Brachiopod genome unveils the evolution of the BMP-Chordin network in bilaterian body patterning

Bone morphogenetic protein (BMP) signalling is crucial in regulating dorsal-ventral patterning and cell fate determination during early development in bilaterians. Interactions between BMP ligands and their main antagonist, Chordin, establish BMP gradients, subdivide embryos into distinct territories and organise body plans. However, the molecular control and evolutionary origins of dorsal-ventral patterning within spiralians, one of the three major bilaterian groups, have been obscured by their unique embryonic development. Here we present the chromosome-level genome of a spiralian with deuterostome-like development, the brachiopod Lingula anatina, and apply functional transcriptomics to study dorsal-ventral patterning under the control of BMP signalling. We uncover the presence of a dorsal-ventral BMP signalling gradient in the L. anatina gastrula with bmp2/4 and chordin expressed at its dorsal and ventral sides, respectively. Using small-molecule drugs, exogenous recombinant BMP proteins and RNA sequencing, we show that a high level of BMP pathway activation inhibits the expression of neural genes during gastrula and larval stages. We also show that BMP signalling splits the developing larval shell field into two valves. The discovery of a BMP-mediated dorsal-ventral patterning system in a spiralian, similar to those observed in deuterostomes and non-spiralian protostomes, suggests deep conservation of this mechanism across all three major bilaterian clades. This is further supported by striking similarities in the gene sets regulated by BMP signalling in brachiopods and the vertebrate model Xenopus. We argue that the spiralian ancestor retained the basal bilaterian mechanism of dorsal-ventral patterning, although downstream components of the BMP-Chordin network have undergone dynamic evolutionary changes.