The octopus mating arm has more talents than expected
The mating arm of a male octopus has more talents than you might expect. Researchers from KTH now show that the arm not only delivers sperm, it also helps the male find the right mate by sensing the hormone progesterone.
How do we choose a partner? For humans, attraction can involve everything from appearance and social cues to voice, scent and signals we may not even notice consciously.
For an octopus, the challenge is different. Most species spend much of their lives alone, rarely crossing paths with potential mates. And when they do meet, finding the right partner comes down to molecules.
At the centre of the encounter is a specialised arm found in male octopuses, known as the hectocotylus. The arm delivers sperm into the female reproductive tract. But exactly how it identifies a mate, or finds the correct location for sperm transfer, has remained a mystery.
A new study involving researchers at KTH now suggests that the hectocotylus is more than a reproductive organ. It is also a sensory organ with the ability to detect progesterone, a hormone found in the ovaries of many animals.
Sensing progesterone
The researchers identified progesterone-sensing receptors at the tip of the specialised arm. To understand how these receptors work, they combined cryo-electron microscopy with molecular simulations.
"One of the important findings is the structure of the receptor that senses progesterone," says Tatiana Shugaeva, researcher at SciLifeLab and KTH.
The structure was determined using cryo-electron microscopy. But progesterone itself was not clearly visible.
"You can see that the molecule is inside the binding site, but you can't tell exactly how it is oriented. In such cases, molecular dynamics simulations can help. That's where our KTH expertise comes in."
The findings also reveal something about octopus courtship.
According to first author Pablo Villar at Harvard University, octopuses can spend 30 to 40 minutes, sometimes even longer, engaged in courtship before mating takes place.
"He was surprised that octopuses use such prolonged chemotactile courtship, despite being known for their colourful visual displays," says Shugaeva.
Mating and predation
In the experiments, male octopuses could extend its specialist arm through a barrier to mate with a female it couldn’t even see.
Another surprise came from the pocket where progesterone binds.
The researchers discovered that progesterone binds to the same receptor pocket already known to recognize molecules involved in prey detection.
"It's unusual for a small part of a protein to specifically recognise and bind different types of molecules that are involved in two very different functions: mating and predation," says Shugaeva.
For her, the project also offered a welcome change of scenery. Much of her previous research has focused on human or bacterial proteins, and a relatively small list of “typical organisms”. Octopuses, she adds, are another matter entirely.
"Simply telling people that I study an octopus mating arm is enough to trigger plenty of adult jokes from family and friends. Personally, I am a big fan of studies that focus on uncommon organisms.”
Text: Anna Gullers