For almost half a century, scientists have been perplexed by a peculiar feature in the genomes of eukaryotes, which include animals, plants, fungi, and protists. Eukaryotic genes are fragmented, with their protein-encoding information interrupted by intervening sequences called “introns.” These introns separate the “exons” that encode parts of the protein.
To express their genes, eukaryotic cells must splice out RNA from introns and stitch together RNA from exons, reassembling the protein recipe. This complex system has puzzled researchers and fueled debate for decades.
The Great Intron Variation Debate
The mystery deepens with the discovery that different branches of the eukaryotic family tree display significant variation in intron abundance. Yeast genes, for example, have few introns, while land plants have many, with introns making up nearly 25% of human DNA.
The evolution of such extreme, enigmatic variation in intron frequency has been a subject of intense debate among scientists.
Recent studies on genetic elements known as introners may provide answers to this longstanding enigma. Introners, regarded by some scientists as genomic parasites, can infiltrate genomes and multiply there, leaving a profusion of introns in their wake.
In November last year, researchers presented evidence that introners have been responsible for the explosive gains in introns throughout the evolution of diverse eukaryotes.
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Furthermore, they demonstrated that introners could explain why these gains were particularly common in aquatic life forms.
Introners are adept at slipping under the radar, evolving to minimize their impact on the host organism. If an introner disrupts the activity of the gene it has embedded itself in, it could harm the host organism, leading to its removal by natural selection.
To avoid this, introners continually evolve to have as little influence as possible, making them efficient genomic parasites.
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An Aquatic Connection: The Role of Horizontal Gene Transfer
Introners are more likely to appear in the genomes of aquatic organisms than in those of terrestrial organisms. This pattern can be explained by horizontal gene transfer, which involves the transfer of genetic sequences from one species to another.
Such transfers tend to occur in aquatic environments, where DNA can be easily taken up by organisms swimming through the water, or when eggs or sperm are released into the water.
The Evolutionary Arms Race and the Impact on Genome Complexity
The ongoing battle between invading genetic elements and host genomes may play a role in the evolution of genome complexity. Parasitic elements compete for genomic space with the host’s genetic elements, driving each other to evolve continually.
Introners could potentially promote the appearance of new genes, helping to drive the evolution of rapidly changing gene families.
The discovery of introners in a wide range of eukaryotes highlights the importance of studying the broader scope of biodiversity, beyond just animals and land plants.
By sequencing more eukaryotic diversity and examining protist lineages, researchers can uncover significant patterns of genomic information that underlie all life.
The study of introners serves as a reminder that focusing on a narrow range of species may hinder our understanding of essential aspects of life’s genetic complexity.
