title: Analysis of Hydra PIWI proteins and piRNAs uncover early evolutionary origins of the piRNA pathway.
abstract: To preserve genome integrity, an evolutionarily conserved small RNA-based silencing mechanism involving PIWI proteins and PIWI-interacting RNAs (piRNAs) represses potentially deleterious transposons in animals. Although there has been extensive research into PIWI proteins in bilaterians, these proteins remain to be examined in ancient phyla. Here, we investigated the PIWI proteins Hywi and Hyli in the cnidarian Hydra, and found that both PIWI proteins are enriched in multipotent stem cells, germline stem cells, and in the female germline. Hywi and Hyli localize to the nuage, a perinuclear organelle that has been implicated in piRNA-mediated transposon silencing, together with other conserved nuage and piRNA pathway components. Our findings provide the first report of nuage protein localization patterns in a non-bilaterian. Hydra PIWI proteins possess symmetrical dimethylarginines: modified residues that are known to aid in PIWI protein localization to the nuage and proper piRNA loading. piRNA profiling suggests that transposons are the major targets of the piRNA pathway in Hydra. Our data suggest that piRNA biogenesis through the ping-pong amplification cycle occurs in Hydra and that Hywi and Hyli are likely to preferentially bind primary and secondary piRNAs, respectively. Presumptive piRNA clusters are unidirectionally transcribed and primarily give rise to piRNAs that are antisense to transposons. These results indicate that various conserved features of PIWI proteins, the piRNA pathway, and their associations with the nuage were likely established before the evolution of bilaterians.
authors.affiliation: Temasek Life Sciences Laboratory, National University of Singapore, Singapore 117604, Singapore; Department of Biological Sciences, National University of Singapore, Singapore 117543, Singapore.
authors.forename: Robyn S M
authors.initials: RS
authors.lastname: Lim
authors.affiliation: Temasek Life Sciences Laboratory, National University of Singapore, Singapore 117604, Singapore.
authors.forename: Amit
authors.initials: A
authors.lastname: Anand
authors.affiliation: Okazaki Institute for Integrative Bioscience, National Institute for Basic Biology, Okazaki 444-8787, Japan.
authors.forename: Chiemi
authors.initials: C
authors.lastname: Nishimiya-Fujisawa
authors.affiliation: Okazaki Institute for Integrative Bioscience, National Institute for Basic Biology, Okazaki 444-8787, Japan.
authors.forename: Satoru
authors.initials: S
authors.lastname: Kobayashi
authors.affiliation: Temasek Life Sciences Laboratory, National University of Singapore, Singapore 117604, Singapore; Department of Biological Sciences, National University of Singapore, Singapore 117543, Singapore. Electronic address: toshie@tll.org.sg.
authors.forename: Toshie
authors.initials: T
authors.lastname: Kai
mesh_terms: D000818:Animals; D005075:Biological Evolution; D020451:Contig Mapping; D003593:Cytoplasm / Q000378:metabolism; D004251:DNA Transposable Elements; D004330:Drosophila / Q000378:metabolism; D029721:Drosophila Proteins / Q000378:metabolism; D005260:Female; D018507:Gene Expression Regulation, Developmental; D020868:Gene Silencing; D005854:Germ Cells / Q000166:cytology; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006829:Hydra / Q000235:genetics* / Q000502:physiology; D051379:Mice; D016376:Oligonucleotides, Antisense / Q000378:metabolism; D010802:Phylogeny; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism*; D013234:Stem Cells / Q000166:cytology
chemical_list: D004251:DNA Transposable Elements; D029721:Drosophila Proteins; D016376:Oligonucleotides, Antisense; D034741:RNA, Small Interfering; D049452:Green Fluorescent Proteins
keywords: Hydra; Nuage; PIWI protein; pirna