Genes that are grouped together on chromosomes in other animals were dispersed in the octopus genome, likely as a result of transposon activity. With large, highly-developed brains, cephalopods are the most intelligent invertebrate and have demonstrated elaborate problem-solving and learning behaviors.
And looking at their genes is starting to reveal some octopus secrets.
The first whole cephalopod genome sequence shows a striking level of complexity with 33,000 protein-coding genes identified, more than in a human.
“What is similar among all cephalopods is probably important to being a cephalopod”, said Dr Brenner, now the founding president of the Okinawa Institute of Science and Technology Graduate University in Japan. These are sequences of DNA that have the ability to “jump” from one genome location to another. However, Ragsdale and his colleagues found no evidence of duplications.
“The octopus nervous system is organized in a totally different way from ours: The central brain surrounds the esophagus, which is typical of invertebrates, but it also has groups of neurons in the arms that can work relatively autonomously, plus huge optic lobes involved in vision”, said Daniel Rokhsar, who co-led the project along with Clifton Ragsdale, of the University of Chicago.
The researchers found the expansion of another gene family called the zinc-finger transcription factors, hundreds of which were unique to the octopus, and were concentrated in its suckers, neural tissues and skin. The other family of genes makes protocadherins, which regulate neuronal development in mammals. It was previously thought that only vertebrates possessed numerous and diverse protocadherins; they are crucial in setting up the wiring of the mammalian nervous system. While limpets and oysters have around 20 protocadherin genes, the researchers found that the octopus has 168.
A unique feature of the octopus genome appears to be widespread genomic rearrangements. In most species, cohorts of certain genes tend to be close together on the double-helix DNA molecule. Of the octopus’s half a billion neurons – six times the number in a mouse – two-thirds spill out from its head through its arms, without the involvement of long-range fibres such as those in vertebrate spinal cords. Here, the arrangement of organs and appendages along the body’s long axis is controlled by clusters of genes called Hox. Different genes are placed in different genomic environments, which mean different regulatory elements affect the genes. Transposons replicate and move around with a life of their own, disrupting or enhancing gene expression and facilitating reshufflings of gene order.
“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 co-lead author Caroline Albertin of the University of Chicago in a statement. “It serves as an example of how to apply modern molecular, cellular and genomic techniques to advance our understanding of the nervous system in general, and will enable others to adapt that knowledge to other aspects of brain-related research”. “One thing that stuck out is that we found hundreds of genes found only in cephalopods”. The suckers, for example, express a set of genes that resemble receptors for the neurotransmitter acetylcholine. But when the octopus stretches out, those same pigments elongate into light-reflecting shields. The findings back up the idea that the octopus and squid lineages broke away from each other about 270 million years ago.
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. Efforts to sequence the genomes of other cephalopods are now underway through the Cephalopod Sequencing Consortium.
Profiling more cephalopod genomes and doing comparative studies will be the key to figuring out which traits are most important when it comes to biological invisibility cloaks. “It is an incredible resource that opens up new questions that could not have been asked before about these remarkable animals”.
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