Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 283

There are practical factors to consider in deciding on the number of crosses to make for a breeding project. These include the ease of making the crosses from the standpoint of floral biology, and the constraints of resources (labor, equipment, facilities, and funds). It will be easier to make more crosses in species in which emasculation is not needed (e.g. monoecious and dioecious species) than in bisexual species. Some breeders make a small number of carefully planned crosses, while others make even thousands of cross combinations.

Generally, a few hundred cross combinations per crop per year would be adequate for most purposes, for species in which the F₁ is not the commercial product. More crosses may be needed for species in which hybrids are commonly produced for the purpose of discovering heterotic combinations. As will be discussed next, breeding programs that go beyond the F₁ usually require very large F₂ populations. Regarding the number of flowers per cross combination, there is variation according to fecundity. Species such as tomato may need only one or two crosses, since each fruit contains over 100 seeds. Plants that tiller also produce large numbers of seed. Each crop species has its own reproduction rate, which may be huge (e.g. tobacco: 1000s of seeds produced per plant, 100s per bowl) or relatively small (e.g. pea: about 100 per plant, about 2–5 per pod).