Wolbachia May Improve Learning and Memory in Fruit Flies

Improving human recall with a tablet or a mild electrical stimulus remains experimental, yet fresh research indicates that a bacterium occurring naturally in as many as 70 per cent of insect species may, in particular circumstances, improve learning and memory.

Wolbachia and insect behaviour

Scientists in China discovered that fruit flies carrying a specific strain of Wolbachia - a bacterial genus first identified in mosquitoes in 1924 - did better than uninfected flies in tests associating particular odours with rewards.

Wolbachia can infect several insect organs, but its frequent presence in reproductive systems has meant that research has largely centred on its effects on reproduction. Work using the bacterium to stop Aedes aegypti mosquitoes transmitting dengue and other viruses to people has delivered important public-health benefits.

However, certain Wolbachia strains are able to infect an insect host's nervous system, including its brain.

Earlier research has found that male flies carrying Wolbachia outcompete uninfected males, and that the bacteria also influence flies' sleep.

To examine further potential effects, biologist Meng-Yan Chen of Central China Normal University and fellow researchers infected fruit flies (Drosophila melanogaster) with a Wolbachia strain known as wMelPop. They then compared the flies' memory with that of uninfected flies.

"As wMelPop has been demonstrated to be the strongest dengue blocking strain known, the behavioral and ecological consequences of wMelPop infection in the insect hosts may be of great importance in controlling insect-borne diseases and agricultural pests," Chen and colleagues write in their published paper.

How wMelPop affects fruit flies

In addition to preventing the dengue virus from replicating, the wMelPop strain reduces the lifespan of infected mosquitoes. As they age, these mosquitoes have difficulty feeding and become 'jittery'. Chen and colleagues say these neurological effects are thought to be connected to the way wMelPop "proliferates massively" within brain tissue.

Given the damage observed in brain tissue, the researchers expected wMelPop to undermine learning and memory in insect hosts, flies included.

"To our surprise," Chen and colleagues write, "we observed that wMelPop infection still improved short-term and long-term memory of D. melanogaster."

The team exposed flies aged between 4 and 35 days to two odours, pairing one with a sugar reward. They assessed the insects' recall of those smells either 2 minutes or 24 hours afterwards.

Young flies infected with wMelPop outperformed uninfected flies of the same age in both tests. The infection also improved memory among older flies, although three-quarters of that group had died by 35 days, again highlighting its lifespan-limiting effects.

Memory changes linked to brain gene expression

Additional analysis identified clear shifts in gene expression in the flies' brain tissue: certain genes were activated, whereas others were reduced.

"These results suggest that wMelPop infection in Drosophila might enhance the insect host's memory by altering host gene expressions in the head," Chen and colleagues report.

The memory improvements associated with wMelPop in Drosophila differ from earlier evidence of memory loss in Wolbachia-infected wasps and woodlice. This suggests that Wolbachia's effect on memory is determined both by the host species and by the bacterial strain.

The scientists propose that this may reflect the way an infection spreads most effectively in each host. Wolbachia, for instance, causes parasitic wasps to lose their memory of laying a clutch of eggs, making them more likely to lay another clutch and allowing the bacteria to spread to more offspring. In Drosophila, by contrast, a better chance of Wolbachia reaching more offspring depends on female flies being more capable of locating food, avoiding threats and finding mates efficiently.

The differing findings could also prompt reflection as awareness grows of possible insect consciousness, alongside numerous studies showing forms of insect intelligence.

The study was published in Animal Behaviour.