In an evolutionary twist, some German cockroaches have developed a genetic mechanism that makes sugar taste bitter – allowing them evade sugary bait set out to kill them.
“Most times, genetic changes, or mutations, cause the loss of function,” said Dr Coby Schal, an entomologist at the State University of North Carolina, and co-author of a study published in the journal Science this week. “In this case, the mutation resulted in the gain of a new function – triggering bitter receptors when glucose is introduced. This gives the cockroach a new behaviour which is incredibly adaptive. These roaches just got ahead of us in the arms race,” he said.
Because the aversion is genetic, it eventually spreads to offspring, meaning increasingly large groups of cockroaches are rejecting glucose and any bait made with it.
Cockroaches do not have taste buds; instead they have taste hairs lining different parts of the body. In the study, scientists focused on those near the mouth areas and receptor cells that detect sweet and bitter sensations, immobilising the roaches and using tiny electrodes to record their activity. They found that instead of triggering just the sweet receptors, the glucose in the cockroaches also triggered the bitter receptors that would normally be set off by caffeine and other bitter compounds.
When roaches from populations known to be glucose-averse were forced to take in sugar they refused, closing their mouths and running away from the glucose. The roaches, however, showed no aversion to other sugars like fructose, which is commonly found in fruit.
But it isn’t all good evolution for the roaches. The researchers found that in bait-free and resource-rich environments, the roaches tended to grow more slowly, as they had more difficulty finding suitable food sources.
There are over 4,000 species of roaches, living in a variety of habitats across the world – from forests to deserts and cities. Only around one per cent of cockroaches, most of which are the German cockroach (Blattella germanica), live among humans, and are considered pests that transmit diseases and trigger allergies.
While the behaviour had been recorded more than 20 years ago, this was the first time researchers had been able to pinpoint why.
The research was carried out by Ayako Wada-Katsumata, Jules Silverman and Coby Schal, at North Carolina State University with the support of the US National National Science Foundation, the US Department of Housing and Urban Development, and the Blanton J Whitmire Endowment at NC State.