Horizontal gene transfer of the Pytheas sequence from Cuscuta to Orobanche via a host-mediated pathway.

Horizontal Gene Transfer (HGT) is a phenomenon of DNA transfer between organisms that does not involve a parent-offspring relationship. HGT is believed to play an important role in all groups of organisms, including plants, and especially in parasites. Here we describe a chain of transfers from Cuscuta (Convolvulaceae) to Orobanche rigens (Orobanchaceae)-both parasitic plants-via a host belonging to tribe Genisteae (Fabaceae). During its "journey" between genomes, the transferred sequence, which we named Pytheas, was altered by deletions, additions of new segments from the current genome, substitutions, and rearrangements. This is the first robustly documented case of a multi-step transfer pathway-involving one IGT and two HGTs-connecting three plant species. Specifically, this system involves a host plant mediated gene flow between two distinct parasites without involvement of transposable elements, the cox1 intron, or other vectors. This case also demonstrates how host-parasite interactions can facilitate the spread of genetic material between evolutionarily distant lineages.