Symbiosis with Bacteria Drives Evolution of Multicellular Life
Insects that survive on plant sap alone offer insights into the likely origin and evolution of all multicellular life.
Suppose you were imprisoned in a room with no food supply except for a huge trough of maple syrup. How long do you think you could survive? Sure, the syrup would provide plenty of energy for basic bodily functions, but it would perhaps be only a few months until scurvy or other nasty diseases of malnutrition ravaged your body. Without the ability to somehow produce vitamins and amino acids necessary for survival, consuming a food composed of just sugar and a few minerals likely wouldn’t sustain you for even a year.
Yet many animals do survive on very limited diets, and they have no more ability than you do to produce the basic building blocks of life. Last week, microbiology researcher Ryan Kitko pointed out that the candy-stripe leafhopper thrives while consuming only the xylem and phloem of plants—sap. So how do sap-sucking insects like leafhoppers and aphids survive? Kitko points to two studies on a type of leafhopper commonly known as sharpshooters. Researchers found cells in sharpshooters that were jam-packed with bacteria, which converted the raw materials from sap into the vitamins and amino acids the insects need to survive.
The glassy-winged sharpshooter has two different resident bacteria, each of which creates different nutrients for the host insect from its base diet of plant sap. The bacteria are transmitted directly from the mother to her eggs, so young insects hatch with all the apparatus they need to live on plant sap alone. The bacteria, in turn, have very limited genomes. They wouldn’t be able to survive without the host insects to provide protection and a ready supply of food. In fact, the two bacteria that provide nutrients for the sharpshooter themselves have complementary genomes, each having lost formerly essential sections of their genome now found in the other. The bacteria not only produce nutrients for the host, but also depend on each other’s presence to get the nutrients they themselves need.
Most biologists now believe that complex cells with nuclei—eukaryotes—originated from simpler cells by combining and integrating the functions of those cells. A eukaryotic cell’s mitochondria, which convert food to more readily usable energy sources, or a eukarotyic cell’s chloroplasts, responsible for photosynthesis, are both believed to derive from what once were separate and distinct organisms. Similarly, the bacteria in leafhoppers have become essential for the leafhoppers’ survival, reproducing along with the insects themselves.
Perhaps not surprisingly, mutualistic relationships between bacteria and host organisms aren’t limited to insects. The biochemist who blogs as “Lab Rat” points to another such relationship, which allows plants to use otherwise inaccessible nitrogen in the atmosphere. While the importance of the relationship between plants and the bacteria in root nodules has been recognized for years, recent advances in genome sequencing have helped shed light on how the relationship may have evolved. Lab Rat cites a review published last month by Christina Toft and Siv Andersson in Nature Reviews Genetics. Toft and Andersson say that as a mutualistic relationship becomes more advanced, genomes in mutualistic bacteria become progressively smaller.In the early stages of a mutualistic relationship, a bacterium must be able to survive on its own in addition to within a host organism. In fact, its genome might become more complex as it develops the means to interact successfully with its host. But once the bacterium is completely dependent on its host, the genes and apparatus that allowed it live independently become unwieldy vestigial baggage. The research on genome size in these bacteria backs this up: Bacteria that are fully dependent on their hosts have significantly smaller genomes than those that are also able to live on their own.
After nearly a billion years of evolution, the mitochondria in your cells bear small resemblance to their bacterial ancestors, but they still possess their own DNA and the ability to reproduce. Their tiny genome contains just enough information to preserve their function, independent of the vastly larger genome of your cell nucleus. Without the protection of your body and the food delivered by your digestive and circulatory systems, your mitochondria couldn’t survive. But without mitochondria, all eukaryotic life as we know it, let alone plants and animals, would be impossible. While it’s impossible to reconstruct the evolution of mitochondria with certainty, other mutualistic bacteria can give us insight into how mitochondria may have evolved.
And while it seems unlikely that humans will ever develop the ability to survive on syrup alone, insects that thrive on an equally simple diet can still show us something about how human life evolved, with all its diverse tastes and needs.
"Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination." Max Planck