An Evaluation of Ideotype Breeding1

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Abstract
Plant breeders have attempted to enhance yield by selecting for individual traits since the beginning of plant breeding. This approach has been broadened to encompass the breeding of model plants or ideotypes. An ideotype is a hypothetical plant described in terms of traits that are thought to enhance genetic yield potential. Ideotype breeding is defined as a method of breeding to enhance genetic yield potential based on modifying individual traits where the breeding goal (phenotype) for each trait is specified. The purposes of this paper are to elaborate on and evaluate ideotype breeding as a method to enhance genetic yield potential and to describe an ideotype for barley (Hordeum vulgare L.). Successes that have occurred in breeding to enhance yield with individual traits, the value of genetic diversity for individual traits, and benefits from goal setting are presented as arguments in support of ideotype breeding. Alternatively, information is presented on the requirement of symmetry in size among plant parts, compensation among plant parts, pleiotropy, and genetic background, all factors that may slow progress in ideotype breeding. Ranges of genetic diversity, heritability estimates, and introgression information are presented for 27 barley traits. A barley ideotype consisting of 14 traits and the target or goal for each trait are described. Ideotype breeding is recommended as a methodology to augment traditional plant breeding, when the breeding goal is enhancing genetic yield potential. Breeding experience and research to date suggest that ideotype breeding is not a suitable substitute for traditional yield breeding.