Chromosome-level genome assembly of burbot (Lota lota) provides insights
into the evolutionary adaptations in freshwater
Abstract
The burbot (Lota lota) is the only member of the cod family (Gadidae)
that is adapted solely to freshwater. This species shows the widest
longitudinal range of freshwater fish in the world. The burbot is a good
model for studies on adaptive genome evolution from marine to freshwater
environment. However, no high-quality reference genome has been
released. Here, the first chromosome-level genome of the burbot was
constructed using PacBio long sequencing and Hi-C technology. A total of
95.24 Gb polished PacBio sequences were generated, and the preliminary
genome assembly was 575.83 Mb in size with a contig N50 size of 2.15 Mb.
The assembled sequences were anchored to 22 pseudo-chromosomes by using
the Hi-C data. The final assembled genome after Hi-C correction was
575.92 Mb, with a contig N50 of 2.01 Mb and a scaffold N50 of 22.10 Mb.
A total of 22,067 protein-coding genes were predicted, 94.82% of which
were functionally annotated. Phylogenetic analyses indicated that burbot
diverged with the Atlantic cod about 44.4 million years ago. In
addition, 377 putative genes that appear to be under positive selection
in burbot were identified. These positively selected genes might adapt
to the freshwater environment. These genome data provide an invaluable
resource for the ecological and evolutionary study of the order
Gadiformes.