The smallest amount of genes ever found for a cellular organism comes from tiny symbiotic bacteria living inside special cells of a small insect, named Carsonella ruddii (photo above).
The bacteria's genome (the complete bulk of DNA for the organism), is only one-third the size of the previously reported "smallest" cellular genome. "It's the smallest genome -- not by a bit but by a long way," said co-author Nancy A. Moran, UA Regents' Professor of ecology and evolutionary biology and a member of the National Academy of Sciences.
"It's very surprising. We would not have predicted such a small size. It's believed that more genes are required for a cell to work."
Carsonella ruddii possesses only 159,662 base-pairs of DNA, which encode only
182 protein-coding genes. The human genome, by comparison, contains about 3 billion DNA base pairs encoding about 35,000 proteins. These bacteria live in plant sap feeding insects. Plant saps is rich in sugar, but lacks proteins, that's way the insects rely on Carsonella to get a balanced diet. The bacteria manufacture amino acids, and share the goodies with their hosts. Carsonella live inside the insect's cells and cannot survive on their own.
This biological relationship is called endosymbiosis and often the insect host cannot survive without its bacteria. The endosymbiosis can be so close and so ancient that the insects shelter their resident bacteria in special cells called bacteriocytes within specialized organs called bacteriomes. The genome of an organism encodes proteins that operate instructions for everything the organism needs to do to survive and reproduce.
Endosymbiotic bacteria live in an extremely stable world, so they need a simple set of instructions. They lost many of the metabolic pathways that free-living bacteria need to survive after so many generations of living within insects.
Carsonella lives inside a psyllid insect called Pachypsylla venusta (photo bellow). The genes that the bacteria lack are compensated for by its insect host. "It lost genes that are considered absolutely necessary. Trying to explain it will probably help reveal how cells can work," said Moran.
It is possible that in the bacteria's evolutionary past, some of its genes were incorporated into the insect's genome, allowing the insect to produce some of the metabolites necessaries for the bacteria. This way, the bacteria lost those genes.
Animal and plant cells have specialized structures inside them called organelles, with specific functions, and some are derived from endosymbiotic bacteria incorporated into the cell over the course of evolution. The organelles responsible for energy production, called mitochondria, are thought to have once been free-roaming bacteria that larger cells assimilated long ago. They still possess their own DNA.
Carsonella's small genome could indicate that one day it will lose its identity altogether and become an organelle.