The first zygotic genes activated after maternal gradients are:
Maternal morphogen gradients directly regulate first class of zygotic genes called gap genes which establish broad regional subdivisions along anterior-posterior axis. Gap genes including hunchback, Krüppel, giant, knirps encode transcription factors with overlapping expression domains set by threshold responses to Bicoid, Caudal and Terminal signals. Their mutual repression refines boundaries and they activate pair-rule genes such as even-skipped, fushi-tarazu which divide embryo into parasegments. Segment polarity genes like engrailed act later. Thus gap genes represent initial zygotic inter
Ref: Alberts et al., Molecular Biology of the Cell, Chapter 22: Gap genes as first zygotic genes.
