For the first time, scientists have sequenced the genome of an octopus species, revealing surprising clues about the evolution of the cephalopod’s brain and behaviour.
Researchers from the University of Chicago, the University of California, Berkeley, the University of Heidelberg in Germany and the Okinawa Institute of Science and Technology in Japan assembled and published the first genome of a cephalopod, the group that includes octopuses, squid and cuttlefish. They achieved this using advanced technology that differs from that used for the Human Genome Project, an international research project completed in 2003 to sequence human DNA and map all the genes of the species Homo sapiens.
The researchers chose to sequence the genome of the California two-spot octopus (Octopus bimaculoides), a species found throughout the Pacific Ocean. The octopus genome turned out to be almost as large as a human’s and to contain a greater number of protein-coding genes — some 33,000, compared with fewer than 25,000 in Homo sapiens. The sequencing led to the team discovering that the octopuses possess massive expansions in two gene families previously thought to be uniquely enlarged in vertebrates: the protocadherins, which regulate neuronal development, and the C2H2 superfamily of zinc-finger transcription factors.
The protocadherins gene group, which regulate the development of neurons and the short-range interactions between them, are significantly more abundant in the octopus’ genome. The octopus possesses 168 of these genes, more than twice as many as mammals. This increased amount of protocadherins explains the creature’s unusually large brain and the organ’s peculiar anatomy, where two-thirds of it’s neurons flow from its head through the nerve cords of its arms, without the involvement of long-range fibres such as those in vertebrate spinal cords. The independent computing power of the arms, which can execute cognitive tasks even when dismembered, have made octopuses an object of study for neurobiologists and for roboticists who are collaborating on the development of soft, flexible robots.
The zinc-finger transcription factors, a gene family that is involved in development, is also highly expanded in octopuses. At around 1,800 genes, it is the second-largest gene family to be discovered in an animal, after the elephant’s 2,000 olfactory-receptor genes.
The team also discovered a clue that could be the basis of the octopus’s intelligence. The genome contains systems that can allow tissues to rapidly modify proteins to change their function. Electrophysiologists had predicted that this could explain how octopuses adapt their neural-network properties to enable such extraordinary learning and memory capabilities.
The genome showed a lot of evidence for transposon activity. Transposons are DNA sequences that move locations around the genome and can drive evolution. In comparison to other genomes, the scientists note that the octopus genome looks like it has been “put into a blender and mixed”. They show that these transposons play an important role in driving this mixing of the genome. They also found that transposons are highly expressed in neural tissues. They suggest that these may play an important role in memory and learning as shown in mammals and flies.
On investigation of the octopus’ Hox genes (which usually occur together, clustered in groups, and the order of the genes directly corresponds to the order in which they are activated along the body during development), the scientists found they are completely scattered across the genome, with no two of them occurring together. This scattered nature of the Hox genes provide insights into octopod body plan development and why octopus have a much more unusual body plan than their cousins, such as snails and oysters.
Ultimately, the scientists’ analysis suggests that substantial expansion of a handful of gene families, along with extensive remodelling of genome linkage and repetitive content, has played a critical role in the evolution of cephalopod morphological innovations, including their large and complex nervous systems.
Determining how octopuses’ brains and bodies evolved “represents a first step to understanding these really cool animals at a new level,” said Caroline Albertin, the lead researcher on the study and a graduate student studying evolution of animal development at the University of Chicago. “Having the genome represents having a tool kit that the animal draws on as it builds its really remarkable body and develops all these, really, very cool, behaviors,” Albertin said.
Watch a video explanation of the investigation below: