A microRNA family exerts maternal control on sex determination in .

Gene expression in early animal embryogenesis is in large part controlled post-transcriptionally. Maternally contributed microRNAs may therefore play important roles in early development. We elucidated a major biological role of the nematode family of maternally contributed essential microRNAs. We show that this microRNA family regulates the sex determination pathway at multiple levels, acting both upstream of and downstream from to prevent aberrantly activated male developmental programs in hermaphrodite embryos. Both of the predicted target genes that act downstream from the family in this process, () and (NCL-1, HT2A, and LIN-41 repeat) domain-containing-2 (), encode RNA-binding proteins, thus delineating a previously unknown post-transcriptional regulatory subnetwork within the well-studied sex determination pathway of Repression of by the family is required for not only proper sex determination but also viability, showing that a single microRNA target site can be essential. Since sex determination in requires zygotic gene expression to read the sex chromosome karyotype, early embryos must remain gender-naïve; our findings show that the family microRNAs act in the early embryo to function as a developmental timer that preserves naïveté and prevents premature deleterious developmental decisions.