An FGF Timer for Zygotic Genome Activation

Zygotic genome activation has been extensively studied in a variety of systems including flies, frogs, and mammals. However, there is comparatively little known about the precise timings of gene induction during the earliest phases of embryogenesis. Here we employ high-resolution in situ detection methods, along with genetic and experimental manipulations, to study the timing of zygotic activation in the simple model chordate, Ciona intestinalis with minute-scale temporal precision. We found that two Prdm1 homologs in Ciona are the earliest genes that respond to FGF signaling. We present evidence for a FGF timing mechanism that is driven by derepression of the ERF repressor by ERK activity, which works in concert with localized activators such as Foxa.a. Absence of ERF results in derepression of target genes throughout the embryo. A highlight of this timer is the sharp transition in FGF responsiveness between the 8- and 16-cell stages of development. We propose that this timer is an innovation of chordates that is also employed by other vertebrates.