Chapter 4: ActivityLiterature: Genetic Redundancy |
Article Summary–Genetic Redundancy
Nowak, M. A., M. C. Boerlijst, J. Cooke, j. Maynard Smith. 1997. evolution of genetic redundancy.
Nature 388:167-171.
Gene duplications are generally believed to result in unstable new genes. Because the original gene is still functioning, the new copy will accumulate mutations, potentially resulting in a gene with new functions (Ch. 4, p. 86). One exception to this pattern occurs in the case of concerted evolution, where selection favors the ability to produce many copies of a single product. However, apparently redundant genes are widespread in development, immune systems, neural processes and the cell cycle. It seem unlikely, that these redundant genes all arose through selection for large volumes of product. Nowak and his collaborators describe a series of models that clearly illustrate how combinations of mutation rates, multiple functions, and errors in developmental processes can result in situations where duplicate genes persist without accumulating mutations and diverging in function.
Imagine that there are two genes, A and B, that each perform some function with equal efficiency. If the mutation rates in both genes are exactly equal then both copies of the gene will persist. It seems unlikely that mutation rates would be exactly equal in the two genes. If they are not equal, the gene with the higher mutation rate accumulates mutations and is eliminated from the population (unless it acquires a new function). Note that this can take a long time if the mutation rate is low. Now consider a case where gene B has lower efficiency than gene A but also has a lower mutation rate. In this case, both genes will persist in the population, without accumulating mutations. The paper also shows how pleiotropy and errors during development can lead to conditions where duplicate genes will persist. If there are errors during development that result in death, then selection may favor redundancy. Consider a gene that is in the germ line without mutation but does not work properly in somatic cells or tissues. In a mature animal this might not matter too much, but in a developing embryo, it could lead to a major abnormalities or death. Under these circumstances, selection can favor genetic redundancy. This is an important paper because it presents old and new explanations for genetic redundancy in an elegant analysis.