The first whole genome analysis of an octopus has revealed unique genomic features that are believed to have driven the evolution of some of the animal's most fascinating traits including its complex nervous system and ability to camouflage.
A team of researchers sequenced the genome of the California two-spot octopus and mapped gene expression profiles in 12 different tissues, the University of Chicago Medical Center reported. In their analysis, the scientists observed dramatic differences between the octopus and other invertebrates, including widespread genomic rearrangements and an expansion of a family of genes involved with neural development.
"The octopus appears to be utterly different from all other animals, even other mollusks, with its eight prehensile arms, its large brain and its clever problem-solving capabilities," said co-senior author Clifton Ragsdale, associate professor in the Departments of Neurobiology and Organismal Biology and Anatomy at the University of Chicago. "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."
Octopuses belong to a class of predatory mollusks called cephalopods, and these creatures possess unique adaptations including chemosensory suckers, the ability to regenerate complex limb, and an incredible camouflage system. Cephalopods are also the most intelligent invertebrates, and display sharp problem-solving and learning behaviors.
The analysis showed the octopus' genome contained more than 33,000 protein-coding genes, meaning it is smaller in size but contains more genes than the human genome. The researchers believe the animal's large genome was created by the "expansion of a few specific gene families, widespread genome shuffling and the appearance of novel genes."
One of the most notable gene expansions was seen in the protocadherins, which is a family of genes involved in neural development and short-range interactions between neurons. The octopus genome contains 168 protocadherin genes, which is about 10 times more than other invertebrates and twice as many as most mammals. The researchers believe that since cephalopod neurons do not contain myelin, protocadherins drive the development of the neural system instead. Other genes that showed significant expansion included those involved in, embryonic and nervous tissue development.
Widespread genomic rearrangements were also a feature identified in the octopus' genome. In most species, cohorts of genes tend to be close together on the chromosome, but this is not the case for octopuses. Octopus Hox genes (responsible for body plan development) were found to be clustered throughout the octopus genome without any observed links. The octopus genome contains transposons, or "jumping genes," which have the ability to rearrange themselves on the genome and are linked to the regulation of gene expression and how the genome is shaped. Scientists are still unsure of the roles they play for octopuses, but noted there is elevated transposon expression in neural tissues.
"With a few notable exceptions, the octopus basically has a normal invertebrate genome that's just been completely rearranged, like it's been put into a blender and mixed," said Caroline Albertin, co-lead author and graduate student in the Department of Organismal Biology and Anatomy at the University of Chicago. "This leads to genes being placed in new genomic environments with different regulatory elements, and was a completely unexpected finding."
Six octopus reflectins (genes associated with light processing and camouflage) were identified, and were found to be relatively different from what is seen in other cephalopods. This suggests a single gene was present in a cephalopod ancestor, and dramatically evolved independently across different species. The findings back up the idea that the octopus and squid lineages broke away from each other about 270 million years ago.
"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," Ragsdale said. "It is an incredible resource that opens up new questions that could not have been asked before about these remarkable animals."
The findings were published in a recent edition of the journal Nature.
