(ORDO NEWS) — The new study has pointed to transposable elements that can move and replicate within the genome as the most likely source of the vast majority of introns, non-coding regions of eukaryotic DNA.
The discovery helps explain the huge differences in the number of introns in the genomes of different species.
One of the longstanding fundamental mysteries of modern biology is related to the origin of introns – non-coding regions of eukaryotic DNA that are removed during the splicing process before protein synthesis.
There is a huge difference in the number of introns found in the genomes of different, even closely related species, and as a rule, the length of the intron sequence is 10-100 times the length of the DNA coding sequences.
A new study by scientists at the University of California, Santa Cruz (USA) and published in the journal Proceedings of the National Academy of Sciences (PNAS) points to introners as a putative source of introns.
Introners are a newly discovered type of transposable genome elements that are able to replicate and move along DNA.
According to the authors, introners are also the only likely explanation for the events when thousands of introns appeared in the genome of one species, seemingly at the same time.
The introns themselves allow for alternative splicing, which allows a single gene to encode multiple transcripts and perform multiple cellular functions.
Introns can also affect gene expression, but they also have a negative effect: when splicing is done incorrectly, gene products can be damaged.
The scientists analyzed the genomes of 3,325 eukaryotic species to find out how common introns derived from introners are and in which species groups they are most common.
The researchers found introns derived from introners in 5.2% of the species studied, and they were common in all types of eukaryotic organisms studied, whose last common ancestor lived more than 1.7 billion years ago.
This suggests that introners represent the fundamental and most abundant source of introns.
The study also showed that marine organisms are 6.5 times more likely to have introners in their genome than terrestrial species.
This is likely due to horizontal gene transfer, in which genes are passed from one species to another. This process most often occurs in the marine environment between unicellular organisms.
Since all species are descended from marine life, it is possible that terrestrial species acquired introns very early in their evolutionary history.
In addition, many introners were also found in fungi, which have higher rates of horizontal gene transfer, which once again confirms the scientists’ assumption.
The classical theory of genome evolution states that, at some point in evolution, many species had low effective population sizes, that is, very few organisms produced offspring. This led to the accumulation of useless or harmful elements in their genome.
Thus, according to this theory, useless or slightly harmful introns would be more common in populations with low effective abundance. But the researchers found the opposite.
Thus, the Symbiodinium protist, which is characterized by a higher effective population size than animals and land plants, has the largest number of introns in its genome among the studied species.
The neutral or negative effect of introns is evidenced by their influence on gene expression.
The comparison showed that the level of expression of genes containing introns is lower than that of genes without introns, that is, they are less often turned on to perform functions in the body.
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