Adaptive mutation: A general phenomenon or special case?

A recent article by Galitski and Roth((1)) characterizes adaptive reversion of chromosomal lac(-) mutations in Salmonella typhimurium LT2. Using a classical genetic approach they show that adaptive reversion, as characterized by the appearance of late revertant colonies, is an exception rather than a general phenomenon for reversion of nonsense, missense, frameshift and insertion mutations. For certain mutations, however, the number of late revertants exceeds the predicted number. These excess revertants suggest that adaptive mutability is applicable to chromosomal genes as well as to genetic changes involving F plasmids and lysogenic phages.