Scientists have sequenced the entire genome of an octopus for the first time. It is, in fact, the first time the genome for any cephalopod, which includes the octopus, cuttlefish and nautilus has been sequenced. The octopus has several features, which are very interesting and unique including the suckers on their eight limbs and body plan.
The team, led by graduate student Caroline Albertin, sequenced the entire genome using the ‘whole genome shotgun sequence’ that involves cloning the DNA then splitting it into small fragments. The small fragments are sequenced and the entire genome is assembled by finding areas where the sequence overlaps. The species of octopus used was Octopus bimaculoides or the California Two Spot Octopus.
The octopus genome was found to have about 33,000 genes and 2.7 billion base pairs (the A’s G’s C’s and T’s in DNA). The number of genes is substantially more than in humans of which there are between 20,000 and 25,000. Of these genes, several which were found are unique to the octopus among vertebrates. Most of these unique genes are involved in the highly advanced brains, and in tissues, especially in those around the suckers on their eight legs.
The octopus is highly intelligent, probably the most intelligent of the invertebrates. This fact is due to its very complex neural network. An octopus brain contains almost half a billion neurons, much more than any other organism of comparable size. A big reason for the intelligence of an octopus is the presence of 168 cadherin genes found in its genome. Cadherin is a protein involved in the development of neurons. Most invertebrates have very few cadherin genes. You have to look to mammals and other vertebrates to see higher levels of this gene. The high levels of cadherin evolved independently in the octopus and in mammals like humans.
The highly advanced neurological system in the octopus is also probably due, in part, to the ability of an octopus to change its colour so fast to camouflage itself while it’s stalking prey. It is still mostly unknown how the octopus has this ability but with the entire genome at hand, finding out how will be an easier question to answer.
What the team also found was a unique arrangement of Hox genes, which are involved in the body plan of animals when they are in the embryonic stage. Normally, Hox genes are arranged in order in the DNA sequence from the front of the animal to the back. This is not what was observed in the octopus. They are spread out in different parts of the genome. The researchers think this is because chromosomal fragmentation.
The team also found a clustered group of acetylcholine receptors. Acetylcholine is a neurotransmitter used in activation of muscles. There were alterations to the gene that resulted in a protein that doesn’t appear to be able to bind to acetylcholine, making it useless as a receptor. The high expression of this gene in areas where suckers on the legs are present leads the team to believe that they act as a sensory receptor that the octopus can use to feel its surroundings.
In order to determine approximately when the octopus lineage diverged from the squid, the team used a molecular clock. This involves finding the number mutations in a small sequence of DNA shared by the octopus and squid. By averaging the rate at which mutations occur, an estimation can be made at when the two linages diverged. It was estimated that the octopus and squid diverged about 270 million years ago; this suggests that octopus has been around for quite a long time.
One final interesting find in this study is that the structure of the genome of the octopus is similar to that of limpets and some worms. This means that as the octopus evolved from simpler invertebrates, most of the essential genes were either there or required little alteration.
This study is just the beginning in learning how the octopus evolved and what mechanisms it uses to survive.