In the spirit of Christmas, a time for giving, I find it fascinating that one concept perplexing many scientists is the number of species that help each other. This is called a mutualism in science speak, and is confusing to many scientists because the underlying rules of natural selection (Darwin, 1859) intuitively work against spending energy or valuable resources helping others. Remember, fitness= grow and reproduce (although this kind of fitness is also fun). Any trait involved in spending the currency on an unrelated organism that would otherwise go towards kids theoretically would not last for many generations (because it takes having kids to make a new generation with those traits). But mutualistic traits do last, even when other organisms evolve ways to cheat and take more than they provide, further baffling evolutionary ecologists.
Which is why I think this paper is so cool.
The scientists used a creative strategy to assess how plants deal with the extra loss of sugars without any return of nitrogen when the symbiotic bacteria in their root nodules cheat. The creative part is forcing bacteria to cheat: rhizobia, which take the inaccessible, triple-bonded nitrogen from the air and turn it into a useful, organic molecule for the plant are not able to provide this service when there is no nitrogen in the air! Dr. E. Toby Kiers and her team kicked out all of the nitrogen by flooding chambers with roots and nodules with oxygen, 20% (rhizobia need oxygen too), and argon, 80% – a stable gas that is heavier than nitrogen.
Plants with “cheating” rhizobia on their roots cut off the oxygen supply to those nodules!
This concept of punishing cheaters (aka host sanctioning) to maintain a fitness benefit from the relationship is a huge help in solving the evolutionary questions about two-way beneficial relationships (mutualisms!) in nature.
It’s kind of like how Santa keeps you on your best behavior so you don’t get coal in your stocking.
Merry Christmas (or whatever you celebrate)!