Lettuce is a major vegetable in western countries. Mutations generated genetic variations and played an important role in the domestication of the crop. Many traits derived from natural and induced mutations, such as dwarfing, early flowering, male sterility, and chlorophyll deficiency, are useful in physiological and genetic studies. Mutants were also used to develop new lettuce products including miniature and herbicide-tolerant cultivars. Mutant analysis was critical in lettuce genomic studies including identification and cloning of disease-resistance genes. Mutagenesis combined with genomic technology may provide powerful tools for the discovery of novel gene alleles. In addition to radiation and chemical mutagens, unconventional approaches such as tissue or protoplast culture, transposable elements, and space flights have been utilized to generate mutants in lettuce. Since mutation breeding is considered nontransgenic, it is more acceptable to consumers and will be explored more in the future for lettuce improvement. 1. Introduction There is an increasing demand by consumers for nutritious foods that improve physical performance, reduce risks of diseases, and increase the life span. Lettuce (Lactuca sativa L.), commonly found in salad mixtures and sandwiches, is an important component in western diet and nutrition. Lettuce was the second most consumed fresh vegetable in the USA at 28.0 pounds per capita in 2008, behind potato at 36.7 pounds [1]. Lettuce belongs to the family Asteraceae (Compositae), tribe Cichorieae. Of about 100 species of Lactuca, only three (L. serriola L., L. saligna L., and L. virosa L.) can be crossed to lettuce by conventional hybridization methods and thus form the most important breeding group. They are all self-fertilized diploids with 2n = 2x = 18 chromosomes. There are five major types of lettuce: crisphead (iceberg), romaine, leaf, butterhead, and stem. Stem lettuce is mainly produced in China. In 2010, 58% of the lettuce production in the USA was of the head type, 29% was romaine, and 13% was leaf types [2]. Genetic variation is very important for any crop breeding program. The inbred nature of lettuce dictates the relatively limited genetic variability in the crop as compared to cross-pollinated crops. Mutation is a valuable tool to create novel traits for lettuce plants and can be classified as natural mutations and induced mutations (Table 1). Natural mutations are still occurring in the crop and its wild relatives, though at a low rate, and resulting beneficial characters can be selected for human needs. Mutagenic
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