Experiencing the cold helps fruit flies survive frigid temperatures

Over the course of time, many animals that started off in the tropics have made their way towards the poles and colonized places that are chillier than their original home. But settling in a cooler climate comes with its own set of problems. These animals now have to contend with cold nights and freezing winter temperatures for the first time. For animals such as fruit flies (Drosophila), the ability to deal with the cold could mean the survival of their species. Luckily for fruit flies, their exodus northwards has taken generations, giving them enough time to solve their conundrum: how to survive when the temperature drops. This monumental feat is probably accomplished in different ways by different species, so Martin Moos, Petr Hůla and Vladimír Koštál of the Czech Academy of Sciences, Czech Republic, along with Johannes Overgaard and Garfiel Byrge of Aarhus University, Denmark, and Petr Šmilauer and Oldřich Nedvěd of the University of South Bohemia, Czech Republic, decided to test what abilities have allowed fruit flies to colonize the North.

After collecting 43 different species from different environments all over the globe, Moos and colleagues began the herculean task of figuring out what exactly each species was doing to keep themselves alive in the cooler temperatures. First though, the researchers needed to find out how much cold each species could handle. They discovered that the flies could tolerate temperatures far below those they would experience in their normal environment (∼25°C below the average temperature), even if they come from the tropics.

The researchers then checked how well the different species of fruit flies tolerated the cold when they were used to being warm (19°C). Afterwards, the team gradually lowered the temperature (from 19°C to 3°C) over 3 weeks to simulate the cold that northern species normally experience at the start of winter. As they expected, species from more northern regions could deal with the cold better once they had some experience with it. This suggests that something must happen inside the flies the first time they experience cold weather that helps them in the future; but what could it be?

Normally, insects that live in cold environments accumulate cryoprotectants – chemicals such as sugars and amino acids – which act like antifreeze. So, Moos and colleagues began their search there, looking for chemicals that would protect the flies’ cells from the cold. Interestingly, some species of flies had 30 times more cold-protective molecules in their bodies, but most relied much less on these chemicals. This indicated that they may need to look elsewhere to find the answer.

Probing a bit further, the researchers discovered that all the species that could handle cold temperatures were changing the makeup of their cell membranes. This suggests that the flies were replacing some of the chemicals in their cell membranes with others that would help the membranes stay more fluid when the temperature got colder. While Moos and colleagues state that much remains to be done, such as testing how well the cryoprotectants work, the fact that all the fruit fly species the team tested seemed to be changing the makeup of their cell membranes suggests that this may be fundamental in dealing with the cold for the flies that have colonized the North.