No evidence for extensive horizontal gene transfer in the genome of the tardigrade Hypsibius dujardini
Significance Tardigrades, also known as moss piglets or water bears, are renowned for their ability to withstand extreme environmental challenges. A recently published analysis of the genome of the tardigrade Hypsibius dujardini by Boothby et al. concluded that horizontal acquisition of genes from bacterial and other sources might be key to cryptobiosis in tardigrades. We independently sequenced the genome of H. dujardini and detected a low level of horizontal gene transfer. We show that the extensive horizontal transfer proposed by Boothby et al. was an artifact of a failure to eliminate contaminants from sequence data before assembly. Tardigrades are meiofaunal ecdysozoans that are key to understanding the origins of Arthropoda. Many species of Tardigrada can survive extreme conditions through cryptobiosis. In a recent paper [Boothby TC, et al. (2015) Proc Natl Acad Sci USA 112(52):15976 15981] the authors concluded that the tardigrade Hypsibius dujardini had an unprecedented proportion (17% of genes originating through functional horizontal gene transfer (fHGT) and speculated that fHGT was likely formative in the evolution of cryptobiosis. We independently sequenced the genome of H. dujardini. As expected from whole organism DNA sampling, our raw data contained reads from nontarget genomes. Filtering using metagenomics approaches generated a draft H. dujardini genome assembly of 135 Mb with superior assembly metrics to the previously published assembly. Additional microbial contamination likely remains. We found no support for extensive fHGT. Among 23,021 gene predictions we identified 0.2% strong candidates for fHGT from bacteria and 0.2% strong candidates for fHGT from nonmetazoan eukaryotes. Cross comparison of assemblies showed that the overwhelming majority of HGT candidates in the Boothby et al. genome derived from contaminants. We conclude that fHGT into H. dujardini accounts for at most 1 2% of genes and that the proposal that one sixth of tardigrade genes originate from functional HGT events is an artifact of undetected contamination.
Proceedings of the National Academy of Sciences