Two opposing roles for Bmp signalling in the development of electrosensory lateral line organs

The lateral line system enables all fishes and aquatic-stage amphibians to detect local water movement via mechanosensory hair cells in neuromasts, and many species to detect weak electric fields via electroreceptors (modified hair cells) in ampullary organs. Both neuromasts and ampullary organs develop from lateral line placodes. However, the molecular mechanisms underpinning ampullary organ formation are understudied relative to neuromasts, as the ancestral lineages of zebrafish (teleosts) and Xenopus (frogs) independently lost electroreception. We identified Bmp5 as a promising candidate via differential RNA-seq in an electroreceptive ray-finned fish, the Mississippi paddlefish ( Polyodon spathula ; Modrell et al., 2017, eLife 6: e24197). In an experimentally tractable relative, the sterlet sturgeon ( Acipenser ruthenus ), we found that Bmp5 and four other Bmp pathway genes are expressed in the developing lateral line, and that Bmp signalling is active. Furthermore, CRISPR/Cas9-mediated mutagenesis targeting Bmp5 in G0-injected sterlet embryos resulted in fewer ampullary organs. Conversely, when Bmp signalling was inhibited by DMH1 treatment shortly before the formation of ampullary organ primordia, supernumerary ampullary organs developed. These data suggest that Bmp5 promotes ampullary organ development, whereas Bmp signalling via another ligand(s) prevents their overproduction. Taken together, this demonstrates two opposing roles for Bmp signalling during ampullary organ formation. ### Competing Interest Statement The authors have declared no competing interest. The publication and associated supplementary figures include representative example images of embryos/larvae from each experiment. Additional data underlying this publication consist of further images of these and other embryos/larvae from each experiment. Public sharing of these images is not cost-efficient, but they are available from the corresponding author upon reasonable request. Previously published sterlet transcriptome assemblies (from pooled stage 40-45 sterlet heads; [Minařík et al., 2024a][1]) are available at DDBJ/EMBL/GenBank under the accessions GKLU00000000 () and GKEF01000000 (). Previously published paddlefish RNA-seq data (from pooled paddlefish opercula and fin tissue at stage 46; [Modrell et al., 2017a][2]) are available via the NCBI Gene Expression Omnibus (GEO) database () under accession code GSE92470. [1]: #ref-50 [2]: #ref-54