Researchers at Duke University Medical Center and at the National Institute of Environmental Health Sciences (NIEHS) have shown how broken sections of chromosomes can recombine to change genomes and spawn new species.
"People have discovered high levels of repeated sequences in the genomes of most higher species and spun theories about why there are so many repeats," said Lucas Argueso, Ph.D., a research scholar in Duke's Department of Molecular Genetics and Microbiology. "We have been able to show with yeast that these repeated sequences allow the formation of new types of chromosomes (chromosome aberrations), and represent one important way of diversifying the genome."
The scientists used X-rays to break yeast chromosomes, and then studied how the damage was repaired. Most of the chromosome aberrations they identified resulted from interactions between repeated DNA sequences located on different chromosomes rather than from a simple re-joining of the broken ends on the same chromosome.
Chromosome aberrations are a change in the normal chromosome complement because of deletion, duplication, or rearrangement of genetic material. On rare occasions, the development of one of these new chromosome structures is beneficial, but more often DNA changes can be detrimental, leading to problems like tumors.
"Every so often the rearrangements may be advantageous," Argueso said. "Those particular differences may prove to be more successful in natural selection and eventually you may get a new species."
The radiation-induced aberrations in yeast were initially detected by co-author Jim Westmoreland in the NIEHS Laboratory of Molecular Genetics and the molecular dissection was done by Duke's Argueso.
In the yeast used for this study, the repeated DNA sequences account for about 3 percent of the genome. In higher species, like humans, about half of the genome consists of these repeated sequences, "which makes for an Achilles heel among humans," Argueso said. "If you have a break in this repeated part, you can repair not only from the same chromosome, but also from a similar repeated sequence in many other places in the genome."