Over a series of papers spanning several years, however, Whitehead Institute Member Yukiko Yamashita and colleagues have made the case that satellite DNA is not junk, but instead has an essential role in the cell: it works with cellular proteins to keep all of a cell's individual chromosomes together in a single nucleus.
Now, in the latest installment of their work, published online July 24 in the journal Molecular Biology and Evolution, Yamashita and former postdoctoral fellow Madhav Jagannathan, currently an assistant professor at ETH Zurich, Switzerland, take these studies a step further, proposing that the system of chromosomal organization made possible by satellite DNA is one reason that organisms from different species cannot produce viable offspring.
"Seven or eight years ago when we decided we wanted to study satellite DNA, we had zero plans to study evolution," said Yamashita, who is also a professor of biology at the Massachusetts Institute of Technology and an investigator with the Howard Hughes Medical Institute. "This is one very fun part of doing science: when you don't have a preconceived idea, and you just follow the lead until you bump into something completely unexpected."
The origin of species: DNA edition
Researchers have known for years that satellite DNA is highly variable between species. "If you look at the chimpanzee genome and the human genome, the protein coding regions are, like, 98 percent, 99 percent identical," she says. "But the junk DNA part is very, very different."
"These are about the most rapidly evolving sequences in the genome, but the prior perspective has been, "Well, these are junk sequences, who cares if your junk is different from mine?'" said Jagannathan.
But as they were investigating the importance of satellite DNA for fertility and survival in pure species, Yamashita and Jagannathan had their first hint that these repetitive sequences might play a role in speciation.
When the researchers deleted a protein called Prod that binds to a specific satellite DNA sequence in the fruit fly Drosophila melanogaster, the flies' chromosomes scattered outside of the nucleus into tiny globs of cellular material called micronuclei, and the flies died. "But we realized at this point that this [piece of] satellite DNA that was bound by the Prod protein was completely missing in the nearest relatives of Drosophila melanogaster," Jagannathan said. "It completely doesn't exist. So that's an interesting little problem."
If that piece of satellite DNA was essential for survival in one species but missing from another, it could imply that the two species of flies had evolved different satellite DNA sequences for the same role over time. And since satellite DNA played a role in keeping all the chromosomes together, Yamashita and Jagannathan wondered whether these evolved differences could be one reason different species are reproductively incompatible.
"After we realized the function [of satellite DNA in the cell], the fact that satellite DNA is quite different between species really hit like lightning," Yamashita said. "All of a sudden, it became a completely different investigation."
Entire article available here.
