Small RNA pathways in mammalian oocytes

RNA silencing pathways use small RNAs to guide sequence-specific repression of endogenous genes, mobile elements, and viruses. It has been 25 years since RNA interference (RNAi) was found in mouse oocytes and became the first identified small RNA pathway in mammals. Today, three distinct small RNA pathways are known to operate in mammalian oocytes-in addition to RNAi, there is the microRNA (miRNA) pathway and the PIWI-associated RNA (piRNA) pathway. These pathways differ mechanistically and functionally and their co-existence in the female germline evolved into different arrangements. This review aims to provide a basic introduction to mammalian RNA silencing pathways with a focus on mouse oocytes and key aspects of the pathways, which influence their biological roles in oocytes and zygotes, across mammals.