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We think of DNA as the vitally important molecules that carry genetic instructions for most living things, including ourselves. But not all DNA actually encodes proteins; now we are finding more and more functions involving non-coding DNA scientists that scientists thought of as “junk”.
A new study suggests that satellite DNA – a type of non-coding DNA organized into long, repeating, seemingly insane chains of genetic material – could be the reason different species cannot reproduce successfully.
Satellite DNA appears to play a vital role in keeping all of a cell’s individual chromosomes together in a single nucleus, through the work of cellular proteins.
According to biologists Madhav Jagannathan and Yukiko Yamashita, authors of the new study, this important role is handled differently in each species, leading to genetic incompatibility. The clash of different strategies between species may be what causes the dispersion of chromosomes outside the nucleus, at least in part, preventing reproduction.
“We provide a unifying framework that explains how the widely observed satellite DNA divergence between closely related species can cause reproductive isolation,” they write in their paper.
This “satellite DNA divergence” has been well established in previous research, leading to suspicion of its role in speciation. In the case of the chimpanzee genome and the human genome, for example, the DNA encoding the protein is almost identical, while the “unwanted” DNA is almost entirely different.
In this new study, the fruit fly experiment Drosophila melanogaster, the researchers noticed that deleting a gene that produces a protein called Prod – which binds to a specific piece of satellite DNA – causes the flies to die because their chromosomes scatter outside the cell nucleus. However, this crucial piece of satellite DNA is completely absent from the closest relatives of flies, who survive very well without it.
This suggests that these important non-coding sequences of DNA material evolved differently between species. To take a closer look, the team looked at hybrid offspring from a D. melanogaster female and one male of the next of kin D. simulans species.
Flies reared this way usually die very quickly or end up sterile. In this case, an examination of the tissues of the hybrid offspring confirmed what the researchers suspected – that the chromosomes (the bundles of DNA needed for reproduction) were also disrupted here.
“When we looked at these hybrid tissues, it was very clear that their phenotype was exactly the same as if you had disrupted the chromosomal organization mediated by satellite DNA from a pure species,” says Yamashita, who works at the Massachusetts Institute. of Technology. (MIT).
“The chromosomes were scattered and not encapsulated in a single nucleus.”
Digging even deeper, the researchers produced healthy hybrid flies by removing genes known to damage hybrid offspring (called “hybrid incompatibility genes”) from their mother flies. These incompatibility genes are known to localize on satellite DNA in the pure species.
Satellite DNA mutates fairly regularly, and researchers believe the proteins that bind to satellite DNA to hold chromosomes together must evolve to keep pace. This then gives each species its own different strategy when it comes to satellite DNA operations.
Next, the team wants to try to design a protein that successfully binds to the satellite DNA of two species, keeping the chromosomes where they should be. This could allow viable offspring between these species, but it will take years to achieve.
“Our study lays the foundation for understanding hybrid incompatibility at the cellular level in Drosophila as well as other eukaryotes, ”the researchers write.
The research was published in Molecular biology and evolution.
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