Principles of Plant Genetics and Breeding

480 CHAPTER 27
Completion of the trait introgression to the BC 6 generation not only provides superior cultivars for immediate use by growers,
but also provides a unique set of near-isogenic experimental materials to rigorously address scientific questions in wheat breeding
and wheat genetics. One such question regards the potential costs of pest-resistance genes in the absence of pest infestation.
Several experiments have shown that some disease-resistance genes in plants come at a cost in fitness, or yield potential. In
general, however, measurement of cost has been difficult due to the lack of defined genetic populations. The development of
isogenic lines carrying different genes affecting quality in the same genetic background will facilitate studies of epistatic interactions.
A cross between two isogenic lines in the same recurrent parent generates a mapping population segregating only for the
targeted genes in an isogenic background. Current public awareness of biotechnology has been shaped by the GMO debate. This
is unfortunate because genetically modified organisms are only one aspect of biotechnology. MAS is a valuable biotechnological
tool for selection and reassembly of genes that already exist in Triticeae.
References
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Chen, X., M.A. Soria, G. Yan, J. Sun, and J. Dubcovsky. 2003. Development of user-friendly PCR markers for wheat stripe rust
resistance gene Yr5. Crop Sci. 43:2058–2064.
Dubcovsky, J., and M.A. Soria. 2005. MASWheat: Bringing genomics to the wheatfields. Available at http://
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Verified 8-Jul-05.
Field nursery
Establishing a breeding nursery
Layout
A wheat breeding nursery may contain various materials
such as parental genotypes with desired traits, elite lines,
special genetic stock, and sterility sources. It is advantageous
to locate the crossing block close to the F1 hybrid
nursery to facilitate backcrossing, topcrossing, and hybrid
parent comparisons. Wide between-row and within-row
spacing facilitates crossing and promotes good plant
growth and development.
Planting
Researchers often raise their seedlings at a plant spacing
of 30–60 cm between rows and 15 cm within rows for
good tillering. Planting dates should be selected such that
flowering is synchronized for crossing. The growth environment
may also be modified to synchronize flowering.
Some breeders of, in particular, winter wheat, clip the
plants (not below the spike primordial to avoid killing it).