Recurrent acquisition of alien ribotoxin encoding genes by insect genomes

Ribosome inactivating proteins (RIPs) are RNA N-glycosidases that depurinate a specific adenine residue in the conserved sarcin/ricin loop of 28S rRNA. These enzymes are widely distributed among plants and bacteria. Recently, we have described RIP genes in mosquitoes belonging to the Culicinae subfamily (Aedini and Culicini tribes). We have also shown that these genes are derived from a single event of horizontal gene transfer (HGT) from a prokaryotic donor. In the present work, we show the existence of two RIP encoding genes in the genome of the whitefly Bemisia tabaci (a hemiptera species belonging to the Aleyrodidae family, distantly related to mosquitoes). Contamination artifacts were ruled out analyzing three independent Bemisia tabaci genome databases. In contrast to mosquitoes RIPs, the whitefly genes harbor introns and, according to transcriptomic evidence, are transcribed and spliced. Interestingly, phylogenetic inference combined with taxonomic distribution strongly supports that whitefly RIP genes are derived from an independent HGT event from a plant source. Our results suggest that RIP genes fill a functional niche in insects.