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Which organisms have the largest and smallest genomes?
Dr. Matic Broz Computational chemist
Table of contents
Genomes vary across life by more than a million-fold. The smallest known cellular genome sits inside a bacterium that lives only inside insects and spans just 112,000 base pairs. The largest, reported in 2024 inside a modest fork fern from New Caledonia, contains about 160 billion base pairs. Between those extremes sit the human genome at roughly 3.2 billion base pairs, the lab bacterium E. coli at 4.6 million, and everything else.
Which organism has the largest genome?
The current record holder is the fork fern Tmesipteris oblanceolata (also recorded as Tmesipteris truncata), a small plant native to New Caledonia and eastern Australia. In May 2024, researchers reported that its genome contains roughly 160 billion base pairs (160 Gbp). That is more than 50 times the size of the human genome.[1]
Before this discovery, the largest known genome belonged to the Japanese canopy plant Paris japonica, a slow-growing woodland flower from sub-alpine Japan, at about 150 billion base pairs. That record stood from 2010.[2]
The largest animal genome belongs to the marbled lungfish (Protopterus aethiopicus), at roughly 130 billion base pairs. The largest known insect genome - that of the migratory locust (Locusta migratoria) - is about 6.5 billion base pairs, roughly twice the human genome.[8]
| Organism | Type | Genome size |
|---|---|---|
| Tmesipteris oblanceolata (fork fern) | Plant | 160,000,000,000 |
| Paris japonica (canopy plant) | Plant | 150,000,000,000 |
| Protopterus aethiopicus (marbled lungfish) | Animal (fish) | 130,000,000,000 |
| Locusta migratoria (migratory locust) | Animal (insect) | 6,500,000,000 |
| Homo sapiens (human) | Animal (mammal) | 3,200,000,000 |
| Escherichia coli | Bacterium | 4,600,000 |
An older study suggested that the amoeba Polychaos dubium might have a genome of roughly 670 billion base pairs - which would be more than four times larger than the fork fern - but that measurement was made before the advent of modern genomic techniques and has not been confirmed. Most researchers treat it with caution.[6]
The reason plants dominate the top of this list is partly that many of them are polyploid - they carry multiple complete copies of their genome. Paris japonica is thought to be octoploid (eight copies). Additionally, plant genomes accumulate large amounts of repetitive DNA and transposable elements without the same selective pressure to streamline that operates in many animal lineages.
Which organism has the smallest genome?
Among cellular organisms, the smallest known genome belongs to Nasuia deltocephalinicola, a bacterial endosymbiont that lives inside leafhoppers. Its genome spans just 112,031 base pairs and contains only 137 protein-coding genes.[3]
The runner-up is Carsonella ruddii, another bacterial endosymbiont found in psyllid insects, with a genome of 159,662 base pairs and 182 predicted genes - the fewest of any known cellular organism. Many genes considered essential for free-living bacteria are missing from its genome, and researchers suggest it may be evolving into an organelle rather than remaining an independent organism.[4]
The smallest known genome of a free-living organism belongs to Mycoplasma genitalium, a parasitic bacterium, at roughly 580,000 base pairs with about 521 genes. The smallest free-living bacterium not dependent on a host, Pelagibacter ubique - the most abundant organism in the ocean - has a genome of 1.3 million base pairs.
| Organism | Type | Genome size |
|---|---|---|
| Nasuia deltocephalinicola | Bacterial endosymbiont | 112,031 |
| Carsonella ruddii | Bacterial endosymbiont | 159,662 |
| Tremblaya princeps | Bacterial endosymbiont | 139,000 |
| Mycoplasma genitalium | Parasitic bacterium | 580,000 |
| Pelagibacter ubique (SAR11) | Free-living bacterium | 1,300,000 |
| Escherichia coli | Free-living bacterium | 4,600,000 |
Among eukaryotes (organisms with complex cells), the smallest known genome belongs to the microsporidian parasite Encephalitozoon cuniculi at roughly 2.9 million base pairs across 11 chromosomes. This fungus-like parasite has lost mitochondria and many metabolic pathways, relying entirely on its host for survival. It contains about 2,000 protein-coding genes, and many of its proteins are unusually short compared to their counterparts in other eukaryotes.[5]
Among viruses - if they are counted - the single-stranded DNA porcine circovirus type 1 has a genome of just 1,759 nucleotides. The well-studied bacteriophage Phi-X174 has a genome of 5,386 nucleotides.[10]
The tiniest genomes all belong to organisms that have abandoned free-living existence. Genome reduction occurs when a microbe becomes an obligate intracellular symbiont or parasite: genes for metabolism, DNA repair, and environmental sensing become unnecessary when the host provides those services, and they are gradually lost through deletion.[3]
Why do genome sizes vary so dramatically?
