The 48-hour cycle of the great black cockchafer

Life on Earth follows a 24-hour cycle as the planet rotates. Animals and plants have built-in circadian clocks that synchronize their metabolism and behavior to this daily cycle. But a beetle is not in sync with the rest of nature.

A new study, published January 18 in Current biology, looks at a beetle with a unique 48-hour cycle. The large black cockchafer, Holotrichia parallela, is an agricultural pest in Asia. Every other night, females emerge from the ground, climb a host plant and release pheromones to attract males.

This mating behavior of females is under the control of a 48-hour “circadian” clock, for reasons that remain mysterious. A team led by Walter Leal, a professor of molecular and cellular biology at the University of California, Davis, and Jiao Yin of the Chinese Academy of Agricultural Sciences in Beijing, wanted to know whether the ability of male beetles to smell females was also in question. a 48 hour clock.

Leal’s lab at UC Davis studies chemical sensing in insects. Many insects, from moths to mosquitoes, use their scent to attract a mate. Insects “smell” with their antennae, which contain specialized receptors that respond to specific chemicals floating in the air.

Track pheromones

The team’s first step was to identify the gene in large black cockchafers for the receptor that responds to the female pheromone, which goes by the enticing name L-isoleucine methyl ester, or LIME. The researchers initially cloned 14 candidate genes. A series of experiments led them to a gene called HparOR14 as a sex pheromone receptor – in fact, the first to be identified in a beetle species.

After identifying the receptor gene, they were able to measure transcription levels of the HparOR14 gene throughout the beetle’s life and its activity over 48 hours. They found that during “date nights,” when females climbed plants to release their scent, HparOR14 transcription was higher after dark. But receptor activity was low on alternate days. (In a control experiment, the response to a chemical signal from damaged leaves, indicating food for the beetle, remained constant day after day.)

The results show that the ability of male chafers to detect the female sex pheromone occurs according to a 48-hour circadian cycle which corresponds to the mating behavior of the female.

We do not know why and how big black cockchafers have these 48-hour cycles. Circadian (24-hour) clocks are synchronized by signals that change on a 24-hour cycle, the most obvious being sunrise or sunset. But there are no 48-hour clues in nature, so exactly how the circadian cycles of large black chafers play out – including how males and females can synchronize with each other , so they all know which night is date night – remains a mystery to be solved. to be determined.

“24-hour rhythms in physiology and behavior are commonly observed in organisms from bacteria to humans, but observations of 48-hour rhythms in nature are rare,” said Professor Joanna Chiu, chair of the Department of entomology and nematology at UC Davis and an expert on circadian rhythms, who was not involved in the work. “This elegant study by Professor Leal and colleagues has provided us with an in-depth description of how the circadian rhythm of pheromone detection is generated in this beetle.”

Other authors of the study are: Yinliang Wang, Huanhuan Dong, Yafei Qu, Jianhui Qin, Kebin Li, Yazhong Cao and Shuai Zhang, Chinese Academy of Agricultural Sciences, Beijing; Yuxin Zhou and Bingzhong Ren, Northeast Normal University, Changchun, China; and Chen Luo of the Beijing Academy of Agricultural and Forestry Sciences.