There are probably around 20,000 genes in the human genome that code for protein, and cells have to transcribe DNA sequences into RNA, which is then processed before it is translated into proteins by the cellular machinery. Part of that processing involved the removal of introns, which are portions of gene sequences that have to be spliced out before being translated into protein. Errors in splicing can lead to serious health problems.
But where do those seemingly unnecessary intronic genetic sequences come from? Scientists have been trying to learn more about the origins and importance of introns.
Introns are not the only unusual parts of the genome. Transposable elements (TEs), or so-called jumping genes, are another part of the genome that scientists are still learning about. TEs have ancient origins, and probably were once instrumental and essential to evolution, as the genome tested various types of sequence rearrangements, so nature could select the best changes.
One type of TE, so-called introners, can create introns when they are inserted into a section of the genome. Scientists have now learned that introners are able to move easily from one species to another in a type of horizontal gene transfer. Giant viruses may help these introners move. The findings have been reported in the Proceedings of the National Academy of Sciences (PNAS), and they could help explain the complexity of genomes and how we may be able to harness that complexity to benefit human health.
"[Introners are] a way that genome architectures and complexity arise, but not necessarily because there is natural selection that favors this complexity," said senior study author Russ Corbett-Detig, a professor at the University of California, Santa Cruz. "A few may ultimately benefit the host, but most are just cheaters that found a really good way to hide in the genome."
Introns are spliced out of a gene before a protein is made, but in many instances, there are multiple ways for one genetic transcript to be spliced. This so-called alternative splicing mechanism enables one genetic sequence to be used to make multiple different proteins. This dramatically expands the complexity of the genome, but it can also lead to problems when transcripts are not properly spliced.
After searching through the introns of many species, this study determined that there were almost 1,100 introner families in nearly 9,000 genomes that were analyzed. There may be many types of introners that spread introns from one species to another. These introners were found most often in algae, fungi, and single-celled eukaryotic organisms like a sea urchin.
The study identified direct evidence for the horizontal transfer of introners between a sea sponge and a type of marine protist known as a dinoflagellate. There is genetic data that about 40 million years ago, one of these species transferred an introner to the other. This may have happened with help from a giant virus, the researchers suggested. Another handful of examples were found, but the investigators noted that this is probably only the tip of the iceberg.
"Because transposons are insanely diverse and present in basically every eukaryote, it implies that this really can be a very general way that new introns arise in different lineages," Corbett-Detig said. "Given how little of eukaryotic diversity we've sampled, I promise you that if we sampled the rest of them, we'd find many more."
Sources: University of California - Santa Cruz, Proceedings of the National Academy of Sciences (PNAS)