European Bioinformatics Institute, Cambridge, UK
A computational geneticist looks at mechanisms of chromosomal evolution.
Humans are unique among great apes in having 23 pairs of chromosomes, a result of the fusion of two ancestral chromosomes: chimpanzees and other great apes have 24. Although the differences between the human and chimp genomes greatly exceed this one event, I find it hard to resist the idea that this single, major change kicked off an evolutionary process that eventually led to Homo sapiens.
The evolution of karyotype — the name given to a species’ collection of chromosomes — is normally a slow process. Although mammals exhibit an inter-chromosomal shuffling event on average once every 4 million years, not all primates have exhibited such restraint. Gibbons, humans’ closest relatives beyond the great apes, have a rate of chromosomal rearrangement that is 20 times higher than that of other primates. So why does this reorganization happen so much more often for some species than others?
Lucia Carbone of the Children’s Hospital and Research Center Oakland in California and her team have proposed an intriguing answer. They analysed the human and gibbon genomes and found specific sites at which gibbons had lower levels of DNA methylation (the addition of methyl groups to DNA) than humans (L. Carbone et al. PLoS Genet. 5, e1000538; 2009). The researchers theorize that this lack of methylation led to the rapid evolution of the gibbon karyotype by creating more open regions of the chromosome, which are more likely to recombine with other genetic elements.
This result could affect our understanding not only of genome evolution, but also of the pathogenesis of diseases such as cancer. The fundamental methylation mechanisms behind rapid chromosomal evolution may also be linked to the karyotype disruptions that are associated with some cancers.