Horizontal gene transfer in schistosomes: A critical assessment

Horizontal gene transfer (HGT), the movement of genetic material between distinct evolutionary lineages, has long been known as a principal force of diversification and adaptation of prokaryotes. More recently, genomic and transcriptomic datasets have suggested gene transfers among various eukaryotic taxa (e.g., Porifera, Cnidaria, Nematoda, Arthropoda, Rotifera, Craniata, and Plantae). Although the exact mechanism of eukaryotic HGT is often unknown, host-parasite interactions may provide ample opportunities for HGT. Schistosomes are trematode blood parasites with complex life cycles that have been repeatedly implicated in HGT. We employed molecular, bioinformatic and phylogenetic approaches to critically analyze 13 published reports of direct HGTs between schistosomes and their hosts to better understand host-parasite co-evolution. Our research suggests that reported cases of schistosome-associated HGT may be due to technical artifacts as opposed to biological reality as we were unable to substantiate them. HGT clearly occurs in eukaryotic organisms, but the burden of proof is high and we emphasize the importance of multiple lines of evidence to conclusively document HGT.