NATURE OF DELETERIOUS MUTATION LOAD IN DROSOPHILA

Much population genetics and evolution theory depends on knowledge of genomic mutation rates and distributions of mutation effects for fitne ss, but most information comes from a few mutation accumulation experi ments in Drosophila in which replicated chromosomes are sheltered from natural selection by a balancer chromosome. I show here that data fro m these experiments imply the existence of a large class of minor viab ility mutations with approximately equivalent effects. However, analys is of the distribution of viabilities of chromosomes exposed to EMS mu tagenesis reveals a qualitatively different distribution of effects la cking such a minor effects class. A possible explanation for this diff erence is that transposable element insertions, a common class of spon taneous mutation event in Drosophila, frequently generate minor viabil ity effects. This explanation would imply that current estimates of de leterious mutation rates are not generally applicable in evolutionary models, as transposition rates vary widely. Alternatively, much of the apparent decline in viability under spontaneous mutation accumulation could have been nonmutational, perhaps due to selective improvement o f balancer chromosomes. This explanation accords well with the data an d implies a spontaneous mutation rate for viability two orders of magn itude lower than previously assumed, with most mutation load attributa ble to major effects.