The elaborate brains of octopuses could be linked to the way their DNA folds and brings normally distant regions close together.
Octopuses, cuttlefish and squid are mollusks, like snails. Yet they possess a highly developed nervous system and can display astonishing behaviors. To understand this evolution, researchers no longer focus solely on which genes these animals possess. They also study how those genes are used.
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DNA often resembles a long, coiled ladder, but this representation shows only a small part of its organization. Inside cells, this molecule folds back on itself to fit inside the nucleus. Two portions that are far apart along the chain can therefore end up next to each other in space.
This proximity has consequences. Some portions of DNA do not produce proteins but help control other genes. When a fold brings them into contact with the right regions, they can modify the activity of those genes, particularly during the development of an organ.
A team compared this organization in three cephalopods: an octopus, a cuttlefish and a small squid. It mapped contacts between different regions of their DNA, then examined which genes were active in several tissues and at different stages of development.
The researchers found that many large regions of the genome retain a common organization. However, hundreds of smaller loops differ depending on the species, tissue or growth stage. These loops correspond to contacts between regions that are sometimes very far apart along the DNA chain.
These differences could stem from ancient changes in the chromosomes. During cephalopod evolution, large pieces of DNA changed position or were assembled differently. New sections could thus come into contact and help control genes that had not previously been regulated in this way.
The team also directly tested the role of a regulatory region through a genetic modification. Its removal disrupted processes related to nervous system development. This experiment shows that at least some of these contacts between DNA regions play a real biological role, beyond being a mere structural peculiarity.
A clue to the origin of octopus intelligence? Their behavior depends on many factors and a long evolutionary history. Future studies will need to determine which DNA loops are specifically involved in forming their various neural circuits, and how they act on genes during development.