Two men walk into a bar. The first man says “I would like a glass of H2O.” The second man says “I would like a glass of H2O too.”
It’s funny because H2O2 is the chemical name for hydrogen peroxide, and hydrogen peroxide is inimical to life. I don’t recommend drinking it.
Is this joke chemical or biological? I think it is both: it is about the interaction between chemical toxins and this thing called life. But there is another level of biological organisation that is important to the outcome. The person that tends the bar is an important component of the equation.
What will the barmaid do? Will she hear the “too” and give the second man water? Or will she hear the “two” and serve a clear liquid that is lethal? Perhaps she has good reason to serve water instead of peroxide. She might not have any on the shelf, or the bar may have a policy of locking up dangerous liquids, so as not to harm themselves or their customers.
A recent paper describes a natural situation where the barmaid’s decision renders the environment benign. The story takes place in the open ocean, where single cells known as plankton produce energy and oxygen through photosynthesis. Excess oxygen tends to increase the levels of H2O2. These tiny organisms protect themselves by releasing a protein called catalase-peroxidise, which breaks down hydrogen peroxide.
But one crafty bacterioplankton known as Prochlorococcus takes advantage of the other plankton by letting them do the costly energetic work of making the open ocean safe for cells. Prochlorococcus has lost the gene for catalase-peroxidase.
Gene loss is usually an undesirable outcome, most often occurring through a process known as drift. Drift happens by accident, and rarely increases the survival prospects of the organism. But this is a case in which the gene loss is adaptive, and not accidental. Natural selection has responded to the low nutrient environment by choosing cellular efficiency. This decision is possible because of other plankton species alter the chemistry of the ocean to make it safe for themselves.
The authors call it the Black Queen Hypothesis, after the Queen of Spades in the game of Hearts. Getting passed the queen drastically raises your points in a game where the low score wins. Prochlorococcus has lowered their own score by reducing their energetic output, and they have done this by passing the cost on to their distantly related cousins.
This species could shed an essential gene because the other organisms in the environment act like a friendly barmaid. Customers are served water even when they accidentally order hydrogen peroxide.
Contrast this situation to one where the other organisms are hostile. If the barmaid is likely to serve hydrogen peroxide, then species need to keep on their toes and hold onto their genes. Too often the biological component of the environment is antithetical to a species’ survival. Predators, competitors and parasites are always looking for ways to take advantage.
This evolutionary scenario is known as the Red Queen Hypothesis. The Red Queen in Alice in Wonderland said, “It takes all the running you can do, to keep in the same place.” The fact that a species’ enemies are evolving as fast as they can to feed on them creates an environment in which every new gene is potentially critical.
Just last year, some researchers found that parasitism may be responsible for sex. And you thought parasites never did anything for you! The Red Queen produces an environment in which new genes are favoured, and sex is important because it is designed to create new genetic combinations at every generation.
Evolution is about adaptation to the environment, but the environment is not limited to air, water, and soil. The environment includes other living things that affect our own survival as surely as the presence of toxins or the availability of water.
It all depends on whether your barmaid is a Black Queen, who takes the Queen of Spades from your hand, or a Red Queen, who looks for every opportunity to do you harm.
So watch what you drink (water is good), surround yourselves with friends to whom you can sometimes pass the costly black card, and try to frequent bars with friendly staff.
Morris JJ. Lenski RE. and Zinser, ER., The Black Queen Hypothesis: evolution of dependencies through adaptive gene loss, mBio 3 March/April 2012.
Levi T. Morran, Olivia G. Schmidt, Ian A. Gelarden, Raymond C. Parrish II, Curtis M. Lively, Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex. Science, 2011; 333 (6039): 216-218