Sex chromosomes in animals are so ancientin the hundreds of millions of years old--that they retain few traces of the historical events that drove their evolution. But the researchers had found in earlier studies that the mating type chromosomes in the self-fertilizing fungus, Neurospora tetrasperma, which are analogous to X and Y in sexually reproducing organisms, have a region of suppressed recombination that is roughly as recent as the split between chimpanzees and homininsless than six million years old.
Suppressed recombination preserves the genomic landscape, because normally, chromosomes recombine during mating, which shuffles the genes like a deck of cards. But suppressed recombination also interferes with natural selection, by forcing genes to be selected or deselected in packages, like the packages of options on new cars that force you to buy the navigation system, the satellite radio, and the MP3 system if you want the side curtain airbags.
The Uppsala researchers' major discovery is that many preferred codons disappeared from regions of the mating type chromosomes where recombination was suppressed. Codons are the "words" of the genetic code. Different codons code for each of the 20 amino acids used in living systems. They code for amino acids, which are the building blocks of proteins, the molecules that form both most of the structure, and most of the machinery cells. Like words in human language, codons often have synonyms. But "Many organisms studied to date preferentially use a specific set of preferred codons which are believed to promote efficient and accurate protein synthesis," says corresponding author Hanna Johannesson. Thus, they are known as "preferred codons."
The suppressed recombination the researchers had found earlier in N. tetrasperma is accompanied by the loss of th
|Contact: Jim Sliwa|
American Society for Microbiology