Yale University, New Haven, Connecticut

An evolutionary geneticist looks at how small genetic changes can have big evolutionary effects.

Vertebrates are diverse in both form and function. What genetic alterations underlie this diversity? Evolutionary modifications in brain size or limb length, for example, involve changes in the complex developmental processes that give rise to these structures. Work in fish elegantly illustrates how a single genetic change can have profound effects on morphological evolution.

David Kingsley at Stanford University in California and his colleagues focused on threespine sticklebacks (Gasterosteus aculeatus), which exist in both marine and freshwater ecosystems (Y. F. Chan et al. Science 327, 302–305; 2010). Most bear pelvic spines on their underside; however, several freshwater populations have lost these structures.

In previous work, the authors suggested that the gene Pitx1 is involved in pelvic reduction in these fish. Now they show that deletion of a sequence that activates Pitx1 expression in the pelvis is directly responsible for the loss of pelvic spines in sticklebacks. In a simple but powerful experiment, they demonstrate that introducing the Pitx1 gene to these pelvic-reduced fish, under the control of the intact Pitx1 regulatory element, is sufficient to restore pelvic spines.

The Pitx1 regulatory deletion has occurred independently in at least nine stickleback populations. This may be because the Pitx1 regulatory element is in a particularly fragile region of the genome that is prone to deletion. Moreover, population-genetic evidence suggests that this recurrent loss of Pitx1 pelvic expression confers a strong fitness advantage — possibly because insects that prey on sticklebacks can grab onto pelvic spines.

This study illustrates the power of laboratory genetics in understanding evolutionary mechanisms, and by doing so provides a conceptual basis for future functional studies of the evolutionary process.

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.