In all fishes and aquatic-stage amphibians, lines of mechanosensory lateral line organs ('neuromasts' containing hair cells), distributed over the head and trunk, detect local water movement. Many species also have electrosensory lateral line organs that detect weak electric fields, such as those surrounding other animals. In electroreceptive non-teleost jawed fishes and amphibians, electroreceptor cells reside within ‘ampullary organs’ (AOs) distributed in fields flanking some or all neuromast lines on the head. Both AOs and neuromasts develop from lateral line placodes that elongate over the head to form sensory ridges. Their shared origin makes the system a useful model to investigate cell fate decisions during development. Electroreception was lost in the lineages leading to teleost ray-finned fishes and frogs, so the electrosensory (AO) vs. mechanosensory (neuromast) fate-choice cannot be studied in the standard anamniote models, zebrafish and Xenopus. To identify genes involved in AO vs. neuromast development, the lab previously used differential RNAseq in late-larval stages of a chondrostean ray-finned fish, the Mississippi paddlefish (Polyodon spathula), to generate a dataset of around 500 genes putatively enriched in lateral line organs. Candidate gene expression analysis suggested electroreceptors and hair cells are closely related cell types.

To gain insight into signalling pathways potentially involved in AO and/or neuromast development, I cloned cDNA fragments of 50 signalling pathway-related genes from the paddlefish gene-set and 26 other signalling pathway-related genes selected via a candidate approach, in an experimentally tractable chondrostean, the sterlet (Acipenser ruthenus, a sturgeon). Overall, I cloned cDNA fragments of 33 genes encoding receptors, 27 encoding ligands and 16 encoding, for example, secreted inhibitors, scaffold proteins and co-receptors. In situ hybridisation identified 32 genes expressed in the developing lateral line system. Of these, five (Dner, Fgf10, Ngfr, Cbln18 and Kit) were expressed within developing neuromasts but not AOs, and four (Dll4, Axin2, Omg and Gpr52) within developing AOs but not neuromasts. (Other genes with specific expression patterns were expressed in non-lateral line structures such as gill filaments and taste buds.)

I explored the Bmp signalling pathway in the most depth. I identified lateral line organ expression of genes encoding two ligands (Bmp5, Bmp4), an activin type II receptor (Acvr2a) that can complex with type I Bmp receptors, and two secreted dual-Bmp/Wnt inhibitors (Sostdc1, Apcdd1). CRISPR/Cas9-mediated mutagenesis targeting Bmp5 in F0-injected sterlet embryos led to fewer AOs forming, primarily in the posterior preopercular fields. Conversely, DMH1-mediated inhibition of Bmp signalling for 20 hours when AOs normally start to develop, led to precocious AO formation generally, plus ectopic AOs in the three dorsal-most fields. This suggested that Bmp signalling inhibits, while Bmp5 promotes, AO development.