A nematode retrotransposon in the common shrew: horizontal transfer between parasite and host

Reports of horizontal transposon and gene transfers involving metazoan species has increased with the sequencing of their genomes. Horizontal transfer could be facilitated by the intimate relationship between a parasite and its hosts. To date, two studies have identified horizontal transfer of RTEs, a class of retrotransposable elements, involving parasites: ticks might act as vector for BovB between ruminants and squamates, and AviRTE was transferred between birds and parasitic nematodes. We wanted to know if parasitic nematodes are involved in other cases of horizontal transfer of RTEs. We searched 33 mSammalian RTEs in 81 nematode assemblies, and 10 nematode RTEs in 98 mammalian assemblies. We identified RTE1õ Sar from Sorex araneus, the common shrew, in parasitic nematodes and show that it originates from nematodes. To exclude contamination of the S. araneus assembly, we developed an approach that uses long reads and paired-end reads. With phylogenetic analysis and copy age estimation, we show that RTE1_Sar was horizontally transferred from nematodes to S. araneus. We confirm horizontal transfer of RTEs in host-parasite interactions, and we present a new method to distinguish between contamination and horizontal transfer.