Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 288

A breeding program starts with an initial population that is obtained from previous programs, and existing variable populations (e.g. landraces), or is created through a planned cross. Hybridization may be used to generate a wide variety of populations in plant breeding, ranging from the very basic two‐parent cross (single cross) to very complex populations in which hundreds of parents could be involved. Single crosses are the most widely used in breeding. Commercial hybrids are mostly produced by single crosses. Complex crosses are important in breeding programs where the goal is population improvement. Hybridization may be used to introgress new alleles from wild relatives into breeding lines. Because the initial population is critical to the success of the breeding program, it cannot be emphasized enough that it be generated with much planning and thoughtfulness.

Various mating designs and arrangements are used by breeders and geneticists to generate plant populations. These designs require some type of cross to be made. Factors that affect the choice of a mating design include: (i) the predominant type of pollination (self‐ or cross‐pollinated); (ii) type of crossing used (artificial or natural); (iii) type of pollen dissemination (wind or insect); (iv) presence of male sterility system; (v) purpose of the project (for breeding or genetic studies); and (vi) size of the population required. In addition, the breeder should be familiar with how to analyze and interpret or use the data to be generated from the mating.

The primary purpose of crossing is to expand genetic variability by combining genes from the parents involved in the cross to produce offspring that contain genes they never had before. Sometimes, multiple crosses are conducted to generate the variability in the base population to begin the selection process in the program. Based on how the crosses are made and their effects on the genetic structure of the plants or the population, methods of crossing may be described as either divergent or convergent.