"We found that one day fruit flies are invincible to specific pathogens, and the next day they're very susceptible," says study author Brian Lazzaro, an associate professor of biology at Virginia Tech. "That variation is related to the time of day."
The fruit fly's immune system is controlled by a circadian rhythm, which is an internal clock that regulates the body's sleep-wake cycle and other daily activities. The circadian rhythm is controlled by a group of genes that produce proteins called clock proteins. These proteins fluctuate in abundance throughout the day, and they control the expression of other genes, including those involved in the immune system.
Lazzaro and his colleagues found that the fruit fly's immune system is strongest at dawn and weakest at dusk. This is because the clock proteins that control the immune system are at their highest levels at dawn and their lowest levels at dusk.
The researchers also found that the fruit fly's immune system is more responsive to some infections at certain times of day than others. For example, the fruit fly is more resistant to a bacterial infection at dawn than at dusk.
"This is a big change in their susceptibility to infection," Lazzaro says. "It's almost like they're a completely different organism at different times of day."
The researchers believe that the fruit fly's circadian rhythm-controlled immune system helps it to survive in its natural environment. During the day, when there is more sunlight, the fruit fly is more likely to be exposed to harmful pathogens. The fruit fly's strong immune system at dawn helps protect it from infection. The researchers also think that the fruit fly's increased susceptibility to infection at dusk helps it to avoid infecting other fruit flies. The fruit fly is most likely to come into contact with other fruit flies at dawn or dusk, when they are both looking for food.
