Evolution: An ancient balancing act


the gene pool of a population by balancing selection (in response to local adaptations, for example). Hittinger et al. now show that balancing selection also operates at the level of whole gene-networks. In the example studied, the well-characterized galactose (GAL) gene network in yeast is maintained in alternative genetic states within a species, a phenomenon the authors call a ‘balanced unlinked gene network polymorphism’ (BuGNP). Saccharomyces cerevisiae catabolizes galactose by using a network of seven genes. This network is conserved in the close yeast relative Saccharomyces kudriavezii, but it was thought to be non-functional in this species. To their surprise, Hittinger and colleagues found that whereas the Japanese strains of S. kudriavezii have indeed lost functionality, Portuguese strains have retained it. Having verified that they were still dealing with a single species, the authors showed that this functional polymorphism is very old indeed — about 89% as old as the species itself. Importantly, the authors showed both recent and extensive gene flow between the strains and elevated divergence peaks at the GAL network genes. Moreover, given the age of this network polymorphism, the persistence of GAL pseudogenes in the Japanese strains indicates that they are not just waiting to be removed by purifying selection. Instead, the authors propose that the non-functional alleles are fitter under some environmental conditions. The authors also provide insights into the selective pressure that maintains the network polymorphism: the introduction of pseudogenes into an otherwise functional network leads to a substantial disadvantage in the presence of galactose and, conversely, functional alleles are at a disadvantage in backgrounds in which only partial gene networks are constitutively expressed. Ways in which alternative allelic states could be maintained at multiple loci have been described previously and include tight linkage, chromosomal inversions, reduced gene flow and inbreeding. With this study, Hittinger et al. have added a new mechanism to this list and argue that, given the many examples of long-term balancing selection and complex genetic interactions, BuGNPs could be important for explaining the evolution of complex traits. The authors suggest that the mechanism is most likely to occur in species that have broad geographic distribution, strong population structure, large effective population size and limited sexual reproduction, as found in the malaria parasite Plasmodium, or limited outcrossing, as found in Arabidopsis species. Magdalena Skipper, Senior Editor, Nature

DOI: 10.1038/nrg2770

Cite this paper

@article{Skipper2010EvolutionAA, title={Evolution: An ancient balancing act}, author={Magdalena Skipper}, journal={Nature Reviews Genetics}, year={2010}, volume={11}, pages={238-239} }