Mechanisms of DNA sequence amplification and their evolutionary consequences.

DNA sequence amplification is a phenomenon that occurs predictably at defined stages during normal development in some organisms and has been shown to occur spontaneously, but sporadically, in a variety of cells, including mammalian cells, selected for overproduction of a gene product. Developmentally programmed gene amplification includes rDNA amplification during oögenesis in amphibia, chorion protein gene amplification in Drosophila and the chromosomal changes accompanying macronuclear formation in ciliates. Selected gene amplification is illustrated by mutant mammalian cells which have been selected in vitro or in vivo for the overproduction of a gene product. In these cells the unit of DNA that is amplified is much larger than the gene under selection, and appears to be formed by multiple recombination events, which bring together sequences not normally adjacent to each other. Often the product of amplification can be seen microscopically as aberrant chromosome forms. The vast majority of DNA amplification events occur in somatic nuclei, and thus would not have any direct effect on the evolution of a genome. However, the ability to amplify DNA in somatic cells does have consequences for the composition of the genomes of the organisms in which it can occur, and should DNA amplification occur, even sporadically, in germ-line cells the potential effect on evolution would be great.