# Placenta-Driven Evolution: Viral Gene Acquisition and PEG10’s Essential Roles in Eutherian Placenta

## TL;DR

The placenta's evolution in mammals was driven by the integration of viral genes, which played key roles in the development of viviparity and the diversification of placental types.

## Contribution

The paper introduces the concept of 'placenta-driven evolution,' linking viral gene acquisition to major evolutionary transitions in placental development.

## Key findings

- Viral-derived genes like PEG10 and PEG11/RTL1 were essential in the evolution of therian viviparity and placental differentiation.
- Extraembryonic tissues like the placenta and yolk sac allowed for the functional domestication of virus-derived genes.
- The placenta may have acted as a driver of eutherian evolution by enabling the co-option of additional viral genes.

## Abstract

Mammalian placentation represents one of the most striking evolutionary innovations among vertebrates, and accumulating evidence indicates that virus-derived genes—particularly the metavirus-derived PEG10 and PEG11/RTL1—have played indispensable but distinct roles: PEG10 in the emergence of therian viviparity and PEG11/RTL1 in the subsequent differentiation between marsupial and eutherian placental types. Notably, the metavirus-derived SIRH/RTL gene group, which includes PEG10 and PEG11/RTL1, exhibits unique and diverse functions not only in placenta development but also within microglia of the brain. Because microglia originate from yolk sac progenitors, these findings suggest that extraembryonic tissues such as the placenta and yolk sac provided permissive environments that enabled the retention, expression and functional domestication of virus-derived sequences. Once the placenta itself was established through viral gene integration, it may in turn have acted as a powerful driver of eutherian evolution through recurrent acquisition and co-option of additional virus-derived genes—a process we refer to as “placenta-driven evolution.” This perspective offers a unified framework in which viral gene acquisition is viewed as a key driver of genomic innovation, tightly intertwined with the emergence of viviparity, subsequent divergence at the marsupial–eutherian split, and continued diversification of placental structure and function across eutherian lineages.