Octopuses, along with squids, cuttlefish and nautiluses, are cephalopods, a class of predatory molluscs with an evolutionary history spanning more than 500 million years, even long before plants moved onto land. With large, highly-developed brains, cephalopods are the most intelligent invertebrates and have demonstrated elaborate problem-solving and learning behaviors. An international team of researchers have sequenced the whole genome of an octopus, revealing unique genomic features that likely played a role in the evolution of traits such as large complex nervous systems and adaptive camouflage. They sequenced the genome of the California two-spot octopus, the first cephalopod ever to be fully sequenced, and mapped gene expression profiles in 12 different tissues.
The research was conducted by teams from University of Chicago, University of California, Berkeley and Okinawa Institute of Science and Technology as part of the Cephalopod Sequencing Consortium. Study co-senior author Clifton Ragsdale, associate professor at University of Chicago, said, "The octopus appears to be utterly different from all other animals, even other molluscs, with its eight prehensile arms, its large brain and its clever problem-solving capabilities. The late British zoologist Martin Wells said the octopus is an alien. In this sense, then, our paper describes the first sequenced genome from an alien."
The research team discovered striking differences from other invertebrates, including widespread genomic rearrangements and a dramatic expansion of a family of genes involved in neuronal development that was once thought to be unique to vertebrates. They identified hundreds of octopus-specific genes, with many highly expressed in structures such as the brain, skin and suckers. The authors said, "The results serve as an important foundation for evolutionary studies and deeper investigations into the genetic and molecular mechanisms that underlie cephalopod-specific traits."
The researchers estimates the octopus genome is 2.7 billion base-pairs in size, with numerous long stretches of repeated sequences. They have identified more than 33,000 protein-coding genes, placing the octopus genome at slightly smaller in size, but with more genes, than a human genome. A unique feature of the octopus genome is the widespread genomic rearrangements. The octopus genome is enriched in transposons, also known as 'jumping genes', which can rearrange themselves on the genome.
The research team also found evidence of extensive RNA editing, which allows the octopus to alter protein sequences without changing the underlying DNA code. Ragsdale said, "The octopus genome makes studies of cephalopod traits much more tractable, and now represents an important point on the tree of life for comparative evolutionary studies. It is an incredible resource that opens up new questions that could not have been asked before about these remarkable animals."
The study is published in Nature.