A transgenic wnt8a:PAC reporter reveals biphasic regulation of vertebrate mesoderm development

Vertebrate wnt8a links anteroposterior and dorsoventral axis patterning, but the regulation of wnt8a expression and its relationship to mesoderm induction and maintenance pathways is unclear. To address this, we have generated zebrafish transgenic for a modified genomic PAC clone that expresses EGFP from the wnt8a locus. The EGFP reporter transgene is expressed in a pattern nearly identical to wnt8a, including maternal deposition, expression in the ventrolateral mesoderm and in the yolk syncytial layer. Loss of function studies show that wnt8a expression is under biphasic control by Nodal and No Tail/Brachyury, whereby early phase expression is Nodal-dependent but late phase expression is Ntl/Bra dependent. EGFP fluorescence persists in cells that transcribe the reporter, thus comprising a tracer for ventrolaterally derived mesodermal lineages. We use this property to show that wnt8a expression marks Nodal-independent tail mesoderm formation and that Ntl/Bra predominantly regulates wnt8a in paraxial mesoderm progenitors.