Life Lines 59

SWAPPING GENES HELPS A PARASITE AND ITS HOST


Aphids are sometimes called plant lice. They are sometimes seen in large numbers sucking sap out of stems of leafy plants. The aphids don’t reveal to anyone but a scientist that they are also being parasitized by a bacterium. The bacterium, called Buchnera, has one of the smallest genomes of any cellular organism, a mere 460 genes. It lives in the cells of the aphid. The aphid, curiously, has one of the largest genomes of any cellular organism, over 35, 000 genes, almost twice that of a human. Only a few of its genes have been given to it by Buchnera. Most of its genes are due to numerous duplication of genes. Among the genes given by Buchnera to the aphid are genes for making amino acids. Eleven of these genes came from Buchnera to help make six amino acids. But for two of these amino acids the last step in making the amino acid is done by a gene of the aphid. This makes those two amino acids examples of a mutually beneficial system. Each helps the other for amino acids they both need and neither can survive without the presence of the other.

We often think of animals at war with each other and for predator and prey relations that is true. But many organisms form mutually beneficial relations. Most ecologists would argue that the world of life is interdependent, with all animals dependent ultimately on the foods made by plants through photosynthesis. There are more intimate relations than the groceries we select for our meals. Most trees have roots penetrated with bacteria or fungi that make nitrogen or other essential nutrients for the plant. In return the fungi or bacteria living in the roots have protection and nourishment.

I have often wondered why the public image [“nature red in tooth and claw”] of evolution is based on the predator-prey relation and not the mutualistic or beneficial interplay of organisms with one another. We need a Tennyson to interpret that more positive image of evolution for us. My own feeble effort of “nature rich in share and change” wouldn’t resonate to our emotions nor would it convey the powerful imagery of that bloody evolution as we imagine dinosaurs tearing hunks of flesh from their victims. There is far more mutual benefit going on in nature than an unceasing warfare between species. When we cut up land with roads and fences we quickly eliminate the migration of many species that will perish because they do not have territory to breed or forage. Our image is the hunter with the rifle but the reality for most deer, pheasants, and small mammals is the difficulty of finding food when human barriers isolate them. Organisms die from diseases and malnutrition more often than they do from carnivorous predators devouring them. Evolution is an outcome of both beneficial and horrifying ways to live. A grandmother making clothing is as beneficial as a young hunter bringing home game for all to eat. What is remarkable in the Buchnera-aphid story is the cooperation in amino acid synthesis that developed at a molecular level. It is a relatively recent event, not as ancient as our own cellular ancestry. All of our cells have mitochondria that were derived from bacteria in the early emergence of eukaryotic cells (cells with nuclei). Most of those bacterial genes entered the nuclei of our cells or were permanently lost as our mitochondria became transformed into oxygen breathing and energy producing organelles of our cells.