With their giant brains, hundreds of suckers lining eight prehensile arms, and rapid changes in color, octopuses are not your basic mollusks. And now researchers have sequenced the complete genome of an octopus for the first time! The findings are published in Nature this week.
Cephalopods, which include the octopus, squid, cuttlefish, and nautilus, emerged as predators throughout the ancient seas half a billion years ago. They were likely the first intelligent life-forms on Earth, and these days, the list of octopus innovations is long and impressive: camera-like eyes, the ability to regenerate complex limbs, a propulsion system, and three hearts that keep blood pumping across the gills, to name a few.
Now, to investigate the molecular basis of the cephalopod brain – the largest nervous system among invertebrates – as well as their cool innovations, a team led by University of Chicago’s Caroline Albertin isolated and sequenced genomic DNA from a single male California two-spot octopus, Octopus bimaculoides. These clever problem solvers have a blue eyespot (pictured above) on either side of their heads. A juvenile female is pictured below to the right.
The octopus genome is about 2.7 billion base pairs in size, with long stretches of repeated sequences and more than 33,000 protein-coding genes. This means that their genome is slightly smaller than ours, but they have more genes. Researchers used to think that the large size of the octopus genome was due to whole genome duplication events during their evolution. But while these can result in increased genomic complexity, the team found no evidence of duplications.
Rather, a couple of gene families expanded, novel genes appeared, and the whole genome was shuffled around. "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," Albertin says in a statement. "This leads to genes being placed in new genomic environments with different regulatory elements."
Even their developmental and neuronal gene repertoire seems similar to that of other invertebrates, except for major expansions in two gene families that were thought to be enlarged in vertebrates only: The family of signaling molecules called protocadherins that regulate neuronal development and short-range interactions between neurons, and the family of transcription factors called zinc fingers that are involved in brain development. “We think they play a critical role allowing a new level of neuronal complexity to be reached in invertebrates," study coauthor Daniel Rokhsar of UC Berkeley says in a statement.
Furthermore, the team identified hundreds of octopus-specific genes. Many of them are highly expressed in the brain, in their long grasping arms with suckers that can also sense chemicals, and of course, in their specialized skin with its adaptive coloration. "We've found hundreds of novel genes that don't have counterparts in other animals and may be involved in this unique camouflage process," Rokhsar adds.
Middle Image: Michael LaBarbera
Video (below): Nature Video