What determines the size of species’ genomes?

The genome, the entire genetic material of an organism, is one of the keys to biological diversity. However, their size can vary surprisingly between species, from viruses with just a few dozen genes to plants with massive genomes. But what factors determine the size of genomes? Is size an indicator of complexity?

What is genome size?

Genome size is measured in base pairs (bp), the fundamental units of DNA. Some notable comparisons include:

• Virus: Viruses have small genomes; For example, that of the influenza virus is around 13,500 bp.
• Bacteria: Their genomes are larger, generally between 1 and 10 million bp.
• Humans: The human genome contains approximately 3.2 billion bp.
• Plants like Paris japonica: They have gigantic genomes, with more than 148 billion bp.

Surprisingly, genome size is not directly related to the complexity of an organism, a phenomenon known as C-value paradox.

Factors That Determine Genome Size

1. Coding Gene Content

Protein-coding genes make up a small fraction of the genome in most organisms. For example, in humans, only about 1.5% of the genome encodes proteins. In simpler organisms, such as bacteria, this proportion is much higher, which contributes to making their genomes more compact.

2. Non-Coding DNA

Non-coding DNA, often called “junk DNA”, plays a crucial role in genome size. This DNA includes elements such as:

• Introns: Non-coding sequences within genes.
• Repetitive sequences: Like microsatellites or transposons, which are DNA fragments that can replicate and move within the genome.
• Regulatory regions: Areas that control gene expression.

In complex organisms, non-coding DNA can make up more than 90% of the genome.

3. Transposons and Repetitive Elements

Transposons, also known as “jumping genes,” are segments of DNA that replicate and move within the genome. Its activity can significantly increase the size of the genome, as occurs in some plants and amphibians.

4. Gene and Genome Duplications

The duplication of genes or even entire genomes is another key factor. These duplications, common in plants, generate additional copies of DNA that can evolve to fulfill new functions or remain as redundancies.

5. Evolutionary Pressure and Environment

Genome size is also influenced by evolutionary needs. In organisms with rapid life cycles, such as bacteria, replication efficiency favors compact genomes. In contrast, organisms with less pressure to optimize resources, such as certain plants, can tolerate larger genomes.

Is Genome Size Indicative of Complexity?

Not necessarily. A large genome does not imply a more complex organism. For example:

• The genome of Paris japonica It is almost 50 times larger than the human, but this plant is not more complex than humans.
• Simple organisms, such as certain bacteria, have small genomes, but can be extremely efficient and adaptable.

Scientific Implications of Genome Size

Understanding what determines genome size has practical applications in biology and genetics:

1. Medicine: It helps identify genes related to diseases and design gene therapies.
2. Agriculture: It allows crops to be improved by selecting specific genetic characteristics.
3. Evolution: Provides information about the evolutionary history of species and adaptation mechanisms.