The million-fold range in genome sizes among cellular organisms has no simple relationship with organismal complexity. This is known as the C-value paradox - a term coined in the 1970s when it became clear that genome size does not predict how complex an organism is.
A common garden onion (Allium cepa) has a genome of about 16 billion base pairs - roughly five times the size of the human genome. A pufferfish (Takifugu rubripes) gets by with a genome roughly one-eighth the size of the human genome while having a similar number of genes.
Most of the variation comes from non-coding DNA: transposable elements, repetitive sequences, and ancient viral insertions. In the largest plant genomes, protein-coding genes make up a tiny fraction of the total DNA. The fork fern Tmesipteris oblanceolata and Paris japonica are not thought to have dramatically more genes than plants with far smaller genomes; they simply carry far more non-coding DNA.
At the other extreme, genome reduction is driven by the loss of unnecessary genes when an organism becomes dependent on a host. Nasuia deltocephalinicola has lost the genes needed to synthesize ATP through oxidative phosphorylation. Carsonella ruddii lacks many genes once thought essential for life. In both cases, the host cell compensates for these losses.[3][4]
There is also a loose correlation between genome size and cell size: organisms with larger genomes tend to have larger cells and slower cell division rates. This may explain why the largest genomes are found in slow-growing plants rather than in animals that require rapid cell turnover.
How does the human genome compare?
The human genome contains roughly 3.2 billion base pairs per haploid copy. It is about 50 times smaller than the largest known plant genome and roughly 29,000 times larger than the smallest known cellular genome.
In absolute terms the human genome is unremarkable. Many amphibians have larger genomes - the marbled lungfish carries roughly 40 times more DNA per cell than a human. Many plants carry genomes that dwarf ours.
What makes the human genome notable is not its size but the fact that it was the first mammalian genome fully sequenced, a project completed in 2003 after 13 years of work, and that it remains the best-annotated large genome on Earth.
Even within vertebrates, genome size varies substantially. Birds tend to have compact genomes, typically around 1 to 1.5 billion base pairs. Some salamanders carry genomes exceeding 100 billion base pairs. Among mammals, genome size is relatively stable, with most species falling between 2 and 6 billion base pairs.[8]
The human genome is neither large nor small by the standards of the tree of life. It sits in the middle - a reminder that biological complexity does not require an exceptionally large or small instruction set, just the right one.
Sources▼
- A 160 Gbp fork fern genome shatters size record for eukaryotes iScience · 2024. https://www.cell.com/iscience/fulltext/S2589-0042(24)01111-8
- The largest eukaryotic genome of them all? Botanical Journal of the Linnean Society · 2010. https://doi.org/10.1111/j.1095-8339.2010.01072.x
- Small, Smaller, Smallest: The Origins and Evolution of Ancient Dual Symbioses in a Phloem-Feeding Insect Genome Biology and Evolution · 2013. https://doi.org/10.1093/gbe/evt118
- The 160-kilobase genome of the bacterial endosymbiont Carsonella Science · 2006. https://doi.org/10.1126/science.1134196
- Genome sequence and gene compaction of the eukaryote parasite Encephalitozoon cuniculi Nature · 2001. https://doi.org/10.1038/35106579
- Genome diversity in microbial eukaryotes Trends in Ecology & Evolution · 2004. https://doi.org/10.1016/j.tree.2003.10.007
- Genome National Human Genome Research Institute · June 30, 2026. https://www.genome.gov/genetics-glossary/Genome
- Animal Genome Size Database Gregory TR · June 30, 2026. https://www.genomesize.com/
- Plant DNA C-values Database Royal Botanic Gardens, Kew · June 30, 2026. https://cvalues.science.kew.org/
- Porcine circovirus type 1 NCBI Genome · June 30, 2026. https://www.ncbi.nlm.nih.gov/genome/?term=Porcine+circovirus+1
