Chromosome-level genome assembly of the primitive loach goby, Rhyacichthys aspro , reveals mechanisms underlying Gobioidei diversification

The percomorph fish clade Gobioidei are a suborder that comprises over 2,200 species distributed in nearly all aquatic habitats. To understand the genetics underlying their diversification, we sequenced and annotated the genome of the loach goby, Rhyacichthys aspro , the basal most group, and compared it with nine additional Gobioidei species. Within Gobioidei, the loach goby possesses the smallest genome at 607 Mb, and a rise in species diversity from basal to derived lineages is mirrored by enlarged genomes and a higher presence of repeat elements (REs), particularly DNA transposons. These transposons are enriched in coding and regulatory regions and their copy number increase is strongly correlated with mutation rate, suggesting that DNA repair after transposon excision/insertion leads to nearby mutations. Consequently, the proliferation of DNA transposons might be the crucial driver of Gobioidei diversification and adaptability. The loach goby genome also points to mechanisms of ecological adaptation. It contains relatively few genes for lateral line development but an over representation of synaptic function genes, with genes putatively under selection linked to synapse organization and calcium signaling, suggesting a sensory system distinct from other Gobioidei species. We also see an overabundance of genes involved in neurocranium development and renal function, adaptations likely connected to its flat morphology suited for strong currents and an amphidromous life cycle. Comparative analyses with hill-stream loaches and the European eel reveal convergent adaptations in body shape and saltwater balance. These findings shed light on the loach goby’s survival mechanisms and the broader evolutionary trends within Gobioidei.