Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 266

Crop improvement typically involves the transfer of genes from one source or genetic background to another, or combining genes from different sources that complement each other, with the hope that the new cultivar will combine the best of both parents, while being distinct from both. When a plant breeder has decided on the combination of traits that he wishes to be incorporated in the new cultivar to be developed, the next crucial step is to find one or more sources of the appropriate gene(s) for such characters. In flowering species, the conventional method of gene transfer or gene combination is by crossing or sexual hybridization. This procedure causes genes from the two parents to be assembled into a new genetic matrix. It follows that if parents are not genetically compatible, gene transfer by sexual means cannot occur at all, or at best, may be fraught with complications. The product of hybridization is called a hybrid.

Sexual hybridization can occur naturally through agents of pollination. Even though self‐pollinating species may be casually viewed as “self‐hybridizing,” the term hybridization is reserved for crossing between unidentical parents (the degree of divergence is variable). Artificial sexual hybridization is the most common conventional method of generating a segregating population for selection in breeding flowering species. In some breeding programs, the hybrid (F₁) is the final product of plant breeding (see hybrid breeding in Chapter 19). However, in most situations, the F₁ is selfed (to give an F₂) to generate recombinants (as a result of recombination of the parental genomes) or a segregating population, in which selection is practiced. In clonally propagated crops the F₁ usually segregates sufficiently, and its clonally produced descendants will be submitted to selection without further crossing or selfing.

The tools of modern biotechnology now enable the breeder to transfer genes by circumventing the sexual process (i.e. without crossing). More significantly, gene transfer can transcend natural reproductive or genetic barriers. Transfers can occur between unrelated plants and even between plants and animals (by genetic transformation, see Chapter 24).