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Formation of spermatogonia and fertile oocytes in golden hamsters requires piRNAs

In a new report in Nature Cell Biology, the laboratory of Petr Svoboda from the Institute of Molecular Genetics (IMG) of the Czech Academy of Sciences, in collaboration with the laboratory of Atsuo Ogura from the RIKEN institute, Japan shows how genome protection in mammalian gametes works.

The stability of the genome, i.e., the complete genetic information of an organism, is threatened by an internal enemy known as transposable elements. These are parasitic DNA sequences that can copy themselves and insert into other places in the host’s genome. New insertions occurring in germ cells and early embryos are then passed on to future generations. Transposable elements thus fundamentally affect the stability and evolution of the genome. For example, they cause growth of the genome size – in humans or mice, the recognizable remains of transposable elements make up about half of their DNA. Sometimes, transposable elements can contribute to evolution in a good way, but more often they damage the genetic information at their insertion site. Therefore, it is not surprising that defense mechanisms against transposable elements. The key player targeting transposable elements in the germline of animals are the so-called piRNAs, short RNA molecules that can recognize active transposable elements and silence them. Based on studies of mouse mutants, mammalian piRNAs were considered essential for the protection of male germ cells but irrelevant in female germ. In contrast, invertebrate and fish piRNAs are also important for normal female fertility. Petr Svoboda’s laboratory now reports that in golden hamsters, piRNAs are indispensable for fertility of both sexes. This fundamentally changes the notion of functioning of the piRNA pathway in mammals and, together with other results, indicates that in this case, the mouse is not a representative model for studying mammalian biology.