genome came to be selected as beneficial genes by an initial series of chance events that now determine the way the current mammalian developmental system works by becoming fundamental parts of the mammalian genome.
We believe this research provides an excellent example of how genes exapted from LTR retrotransposons/retroviruses contributed to the establishment of mammals. In other words, how mammalian evolution was driven by exapted genes follows a story-line of “chance event to necessity.”

Trajectory of our research