Genome of the ocean sunfish (Mola mola).

The ocean sunfish (Mola mola) which can grow up to a length of 2.7 m and weigh 2.3 tons is the worlds largest bony fish. It exhibits an extremely fast growth rate and possesses an endoskeleton made up largely of cartilage. Another unique feature of the sunfish is the loss of caudal fin which is replaced by a broad and stiff lobe resulting in the characteristic truncated appearance of the fish.
To get an insight into the genomic bases of these derived phenotypes, we sequenced the ocean sunfish genome and performed comparative analysis with other teleost genomes. Several ocean sunfish genes involved in the GH/IGF1 axis signaling pathway were found to be under positive selection or accelerated evolution, which might explain its fast growth rate and large body size. A number of genes associated with the extracellular matrix, some of which are involved in the regulation of bone and cartilage development, have also undergone positive selection or accelerated evolution. The ocean sunfish contains seven Hox clusters similar to fugu but contains more Hox genes than fugu. Thus, it appears that the caudal fin loss in ocean sunfish is not associated with the loss of any specific Hox gene.
Our analyses provide insights into the molecular basis of the fast growth rate and large size of the sunfish. The high-quality genome assembly generated in this study should facilitate further studies of this natural mutant.