Sex and U.

Resolution of several unsettled problems in genetics depends on the genomic rate of deleterious mutation, U. Selection against mutations can be a major factor in evolution only if U > or =1. Recently, significant progress has been made in measuring U in multicellular eukaryotes. An indirect estimate, based on a human-chimpanzee pseudogene comparison, produced U>3 for hominoids. By contrast, an estimate for Drosophila based on comparison of synonymous protein-coding sites produced U<0.1. However, the Drosophila figure might be underestimated because of selection at synonymous sites. Perhaps, the best way to measure U is to observe mutations shortly after they appear. So far, this direct approach has been applied only to humans and Caenorhabditis elegans, yielding high estimates of mutation rates.