Role of maternal Wnt11b in cortical rotation and axis formation

Asymmetric signalling centres in the early embryo are essential for axis formation in vertebrates. These regions, namely the dorsal morula, yolk syncytial layer, and distal hypoblast/anterior visceral endoderm (in amphibians, teleosts and mammals, respectively), require the localised stabilisation of nuclear Beta-catenin (Ctnnb1), implying that localised Wnt/Beta-catenin signalling activity is critical in their establishment. However, it is becoming increasingly apparent that the stabilisation of Beta-catenin in this context may be initiated independently of secreted Wnt growth factor activity. In Xenopus, dorsal Beta-catenin stabilisation is initiated by a requisite microtubule-mediated symmetry-breaking event in the fertilised egg: “cortical rotation”. Vegetally-localised wnt11b mRNA has been implicated upstream of Beta-catenin in this context, as has the dorsal enrichment of Wnt ligand-independent activators of Beta-catenin, but the extent that each of these processes contribute to axis formation in this paradigm remains unclear. Here we describe a maternal effect mutation in Xenopus laevis wnt11b.L, generated by CRISPR mutagenesis. We demonstrate a maternal requirement for timely and complete gastrulation morphogenesis and a zygotic requirement for proper left-right asymmetry. We also show that a subset of maternal wnt11b mutants have axis and dorsal gene expression defects, and we find that microtubule assembly and cortical rotation are reduced in wnt11b mutant eggs, leading to less organised and directed vegetal microtubule arrays. mRNA sequencing was used to determine the extent of dorsal and other gene dysregulation during gastrulation. Overall design: Bulk mRNA sequencing of gastrula gene expression in maternal-zygotic wnt11b mutants.