Evolution Update

Evolution Update

The Curious Case of the Octopus Brain as Told by Evolution

Michael Olvera September 20, 2015

New interdisciplinary research underlines how traits among the octopus genome allows for their unprecedented intelligence and camouflage abilities.

Among the invertebrates, the octopus is king in terms of camouflage and intelligence. Octopuses are known to have one of the largest brain-to-body ratios among the invertebrates, and have been observed using tools, communicating with one another, and even escaping from a closed glass jar . This intelligence inspired Dr. Daniel S. Rokhsar to collaborate with scientist from around the globe in order to investigate what makes the octopus brain tick. However, rather than directly looking at the brain, Dr. Rockhsar led an effort to sequence and analyze the octopus genome, a task that is much more difficult than it sounds. This would help him paint a picture on the evolution of the octopus and in turn the octopus brain.

Using Octopus bimaculoides as their model octopus, the team got to work sequencing and analyzing the 2.7 gigabase genome, a size almost as large as our own. The team scoured the genome, looking for genes associated with brain activity, neuron growth, and neuronal development. After extensive analysis, the group identified two genes ‘families’ (genes that are similar to one another) associated with brain development that were present in large numbers within the genome. This puzzled the researchers, as only vertebrates have been previously shown to have these gene families in abundance. How did they come to be in an octopus?

A broken copy machine.

To understand where theses genes came from, we have to understand how one gene turns into many. Let’s imagine I have a cookbook that I want to distribute to hundreds of people. One method of copying my book is to use a copy machine, which will make an exact copy of every page. However, imagine that my copy machine is flawed, and every once in awhile my machine will duplicate a page, producing two pages instead of one. So maybe now my book has two copies of the ‘Clam Chowder’ recipe instead of one. In genetics terms, this is called gene duplication; one gene can be accidentally copied and turn into two identical genes. Over time, these genes can acquire mutations and change from one another. So if I went back to my cookbook and changed one of the ‘Clam Chowder’ pages to include broccoli and cheese, I would have two similar but different recipies.

A biological example of this would be the genes that control color vision, called Opsins. Normally, humans have nine different Opsin genes, that all perform similar but different tasks. Some are associated with certain colors, such as detecting blue, green, and red light. With dogs and color blind individuals, some of these genes are missing or not working. Different organisms have different numbers of opsin genes depending on evolutionary pressures. The exact same thing is true for the unique gene families identified in the octopus genome.

Sharpening the saw.

The two gene families identified by Dr. Rokhsar and company were the protocadherins and the C2H2 zinc-finger transcription factor superfamily (let’s just call it C2H2). Protocadherins are a subfamily of the cadherin group of genes. Cadherins, which comes from the words calcium and adhesion, are proteins that help cells stick together. While found mostly in vertebrates, the smaller protocadherin family is found throughout the animal kingdom (thus the prefix ‘proto’ meaning primitive). In nervous tissue, protocadherins help to develop and maintain neurons. When Dr. Rokhsar’s team looked to see where these protocadherin genes were being expressed in the octopus, they found that tissues containing a high percentage of neurons were also making a majority of the protocadherins for the octopus. Thus it is safe to suggest that protocadherins may be playing a part in the neural capacities of octopuses.

On the other side, the C2H2 zinc-finger transcription factor superfamily are proteins that, in the broadest of terms, bind to DNA to turn a gene on. Certain parts of a protein, called amino acids, help form a protein structure that can regulate when a gene is turned on and off. This tight control is important for development; you want some genes to be turned on in applicable cells. For instance, when a human fetus develops, it turns on ‘skin-cell-associated’ genes in skin cells and only skin cells. You don’t want your baby's skin to have the genetic profile of their bone cells. The authors comment on how these genes most likely arose to help the octopus develop its complex nervous system.

The mysterious complexity behind the octopuses keen intelligence will take more than a genome sequence to elucidate however. The work done by Dr. Rokhsar and company help to paint a target on areas of interest for future biologists, but more work is still required to fully understand how the octopus develops its impressive nervous system. A nervous system capable of problem solving, intelligent reaction, and an incredible camouflage ability.

Punnoctopus cordiformis, common octopus photo by Brian Gratwicke

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