Molecular characterization of the recent intragenomic spread of the murine endogenous retrovirus MuERV-L

The mouse genome has been subjected to two successive amplification bursts of the murine endogenous retrovirus MuERV-L after the Mus/Rattus split. The main objective of this work is to characterize in detail the intragenomic spread giving rise to these two murine bursts using full-length MuERV-L proviruses taken from public databases. Phylogenetic analyses led to the identification of elements putatively amplifying during each one of the two burst. Likelihood-ratio tests were used to confirm that elements supposedly arisen during the first burst have been evolving under lower selective constrains, as expected for older insertions. The data reported here suggested an evolutionary dynamics for MuERV-L amplification characterized by the existence of multiple elements simultaneously active during each one of the bursts while only one or very few closely related proviruses from the first burst gave rise to the second one. Finally, more than one third of the proviruses present 100% identity between the 5' and 3' LTRs, strongly indicating that MuERV-L is currently active within the mouse genome.