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A North Carolina State University research team has reported that a mutant strain of German cockroaches have gained a new biological function over the last 20 years: they have developed the capacity to perceive sugar as bitter, enabling them to avoid sugar-coated cockroach traps and increase their chances of survival.
The research team’s original report, which is titled, Changes in Taste Neurons Support the Emergence of an Adaptive Behavior in Cockroaches was jointly authored by Ayako Wada-Katsumata, Jules Silverman, and Coby Schal and was published last week in Science magazine (24 May 2013: Vol. 340 no. 6135 pp. 972-975, DOI: 10.1126/science.1234854).
Science reporter Victoria Gill summarized the scientists’ findings in a BBC News report (Cockroaches lose their ‘sweet tooth’ to evade traps, 24 May 2013):
A strain of cockroaches in Europe has evolved to outsmart the sugar traps used to eradicate them.
American scientists found that the mutant cockroaches had a “reorganised” sense of taste, making them perceive the glucose used to coat poisoned bait not as sweet but rather as bitter…
The scientists immobilised the cockroaches and used tiny electrodes to record the activity of taste receptors – cells that respond to flavour that are “housed” in microscopic hairs on the insects’ mouthparts.
“The cells that normally respond to bitter compounds were responding to glucose in these [mutant] cockroaches,” said Dr Schal.
“So they’re perceiving glucose to be a bitter compound.
“The sweet-responding cell does also fire, but the bitter compound actually inhibits it – so the end result is that bitterness overrides sweetness.”
So it actually seems as if natural selection has changed the way flies perceive tastes, in the short space of 20 years:
…Dr Elli Leadbeater from the Institute of Zoology in London said the work was exciting.
“In the cockroach case, sugar actually tastes bitter – an effective way for natural selection to quickly produce cockroaches that won’t accept the sugar baits that hide poison.”
A recent report by Joseph Nordqvist in Medical News Today explains the mechanism of the changes (Cockroaches Are Evolving To Evade Sugar Traps, 25 May 2013):
Cockroaches have tiny little hair-like sensors on their mouths which they use to “taste” food, activating sensors house gustatory receptor neurons, or GRNs. Certain GRNs activate in the presence of food that is sugary – which makes them feed – as opposed to GRNs that activate in the presence of food that is bitter – making the animal avoid the food…
The research, which started in the mid-1980s, found that German cockroaches given baits incorporating a stimulant (glucose) and a deterrent (insecticide) evolved a behavioral change called “glucose aversion”.
What the scientists found was that the mutant German cockroaches which displayed glucose aversion were given sugar, it actually stimulated their bitter taste response and suppressed their “sweet” response, which prevented them from feeding. The scientists were amazed to observe that when the mutant cockroaches tasted sugary foods such as jam, they actually jumped back, as if they’d been electrocuted. The mutants preferred to eat peanut butter, instead.
The evolutionary significance of this discovery is quite profound. To quote Nordqvist’s report again:
Corresponding author Coby Schal, the Blanton J. Whitmire Distinguished Professor of Entomology at North Carolina State, said:
“Most times, genetic changes, or mutations, cause the loss of function. 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 behavior which is incredibly adaptive. These roaches just got ahead of us in the arms race.”
I think it’s fair to say that a new kind of behavior which aids survival deserves to be called a gain of function. So, does this mean that Darwinian evolution is capable of generating new functions over the short space of 20 years? Not so fast. There’s another possibility: maybe the capacity lay dormant in the cockroaches’ genes all along (don’t forget, they’ve been around for 400 million tears):
Schal noted that it is also possible that the environmental adaptation could have started millions of years ago – before humans were around – when they avoided some plants that are capable of producing toxic, but sweet, compounds.
That’s an avenue of investigation that definitely sounds worth checking out.
One thing is quite clear: if the gain of function is confirmed, it would be the first of its kind in Nature, according to a report by Amina Khan (Los Angeles Times, 24 May 2013) (Scientists uncover a secret to cockroaches’ adaptability):
Losing a function in sight or taste is not uncommon in the animal kingdom — in fact, many marine mammals have lost their ability to taste sweet things, perhaps because they don’t encounter it in their fishy diet. But adding sensory information — setting off a “bitter” alarm for a sweet food — is another story.
“It’s incredibly rare” Schal said. “We don’t know any other example where instead of having a loss of function, you had a gain of a new function — and that’s what happened in this cockroach.”
ID Critics often complain that Intelligent Design theory doesn’t make scientific predictions, so I’ll invite readers to weigh in here. What predictions would you make about the genes of German cockroaches, on the basis of this discovery?
Over to you.