| In most organisms, control of the developmental program involves a regulated transition from maternally supplied mRNAs and proteins to newly synthesized zygotically encoded factors. This phenomena, known as the maternal to zygotic transition (MZT), is observed in a wide range of embryos in the animal and plant kingdoms; in chordates, the MZT typically occurs during midblastula stages, and therefore is often referred to as the midblastula transition (MBT). Early development of most organisms is exclusively maternally controlled, and the zygotic genome of the embryo remains transcriptionally silent until after the MBT, when the transition to zygotic control culminates. Recent work in a number of organisms has identified several genes that are activated prior to the MBT, but whether precocious expression of specific mRNAs is important for later development has not been examined in detail. In this work, I characterize the role of a maternal transcription factor in preMBT transcription, and identify a developmentally significant function for the expression of specific transcripts before the MBT. I identify a class of protein coding transcripts activated before the MBT by the maternal T-box factor VegT, all of which are components of an established transcriptional regulatory network required for mesendoderm induction in Xenopus laevis, including the Nodal related ligands xnr5, xnr6, and derriere and the transcription factors bix4, and sox17alpha. Phosphorylated Smad2, a hallmark of active Nodal signaling, is present before the MBT and preMBT transcription of xnr5 and xnr6 is required for this activation. Furthermore, preMBT activation of the Nodal pathway is essential for mesendodermal gene expression and patterning of the embryo. Together, this work demonstrates that regulated transcription before the MBT is important for the establishment of dorsal mesoderm and the development of the embryo. |
