Scientists have recently achieved something quite remarkable by sequencing the genome of the South American lungfish, which boasts the largest animal genome ever discovered. With over 90 billion base pairs, this genome is not only astonishingly large but also approximately 30 times the size of the human genome.
This groundbreaking work was carried out by an international team led by Axel Meyer from the University of Konstanz and Manfred Schartl from the University of Würzburg. Their findings were published on August 14, 2024, in the prestigious journal Nature.
The lungfish, belonging to the order Dipnoi, is often referred to as a 'living fossil' because its physical traits have remained largely unchanged for about 400 million years. Three extant lineages exist today, found across Africa, South America, and Australia.
The significance of this species lies not just in its size but also its evolutionary history. It is considered the closest relative to the earliest four-limbed vertebrates, linking modern terrestrial animals with their aquatic ancestors.
Researchers are fascinated by the historical transition of lungfish from water to land, dating back to the Devonian period approximately 420 million years ago. During this era, these fish evolved to possess lungs, enabling them to breathe air and transition between aquatic and terrestrial life.
Through their extensive analyses, the research team discovered autonomous transposons playing a key role in the lungfish’s substantial genome expansion. These DNA sequences are known to replicate themselves and shift locations within the genome, prompting significant growth.
Interestingly, the South American lungfish has shown the fastest genome expansion rate recorded, growing the equivalent size of the entire human genome approximately every ten million years. This active genomic growth is attributed to the continued activity of these transposons, which remain present even today.
Each chromosome of the South American lungfish is massive, with 18 out of 19 chromosomes being larger than the entire human genome. This presents exciting opportunities for researchers to explore genetic traits and ancestral links through comparative genomic studies.
Despite the lungfish’s immense genetic material, the study provided surprising results concerning genome stability. Unlike what might be expected from such immense genome size and the presence of jumping genes, no connection was found between transposons abundance and instability, indicating a surprisingly conservative gene arrangement.
This genome analysis has advanced our insights on the genetic basis for various traits across existing lungfish lineages. For example, the Australian lungfish retains the limb-like fins of its ancestors, which facilitated their movement onto land, unlike its relatives from Africa and South America, which have evolved more slender, filamentous fins.
The researchers utilized modern experimental techniques, including CRISPR-Cas technology, to investigate the genetic underpinnings of these anatomical changes. They discovered alterations within the Shh-signaling pathway, which governs the development of digits and limbs.
Overall, the research team views their work as just the beginning of what can be achieved through more comprehensive analyses of lungfish genomes. With the groundwork laid for future studies, they are excited about untangling the mysteries surrounding the evolutionary progress of vertebrates transitioning from aquatic to land-based life.
Certainly, the remarkable timeline of the lungfish and its genome holds great potential for enlightening our comprehension of life’s evolutionary processes. Scientists expect continued exploration will lead to more discoveries about the origins of tetrapods and their adaptation strategies over the eons.
The funding for this extensive effort was provided by organizations including the German Research Foundation (DFG) and the Next Generation Sequencing Competence Network (NGS-CN). This collaborative endeavor emphasizes how international partnerships can lead to significant advancements within the field of genetics.
