Dramatic differences between the octopus and other invertebrates have been discovered by scientists. The first whole genome analysis of an octopus also shows significant genomic rearrangements, as well as a striking expansion of a family of genes involved in neuronal development once thought to be unique to vertebrates.
As reported in Nature, the team uncovered genomic features that likely played a role in the evolution of traits such as large complex nervous systems and adaptive camouflage, including hundreds of octopus-specific genes. Many have highly expressed in structures such as the brain, skin, and suckers.
Co-senior author Clifton Ragsdale, associate professor in the departments of neurobiology and organismal biology and anatomy at the University of Chicago, says:
“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.
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.”
Along with squids, cuttlefish, and nautiluses, octopuses are cephalopods, a class of predatory mollusks with an evolutionary history spanning more than 500 million years. They go back to long before plants moved onto land.
Cephalopods evolved unique adaptations, like prehensile arms lined with chemosensory suckers, the ability to regenerate complex limbs, vertebrate-like eyes, and a sophisticated camouflage system. With their large, highly-developed brains, cephalopods are the most intelligent invertebrate and have demonstrated elaborate problem-solving and learning behaviors.
To investigate the genetics of these specialized traits, Ragsdale and his colleagues sequenced the genome of the California two-spot octopus (Octopus bimaculoides) to a high level of coverage. On average, each base pair was sequenced 60 times.
The O. bimaculoides genome, the team estimates, is 2.7 billion base-pairs in size, with numerous long stretches of repeated sequences. They 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.
(Credit: Michael LaBarbera)
The large size of the octopus genome was initially attributed to whole genome duplication events during evolution, which can lead to increased genomic diversity and complexity. This phenomenon has occurred twice in ancestral vertebrates, for example. However, Ragsdale and his colleagues found no evidence of duplications.
Rather, the evolution of the octopus genome was likely driven by the expansion of a few specific gene families, widespread genome shuffling, and the appearance of novel genes.
Why Can Suckers Taste?
The researchers uncovered evidence of widespread RNA editing, which allows the octopus to alter protein sequences without changing underlying DNA code.
Many edited proteins are found in neural tissues, and these proteins are thought to act as a switch to regulate functions such as neural activity.
Hundreds of octopus-specific genes were identified, large numbers of which were found in the nervous system, retina, and suckers. The team notes several specific gene families of interest.
The suckers, for example, express a set of genes that resemble receptors for the neurotransmitter acetylcholine. However, the proteins these genes code for lack the ability to bind acetylcholine, and are suspected to function as chemosensory receptors involved in the octopus’s ability to taste with their suckers.
“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 says. “It is an incredible resource that opens up new questions that could not have been asked before about these remarkable animals.”