De Novo Genome Assembly of the Electric Fish Brachyhypopomus occidentalis (Hypopomidae, Gymnotiformes).

The bluntnose knifefish Brachyhypopomus occidentalis is a primary freshwater fish from north-western South America and Lower Central America. Like other Gymnotiformes, it has an electric organ that generates electric discharges used for both communication and electrolocation. We assembled a high-quality reference genome sequence of B. occidentalis by combining Oxford Nanopore and 10X Genomics linked-reads technologies. We also describe its demographic history in the context of the rise of the Isthmus of Panama. The size of the assembled genome is 540.3 Mb with an N50 scaffold length of 5.4 Mb, which includes 93.8% complete, 0.7% fragmented, and 5.5% of missing vertebrate/Actinoterigie Benchmarking Universal Single-Copy Orthologs. Repetitive elements account for 11.04% of the genome, and 34,347 protein-coding genes were predicted, of which 23,935 have been functionally annotated. Demographic analysis suggests a rapid effective population expansion between 3 and 5 Myr, corresponding to the final closure of the Isthmus of Panama (2.8-3.5 Myr). This event was followed by a sudden and constant population decline during the last 1 Myr, likely associated with strong shifts in both precipitation and sea level during the Pleistocene glacial-interglacial cycles. The de novo genome assembly of B. occidentalis will provide novel insights into the molecular basis of both electric signal productions and detection and will be fundamental for understanding the processes that have shaped the diversity of Neotropical freshwater environments.