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The Earth takes about 24 hours to rotate on its axis. All animals and plants have adapted to this daily rhythm. Except for one beetle, which is completely out of sync.
Or beetles, let’s say beetles. Officially the big black cockchafer Parallel holotrichy called, is a plague in Asia. The female beetles emerge from the ground not every night, but every other night. They then climb a plant and release pheromones to attract males.
Mysterious rhythm
Strangely enough, the females keep a 48-hour rhythm. The reason remains a mystery to scientists. What they now know is that the males are also rather slow and the females can therefore only smell every other night.
It works like this: Many insects, from moths to mosquitoes, use scents to attract a mate. The insects smell with their antennae, which have special receptors with which they react to certain odors in the air.
First the genes
To find out how this stubborn species of beetle works, the Chinese and American researchers first had to find the gene for the receptor that responds to the female pheromone in the beetles. We succeeded and gave it the tantalizing name L-Isoleucine Methyl Ester or LIME. The researchers initially cloned fourteen candidate genes. A series of experiments eventually led them to the gene HparOR14 as a receptor for sex pheromones. It was the first time such a gene had been found in beetles.
Time for a date night
Now the hardest part was over and they could start measuring how the activation of this gene worked in the 48-hour cycle. And they discovered something remarkable. On “date night” – when females climb plants to spread their scent – HparOR14 transcription is higher when it is dark. But on the other nights the activity of the receptor was low. To confirm this, a control experiment was carried out measuring the response to a chemical signal from damaged leaves. This indicates the presence of food for the beetles. But this reaction remained constant day after day. The males follow the same 48-hour rhythm as the female beetles, but only to absorb the pheromones.
Exactly 48 hours
Why large, black beetles have this abnormal rhythm and how it can arise is unclear. Normally, our biological clock synchronizes with the 24-hour cycle of the sun rising and setting. Since there are no signals in nature that allow animals to follow a 48-hour rhythm, it is a mystery how the female beetles manage to emerge every exactly 48 hours to release pheromones and how it the male beetles then manage to be able to smell the scents at exactly the right time.
“24-hour rhythms have been commonly observed in organisms from bacteria to humans, but a 48-hour rhythm has rarely been discovered,” says Professor Joanna Chiu of UC Davis, who was not involved in the study. “This elegant study has provided us with a detailed description of how the two-day pheromone detection rhythm arises in this beetle.”
The circadian rhythm
Almost all plants and animals have a circadian rhythm, i.e. a biological clock of 24 hours. Even in complete darkness, a rhythm that is established in certain mammalian brain cells persists. Many bodily functions follow this rhythm. For example, our body temperature is highest in the late afternoon, about six hours before bed, and lowest two hours after falling asleep. But the release of hormones, our heart rate, our need for food and drink and the volume of our bladder are also influenced by the circadian rhythm.
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