Research: In search of U

A new estimate has been obtained of the rate at which harmful mutations arise. It comes tantalisingly close to explaining why we have sex.

The rate at which deleterious mutations occur is a key concept in evolutionary theory. This parameter, known as U, crops up in studies of genome evolution, the effects of inbreeding and the evolution of sexual reproduction.

In attempts to quantify U, biologists have either estimated the rate of accumulation of genetic errors affecting the fitness of inbred lines or compared related DNA sequences in different species. But both methods are problematic.

­In new work, a group led by Peter Keightley at the University of Edinburgh directly scanned 20 million bases of DNA from three initially inbred lines of Drosophila that had randomly accumulated spontaneous mutations for more than 200 generations. Using estimates of the proportion of mutations likely to be deleterious, the group was then able to calculate a figure for U.

The magic number is, with significant uncertainty, 1.2 (i.e. just over one deleterious mutation per genome per generation). This is higher than previous estimates, suggesting that selection against mutations has a major evolutionary role, and is sufficient, for example, to account for the evolution of recombination.

On the other hand, it is slightly lower than would be expected if, as is widely believed, obligate sexual reproduction has flourished because of its ability to rid the genome of harmful mutations. So sex maintains its mysteries for a little while longer…

External link

• Haag-Liautard C et al. Direct estimation of per nucleotide and genomic deleterious mutation rates in Drosophila. Nature 2007;445(7123):82–5.