A direct role for Wnt8 in ventrolateral mesoderm patterning.

Vertebrate dorsoventral patterning requires both Wnt8 and BMP signaling. Because of their multiple interactions, discerning roles attributable specifically to Wnt8 independent of BMP has been a challenge. For example, Wnt8 represses the dorsal organizer that negatively regulates ventral BMP signals, thus Wnt8 loss-of-function phenotypes may reflect the combined effects of reduced Wnt8 and BMP signaling. We have taken a loss-of-function approach in the zebrafish to generate embryos lacking expression of both Wnt8 and the BMP antagonist Chordin. wnt8;chordin loss-of-function embryos show rescued BMP signaling, thereby allowing us to identify Wnt8-specific requirements. Our analysis shows that Wnt8 is uniquely required to repress prechordal plate specification but not notochord, and that Wnt8 signaling is not essential for specification of tailbud progenitors but is required for normal expansion of posterior mesoderm cell populations. Thus, Wnt8 and BMP signaling have independent roles during vertebrate ventrolateral mesoderm development that can be identified through loss-of-function analysis.