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 have elucidated a major biological role of the nematode
mir-35 family of maternally-contributed, essential microRNAs. We show that this microRNA family regulates the sex determination pathway at multiple levels, acting both upstream and downstream of
her-1 to prevent aberrantly activated male developmental programs in hermaphrodite embryos. The predicted target genes that act downstream of the
mir-35 family in this process,
sup-26 and
nhl-2, both encode RNA binding proteins, thus delineating a previously unknown post-transcriptional regulatory subnetwork within the well-studied sex determination pathway of C. elegans. Repression of
nhl-2 by the
mir-35 family is not only required for proper sex determination but also for viability, showing that a single microRNA target site can be essential. Since sex determination in C. elegans requires zygotic gene expression to read the sex chromosome karyotype, early embryos must remain gender-nave; our findings show that the
mir-35 family microRNAs act in the early embryo to function as a developmental timer that preserves navet and prevents premature deleterious developmental decisions. The
mir-35 family of microRNAs is essential for development. The
mir-35-41(nDf50) allele deleted 7 of 8
mir-35 family members, and presents a hypomorphic phenotype in which embryonic lethality is temperature sensitive. To characterize the molecular phenotype associated with loss of
mir-35 family function, we profiled gene expression in
mir-35-41(nDf50) mutant embryos at both permissive (20) and restrictive (25) temperatures. (Refer to A microRNA family exerts maternal control on sex determination in C. elegans)