''Nasuia deltocephalinicola'' was reported in 2013 to have the smallest

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

of all

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

, with 112,091

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...

s. For comparison, the human genome has 3.2 billion nucleotides. The second smallest genome, from bacteria '' Tremblaya princeps'', has 139,000 nucleotides. While ''N. deltocephalinicola'' has the smallest number of nucleotides, it has more

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

-coding

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s (137) than some bacteria.

Symbiotic relationship

''N. deltocephalinicola'' was discovered when leafhoppers and other

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...

- and

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

-feeding

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

s were investigated for

endosymbiotic An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

bacteria. The phloem and xylem of

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...

s are rich in

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...

s (in the form of sucrose) but lack

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids includ ...

s and

s. Lipids can be synthesized from carbohydrates; however, proteins require

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

, which is not commonly found in

plant sap Sap is a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, or cell sap; it is a se ...

. ''N. deltocephalinicola'' along with other bacterial endosymbionts help the insects by synthesizing 10 essential amino acids that they would not otherwise have. The only insects that can benefit from this relationship are those from the suborder ''

Sternorrhyncha The Sternorrhyncha suborder of the Hemiptera contains the aphids, whiteflies, and scale insects, groups which were traditionally included in the now-obsolete order "Homoptera". "Sternorrhyncha" refers to the rearward position of the mouthparts re ...

'', which feed off phloem, and those from the suborder ''

Auchenorrhyncha The Auchenorrhyncha suborder of the Hemiptera contains most of the familiar members of what was called the "Homoptera" – groups such as cicadas, leafhoppers, treehoppers, planthoppers, and spittlebugs. The aphids and scale insects are the othe ...

'', which feed off xylem. ''N. deltocephalinicola'' can synthesize two of the essential amino acids that these insects require. ''N. deltocephalinicola'' uses the UGA codon in its DNA to specify

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

instead of the

stop Stop may refer to: Places * Stop, Kentucky, an unincorporated community in the United States * Stop (Rogatica), a village in Rogatica, Republika Srpska, Bosnia and Herzegovina Facilities * Bus stop * Truck stop, a type of rest stop for truck d ...

as in most other organisms. The symbiotic relationship between ''N. deltocephalinicola'' and leafhoppers is proposed to have started at least 200 million years ago, when leafhoppers and spittlebugs diverged evolutionarily. This claim is supported by the fact that ''N. deltocephalinicola's'' closest bacterial relative is '' Zinderia insecticola'', which plays the same role for spittlebugs as ''N. deltocephalinicola'' does in leafhoppers. Leafhoppers return the favor by providing shelter in the form of a specialized organ in their abdominal cavity called a

bacteriome A bacteriome is a specialized organ, found mainly in some insects, that hosts endosymbiotic bacteria. Bacteriomes contain specialized cells, called bacteriocytes, that provide nutrients and shelter to the bacteria while protecting the host animal ...

, which they have on both sides of their

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

s. Many types of bacteria can reside in these organs, though the bacteria are completely separated from each other and reside in different sections of the bacteriome. ''N. deltocephalinicola'' is an obligate endosymbiont—it cannot thrive without being in a leafhopper. It is an intracellular endosymbiont, living within bacteriocytes, cells that are specialized for housing endosymbiotic bacteria. These bacteriocytes comprise an organ called a

, whose cells host a variety of bacterial endosymbionts. Intracellular endosymbionts may evolve to depend on the host cells for essential cellular functions. As a result, their genomes often lack genes that would be required for life in an extracellular environment, even one containing abundant nutrients. They have thereby begun the process of evolving from a free-living organism to an intracellular organelle. ''N. deltocephalinicola'' also no longer has genes needed to synthesize ATP through oxidative phosphorylation. It is proposed that this is because of the high sucrose concentration found in xylem and phloem of plants.

References

{{Taxonbar, from=Q18379313 Betaproteobacteria