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The PIWI-interacting RNA (piRNA) pathway is essential for germline development and transposable element repression. Key elements of this pathway are members of the piRNA-binding PIWI/Argonaute protein family and associated factors (e.g., VASA, MAELSTROM, and TUDOR domain proteins). PIWI-interacting RNAs have been identified in mouse testis and oocytes, but information about the expression of the different piRNA pathway genes, in particular in the mammalian ovary, remains incomplete. We investigated the evolution and expression of piRNA pathway genes in gonads of amniote species (chicken, platypus, and mouse). Database searches confirm a high level of conservation and revealed lineage-specific gain and loss of Piwi genes in vertebrates. Expression analysis in mammals shows that orthologs of Piwi-like (Piwil) genes, Mael (Maelstrom), Mvh (mouse vasa homolog), and Tdrd1 (Tudor …