Not all experiments go to plan, but when Jan Lubawy, from the Adam Mickiewicz University in Poznań, Poland, forgot to fish a Madagascar cockroach (Gromphadorinha coquereliana) out of the freezer on time, what could have been a disaster turned into a miracle. After being frozen for 2 h at sub-zero temperatures, the insect still survived, despite never experiencing freezing conditions in its native habitat. However, Lubawy realised that this unexpected superpower could open up northern climes to the risk of invasion by the tenacious insect. Deciding that it is important to understand how the exotic creatures might survive in environments that they would not encounter naturally, Lubawy, Szymon Chowański and Małgorzata Słocińska popped a few into a chilly bath to find out how they coped.
Monitoring the insects’ body temperatures as the ice-bath gradually cooled, the team could see the insects’ bodies supercool as their body water remained fluid down to temperatures ranging from –1.9 to –7.6°C. But then the cockroaches’ body temperatures suddenly increased as the water within began turning to ice. So long as their body fluids did not freeze, the robust insects were capable of surviving sub-zero temperatures; but what would happen to them when they froze solid?
This time, Lubawy cooled some of the insects to the temperature where their body fluids began to freeze, pulling them out of the chiller when they were only partially frozen 5 min later. Meanwhile, the remaining cockroaches were left in the ice-bath until their temperatures reached –6°C and they froze solid. In addition, Lubawy warmed another group of insects to 44°C, a temperature that is known to be harmful for the robust creatures, to find out how they fared.
Impressively, 80% of the partially frozen cockroaches survived the first day of recovery, with 30% making it through 10 d. However, the insects that had been completely frozen suffered more; 70% of the insects died during the first day and the 10% that survived longer than a week showing signs of disabilities. In contrast, none of the overheated insects suffered any long-term ill effects despite struggling with the heat at the time. So the insects weren't completely immune to the damaging effects of freezing, but were sufficiently resilient for some to make it through the most gruelling conditions.
Wondering how time in the chiller might have harmed the insects and knowing that low temperatures can damage DNA, Lubawy collected tiny haemolymph (blood) samples from the insects to check for signs of DNA damage. ‘The haemolymph of this species coagulates rapidly, within 5 to 10 s’, says Lubawy, which made it difficult to extract the blood cells that they needed to investigate. Then Hervé Colinet, from the Université de Rennes, France, suggested that Lubawy and Virginie Daburon (also from Rennes), embedded the blood cells in a gel, before applying an electric field that would cause small strands of damaged DNA to begin moving, while larger intact sections remained immobile, ‘which results in a comet-like shape’, says Lubawy. Remarkably, the insects that had only partially frozen seemed to experience little DNA damage, while the cockroaches that had been entirely frozen suffered more damage than the animals that had frazzled in a heatwave. But Lubawy suggests that the insects are protected from the damaging effects of a chill because all of the heat-stressed insects survived despite incurring high levels of DNA damage. They must have a very efficient DNA repair system, suggesting that DNA damage is not the culprit.