Octopus genetic code reveals 'alien creature'
A new map of the octopus genetic code has revealed a creature so strange that it could be an "alien", say scientists.
The first whole cephalopod genome sequence shows a striking level of complexity with 33,000 protein-coding genes identified, more than in a human.
Octopus DNA is highly rearranged - like cards shuffled and reshuffled in a pack - containing numerous so-called "jumping genes" that can leap around the genome.
US researcher Dr Clifton Ragsdale, from the 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 abilities.
"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 scientists who sequenced the genome of the California two-spot octopus report their findings in the journal Nature.
They discovered unique genetic traits that are likely to have played a key role in the evolution of characteristics such as the complex nervous system and adaptive camouflage.
Analysis of 12 different tissues revealed hundreds of octopus-specific genes found in no other animal, many of them highly active in structures such as the brain, skin and suckers.
Cephalopods, which include the octopus, squid, cuttlefish and nautilus, are a family of predatory molluscs with an evolutionary history stretching back more than 500 million years to a time long before plants colonised the land.
They inhabit every ocean at almost all depths and possess a range of features that call to mind sci-fi aliens. These include prehensile sucker-lined tentacles, highly mobile, camera-like eyes sensitive to polarised light, sophisticated camouflage systems that alter skin colour and patterns, jet-propulsion, three hearts, and the ability to regenerate severed limbs.
The scientists estimate that the two-spot octopus genome contains 2.7 billion base pairs - the chemical units of DNA - with long stretches of repeated sequences.
Although the genome is slightly smaller than a human's it is packed with more genes.
Reshuffling was a key characteristic of the creature's genetic make-up. In most species, cohorts of certain genes tend to be close together on the double-helix DNA molecule. A gene is a region of DNA that contains the coded instructions for making a protein.
In the octopus, there are no such groupings of genes with related functions. For instance, Hox genes - which control body plan development - cluster together in almost all animals but are scattered throughout the octopus genome.
It was as if the octopus genome had been "put into a blender and mixed", said co-author Caroline Albertin, also from the University of Chicago.
The role of the many transposons, or "jumping genes", remains unclear. These are sequences of DNA that have the ability to "jump" from one genome location to another.
Transposons are known to affect the regulation of gene activity and help shape genome structure. In the octopus, transposon activity was especially high in the nervous system.
Evidence of extensive RNA editing - allowing protein structure to be changed without altering the underlying DNA code - was also seen in the octopus.
Dr 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."