Despite being the size of a fingernail and sharing the physical traits of a water balloon, the goldenrod gall fly (Eurosta solidaginis) is one of nature’s toughest creatures. It can live six months without eating or drinking and can survive freezing temperatures down to -80 °C.

New research from Western University has added another feat to the gall fly’s list of tenacious talents: an ability to produce and consume a fat that no other animal on the planet uses to store energy. The findings were published in the Journal of Experimental Biology.

Studying how freezing and thawing through the winter affected the gall fly’s energy reserves, the research team looked more closely at the types and amounts of fats in its body and discovered something completely unexpected.

“Almost every animal on the planet stores their fat energy in molecules called long-chain triacylglycerols,” said co-author Katie Marshall. “The goldenrod gall fly had an enormous amount of a fat we couldn’t immediately identify.”

During investigation, the research team identified the unknown fat as a molecule called acetylated triacylglycerols. These fats are rare in the animal kingdom and have only been found in trace amounts in the antlers of Japanese deer and the udders of domestic cows. To the surprise of the researchers, acetylated triacylglycerols are plentiful in the goldenrod gall fly. Marshall estimates that if gall flies were the size of 70 kg (154 lb) humans, they would hold almost five kilograms of acetylated triacylglycerols in their bodies.

“Like many animals in Canada, the goldenrod gall fly spends the summer eating as much as it can to store up fat for the long winter ahead,” said Brent Sinclair, senior author on the study. “Because they spend the winter unable to eat, using those fat reserves sparingly through the winter is critical for the gall fly’s survival and ability to reproduce in the spring.”

Acetylated triacylglycerols contain less energy than regular long-chain triacylglycerols, so they seemed like an odd tool for the gall fly to use to save energy over winter. Other researchers have even investigated these fats as potential low-calorie oils for use in cooking. Marshall and her colleagues thought that making this fat might be related to some of the gall fly’s extraordinary ability to survive freezing. She found that acetylated triacylglycerols are produced right before winter, and that the gall fly makes more of them after freezing and thawing.

The new fat also remains liquid at much lower temperatures than usual fats, which means the flies could use it while they are frozen.

“The discovery that gall flies can process long-chain triacylglycerols into this low temperature version paves the way for researchers to develop new ways to turn regular fats into biofuels that work in Canada’s cold winters or chilly high altitudes,” said Sinclair.

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– Seasonal accumulation of acetylated triacylglycerols by a freeze-tolerant insect