DNA serves as the instruction manual for building and operating every living cell, guiding an organism’s development and functions. While humans often assume more complex organisms have larger amounts of genetic material, the reality of genome size across the animal kingdom challenges this belief. The animal holding the record for the most DNA is not what many might expect.
Understanding Genome Size and the C-Value Paradox
A genome represents the complete set of genetic instructions found in an organism’s cells. Its size, often referred to as the C-value, is typically measured in base pairs (bp) or in picograms (pg). For instance, the human genome contains approximately 3 billion base pairs.
The C-value paradox describes the observation that the amount of DNA in an organism’s genome does not directly correlate with its perceived complexity. This means organisms considered less complex, such as certain plants or single-celled eukaryotes, can possess significantly larger genomes than more complex ones like humans. Discoveries of non-coding DNA later helped explain this phenomenon, revealing that much of the genetic material does not directly code for proteins.
The Animals with the Largest Genomes
The animal kingdom’s record for the largest sequenced genome is currently held by the South American lungfish (Lepidosiren paradoxa). This air-breathing fish, found in slow-moving rivers and wetlands, possesses a staggering 91 billion base pairs of DNA. Its genome is more than twice the size of its relative, the Australian lungfish, which previously held the record with 43 billion base pairs. To put this into perspective, the South American lungfish’s genome is roughly 30 times larger than the human genome, with 18 of its 19 chromosomes individually exceeding the entire human genome in size.
Before the lungfish, certain salamanders were known for their exceptionally large genomes. The axolotl (Ambystoma mexicanum), a type of salamander, has a genome of about 32 billion base pairs, making it significantly larger than the human genome and a former record holder. Other salamander species, such as the Neuse River waterdog (Necturus lewisi), exhibit genomes up to 38 times the size of a human’s. Salamander genomes generally range from 10 to 120 billion base pairs, making them notable for their genomic gigantism among vertebrates. Their cells are considerably larger due to their expansive DNA content.
Factors Influencing DNA Content
The immense size of these genomes, particularly in animals like the lungfish and salamanders, is primarily attributed to the accumulation of non-coding DNA. This non-coding material includes repetitive sequences and transposable elements, often referred to as “jumping genes”. Transposable elements are DNA sequences capable of copying and inserting themselves into new positions within the genome, leading to significant expansion. In the South American lungfish, for example, autonomous transposons are largely responsible for its colossal genome, which has expanded at an exceptionally rapid rate over evolutionary time.
While some non-coding DNA plays important roles in gene regulation and maintaining genome structure, a large portion consists of these repetitive elements that do not directly code for proteins. The presence of these elements does not necessarily translate to increased biological complexity or a greater number of genes. Rather, the expansion can be due to a reduced efficiency in removing these sequences or less stringent control over their proliferation. In some organisms, polyploidy, a condition where cells contain multiple sets of chromosomes, can also contribute to an increase in overall DNA content.