How heat and drought threaten fruit fly dehydration defences

"Insects are vital to life on Earth, pollinating three-quarters of global food crop species, forming the base of food webs, and cycling nutrients back into the soil," says Alex Gould, who runs a lab at the Crick investigating the impact of environmental stresses on development. "With the backdrop of human-induced climate change, it's important to understand the resilience of insects to increases in aridity and temperature."

Drosophila spends its early days as a larva in a humid environment, feeding and burrowing in decomposing fruit. It then crawls to a drier place and becomes an immobile pupa. The pupa develops into an adult fly through a complex transformation called metamorphosis.

The pupal and adult stages of insects like Drosophila occupy dry land environments and are therefore particularly vulnerable to dehydration. To help prevent this, it is known that the adult fruit fly is covered with a barrier of lipid (fat): a hydrophobic coating made of long hydrocarbon molecules that blocks water loss. Until now, researchers didn't know how the pupa protects itself from dehydration or what kind of hydrocarbon barrier it might have.

The pupa's support system against dehydration

Many of the known hydrocarbons in Drosophila are similar to those found in crude oil, so Lena Lampe, a postdoctoral researcher in the lab, and Clare Newell, a Principal Laboratory Research Scientist in the lab as well as a visiting scientist at the National Physical Laboratory, borrowed methods from the petroleum industry in their experiments. "We were able to separate the hydrocarbons by size, to work out their relative composition in Drosophila melanogaster as it develops," Clare says. "Embracing techniques normally used outside biology helped us to find one of the hidden secrets of metamorphosis."

The surprise was that the pupa doesn't have any hydrocarbons on the surface of its outer body casing (cuticle), whereas the adult has them in abundance. "We wondered why the young fly needs these hyper-long hydrocarbons but the pupa apparently doesn't, despite being equally vulnerable to dehydration," says Lena.

So how does the pupa avoid drying out? The answer to this question came from the discovery, in research published today in Science Advances that the pupa has a different kind of lipid barrier to the adult. It is internal rather than external, coats the underside of the cuticle, and is made of hyper-long hydrocarbons that are much longer than those of the adult.

Next the team traced the origin of the hyper-long hydrocarbons, finding that they are premade by specialised cells in the larva and then stored until needed in the larva's fat reserve. The researchers think that this anticipatory strategy may allow hyper-long hydrocarbons to be deployed very quickly to the underside of the cuticle during the rapid transition from larva to pupa.

Feeling the heat

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By looking at the expression of genes involved in producing hydrocarbons, the team identified a specific elongase enzyme that makes hyper-long hydrocarbons. However, removing this enzyme in standard laboratory conditions didn't have a huge impact, suggesting that other elongase enzymes also contribute to the production of hyper-long hydrocarbons.

To test whether the enzyme might nevertheless become important under environmental stress, the team recreated high temperature and low humidity conditions similar to those of some subtropical and mediterranean climates. Exposure of the pupa to this environment significantly decreased its water content but, without the hyper-long elongase enzyme, dehydration almost doubled to a level incompatible with survival.

"We've identified a key enzyme that ensures the development of a specific lipid barrier in the pupa that protects it from water loss in hot and arid conditions. However, Drosophila has its survivable environmental limits and our studies of this enzyme have helped to define these," says Lena.

An evolutionary tug-of-war

"In adult Drosophila, the hydrocarbons of the lipid barrier play another interesting role," says Alex. "They also function as air-borne sex pheromones, chemical signals that flies use as mating cues."

Long hydrocarbons best protect against dehydration but short ones evaporate into the air more easily and are better suited as sex pheromones. So pupae, which have yet to sexually mature, make hyper-long hydrocarbons to prevent water loss whereas adult flies have settled on a medium length compromise to satisfy both the barrier and sex pheromone functions.

An exception to this rule is a desert species called Drosophila mojavensis. "We found that it retains hyper-long hydrocarbons as an adult, instead of switching to shorter ones like other Drosophila species," says Clare. "In a desert environment, an optimal barrier to dehydration may be more important for survival than air-borne sex pheromones. It's likely that D. mojavensis may use other mating signals such as contact or visual cues."

Alex hopes that this work will inspire research into other insect species. "It will be especially important to look at insects of agricultural importance," he concludes. "This could help us understand the likely impacts of climate change on the essential roles that insects play in the food chain."

The research involved collaboration with the Metabolomics team, the Fly Facility, Cyrille Alexandre in Jean-Paul Vincent's group at the Crick, Ian Gilmore at the National Centre of Excellence in Mass Spectrometry Imaging at the National Physical Laboratory and Li Zhao at the Laboratory of Evolutionary Genetics and Genomics at The Rockefeller University.