Zinc finger homeobox is required for the differentiation of serotonergic neurons in the sea urchin embryo

► Serotonergic neurons differentiate in sea urchin anterior neuroectoderm. ► zinc-finger homeobox, tryptophan 5-hydroxylase (tph) and serotonin are co-expressed. ► Zinc-finger homeobox (Zfhx1/Z81) is required for tph and serotonin. ► zfhx1/z81 depends on FoxQ2, which is required for forming anterior neuroectoderm. ► Zfhx1/Z81 is an important intermediate regulator in serotonergic neurogenesis.