Lost and found: beer belly is a novel, ancient regulator of animal development

Single-cell RNA sequencing (scRNA-seq) can provide new insights into the regulation of developmental processes by revealing unknown or overlooked genes. We identified a novel gene called beer belly ( bebe ) as a marker of a segment addition zone (SAZ) associated cell cluster in a single-cell atlas of the spider Parasteatoda tepidariorum . Here we show that bebe is an ancient toolkit gene encoding a previously uncharacterised protein predicted to be secreted, which likely plays an important role in the development of most animal phyla. However, we could not identify a bebe ortholog in any mammal or holometabolous insect, suggesting it has been lost in these lineages and explaining why it has been overlooked to date. Knock-down of bebe expression in P. tepidariorum demonstrates that it regulates SAZ size and consequently segment size by interacting with Wnt8 . Our findings also suggest that bebe may regulate segmentation in other invertebrates and play additional developmental roles in bilaterian and non-bilaterian animals, likely ancestrally, through modulation of Wnt signalling. This study emphasises the promise of applying single-cell sequencing across a diverse range of species to better understand the regulation of development beyond candidate genes from traditional models, allowing us to better characterise the animal developmental toolkit and its evolution.