Preference for oranges protects fruit flies from parasites

In nature, laying eggs on oranges is advantageous, because parasitoid wasps feeding on the larvae of Drosophila avoid citrus fruits. The same smell that is attractive to the flies also repels the wasps. The scientists used imaging techniques to visualize the activity in certain areas of the flies’ brains while these were stimulated with different odours, and they were able to localize and identify the receptor for citrus. Flies in which this receptor was silenced were no longer able to distinguish oranges from other fruits.

For egg-laying insects, selecting the best site to lay eggs is crucial for the survival of the eggs and larvae. Once the eggs have been deposited, the maternal care of the female flies ends: eggs and larvae are henceforth at the mercy of their environment; their range is usually limited. Suitable and sufficient food sources for the hungry larvae and protection against predators and parasites are important selection criteria for the best oviposition substrates.

First, Marcus Stensmyr, Bill Hansson and their colleagues in the Department of Evolutionary Neuroethology tested the preferred egg-laying substrates of fruit flies by letting insects select among different ripe fruits. They excluded damaged fruits to make sure that the smell of yeast would not influence the flies’ choices (yeast is the flies’ main food source). An analysis of the behavioural assays showed that female flies preferred to lay their eggs on oranges. Further selection experiments helped to identify the odour that was the crucial factor for the flies’ choice: the terpene limonene. Flies were not attracted to limonene-deficient oranges. On the other hand, they were immediately drawn to fruits that had been spiked with synthetic limonene. Although citrus is not an attractant for the flies, it elicits egg laying. Interestingly, evolution has split the perception of odours into two channels: those that guide flies to their food source and those that elicit the oviposition behaviour.

By performing electrophysiological measurements, the scientists were able to quantify the flies' responses to limonene and to localize and identify the olfactory sensory neurons responsible for detecting citrus. Subsequently, they tested the flies’ responses to 450 different odours. Apart from limonene, valencene, another component of citrus fruit, also triggered a strong response. Valencene distinguishes the scent of oranges from that of lemons; lemons are less favoured by flies because of their acidity. Compounds that activated these particular sensory neurons induced oviposition. In vivo calcium imaging of the flies’ brains stimulated with citrus enabled the researchers to identify the corresponding odorant receptor.

"It is fascinating that a complex behaviour, such as choosing an egg-laying site, can be broken down into multiple sub-routines that have such a simple genetic basis," says Marcus Stensmyr. “We were quite surprised that by silencing just this single odorant receptor, flies could no longer localize their preferred egg-laying substrate.”

In nature, a considerable proportion of Drosophila larvae are killed by enemies, mainly parasitoid wasps that lay their eggs inside the larvae. It is astonishing that these wasps are repelled by citrus odours, although citrus should guide them to their food source: Drosophila larvae. The parasitoid wasp Leptopilina boulardii, which specializes in Drosophila melanogaster, is repelled by valencene. In a further choice experiment, the wasps had to choose larvae from two substrates − one with valencene and one without − and they clearly preferred the larvae on the valencene-free substrate. It is still unknown why the wasps avoid citrus. However, it is certain that female fruit flies have learned to let their offspring grow on citrus fruits, because there the larvae are better protected against parasites.

These research results provide important information about the criteria that insects use to select an oviposition site that guarantees the improved development of their offspring. Marcus Stensmyr is convinced that “there are similar processes in other insects and ways to manipulate them.” These insights may lead to new ways to control insects, especially those that destroy crops or transfer diseases.