Sexual reproduction has many barriers, such as hybrid infertility, that prevent the use of wild germplasm in plant breeding. Mutagenesis can be used to induce, or create, new genetic variability.
Radiation, such as gamma-rays, or chemicals such as alkylalkanesulfonates, can be used to generate variation from which plants with desirable characteristics can be selected.
Example 4: The first successful mutant barley with 'stiff straw' (cultivar 'Pallas', 1958) was generated because of a new process: combine harvesting (Lundqvist, 1986). A gamma-irradiated malting-cultivar mutant was selected for its semi-dwarf growth habit and cultivar 'Golden Promise' became the dominant malting quality barley in Scotland throughout the 1970s and 1980s (Forster et al., 1997).
In vitro culture of totipotent plant cells and tissues has increased the importance of the laboratory in plant breeding. In vitro culture of haploid microspores, or 'androgenesis', is used to produce doubled haploids for genetic mapping; this is important when deciphering the relationship between genetic variation and agronomic traits. Doubled haploidy enables complete homozygosis in one generation rather than recurrent backcrossing for six or more generations, as is usually required; many breeding programmes use this technique to accelerate the delivery of homozygous lines for release.
In vitro culture can be used to induce soma-clonal variation such as changes in chromosome number (polyploidy, aneuploidy), chromosome structure (translocations, deletions, insertions and duplications) and DNA sequence (base mutations). This has been useful for introgressing 'alien' genes.
Example 5: A cereal cyst nematode resistance was transferred from rye into wheat by the in vitro culture of wheat-rye monosomic addition lines (Larkin et al., 1989; Banks et al., 1995).
Insertional mutagenesis uses foreign DNA fragments to disrupt gene function. Agrobacterium-mediated gene transfer has been widely applied for generating such mutations and has been used to support gene discovery programmes.
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