FIG 1.

Phylogeny of Euarchontoglires and previously identified Syncytin genes. Euarchontoglires can be grouped into three major orders: Rodentia, Lagomorpha, and Primates (dates are from reference 36). Rodentia can be further subdivided into three major clades: the squirrel-related clade, the mouse-related clade and the guinea-pig-related clade Ctenohystrica (37). Branch length is proportional to time (in million years [My]), and the families where syncytins have been identified to date are indicated with the corresponding gene name.