DNA of lungfish decoded: 30 times the size of the human genome
The decoding of the genome of the South American lungfish promises insights into the evolution and adaptation of vertebrates to life on land.
The South American lungfish has the largest genome of all animal species
(Image: Katherine Seghers, Louisiana State University / Bearbeitung heise online)
An international team of researchers from the USA, Brazil, France, Sweden and Austria, led by evolutionary biologist Axel Meyer from the University of Konstanz and biochemist Manfred Schartl from the University of Würzburg, has decoded the largest animal genome known to date. The scientists sequenced the genome of the South American lungfish (Lepidosiren paradoxa), which with over 90 billion base pairs is about 30 times as large as the human genome (3.1 billion base pairs). Until now, the genome of the Australian lungfish (Neoceratodus forsteri) was considered the largest, but the South American lungfish is more than twice as large.
"Jumping genes" in the huge genome
The results of the study published in the journal Nature promise insights into how the fish-like ancestors of today's terrestrial vertebrates were able to make the transition from aquatic to terrestrial life around 400 million years ago.
"18 of the 19 chromosomes of the South American lungfish are each larger than the entire human genome with its almost 3 billion bases," explains Professor Meyer. Responsible for the enormous size of the genome are so-called transposons – DNA sequences that can multiply in the genome and change their position (transposition). In genetics, transposons are therefore also known colloquially as "jumping genes".
Unchecked genome growth
The researchers discovered that the genome of the South American lungfish is expanding at the fastest rate known to date: Every ten million years, it grew by the size of the entire human genome. "And it continues to grow," explains Meyer. "We have found evidence that the transposons responsible for this are still active."
Surprisingly, despite the enormous genome size and the many jumping genes, the team found that the genome was remarkably stable. The gene arrangement proved to be unexpectedly conservative, according to the researchers. This made it possible to reconstruct the original chromosome structure of the common ancestor of all terrestrial vertebrates.
Evolutionary findings
The comparison of the genomes of various species of lungfish living today also provided insights into the genetic basis of their differences. The Australian lungfish, for example, still has articulated fins that once enabled its ancestors to move on land. In African and South American species, these fins have reverted to thread-like structures over the course of evolution.
"In our research, we were able to show through experiments with CRISPR-Cas transgenic mice that this simplification of the fins is due to a change in the so-called Shh signaling pathway," explains Meyer. Among other things, this signaling pathway controls the number and development of the fingers during embryonic development in mice. The research results thus provide further evidence for the evolutionary connection between the fin rays of bony fish and the fingers of land vertebrates.
According to the researchers, the now available complete genome sequences of all current lungfish families would open up new possibilities to better understand the evolution of the first land vertebrates. Further comparative genome studies should provide additional insights in the future and help to solve the puzzle of how vertebrates came to land.
(vza)