Washington | Scientists have for the first time sequenced the genome of the octopus, uncovering several large gene families that may hold the key to how the ‘intelligent’ animal wires up its complex brain. Researchers sequenced the genome of the California two-spot octopus (Octopus bimaculoides), the first cephalopod ever to be fully sequenced.
They found the octopus genome possesses striking differences from other invertebrates, including a dramatic expansion of a gene family involved in neuronal development that was once thought to be unique to vertebrates. 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 capabilities, said Clifton Ragsdale, an associate professor at the University of Chicago and co-senior author on the study published in the journal Nature.
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, Ragsdale said. The team estimates the O bimaculoides genome is 2.7 billion base-pairs in size, with numerous long stretches of repeated sequences. Researchers 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.
The large size of the octopus genome was initially attributed to whole genome duplication events during evolution. However, Ragsdale and his colleagues found no evidence of duplications. Instead, 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.
The most notable expansion was in the protocadherins, a family of genes that regulate neuronal development and short-range interactions between neurons. The octopus genome contains 168 protocadherin genes 10 times more than other invertebrates and more than twice as many as mammals. It was previously thought that only vertebrates possessed numerous and diverse protocadherin genes.
The research team hypothesised that because cephalopod neurons lack myelin and function poorly over long distances, protocadherins were central to the evolution of a nervous system whose complexity depends on short-range interactions. The octopus genome is enriched in transposons, also known as ‘jumping genes,’ which can rearrange themselves on the genome. While their role in octopuses is unclear, the team found 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, graduate student in the Department of Organismal Biology and Anatomy at the University of Chicago. Researchers also identified six octopus-specific reflectins, genes involved in light manipulation and camouflage, besides hundreds of octopus-specific genes in the nervous system, retina and suckers.
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