Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 268

Artificial hybridization is the deliberate crossing of selected parents (controlled pollination) (Figure 6.1). There are specific methods for crossing that depend on the species in which the cross is being made, which differ according to factors including floral morphology, floral biology, possible genetic barriers, and environmental factors. Methods for selected species are described later in this book. However, there are certain basic factors to consider in preparation for hybridization:

 Parents should belong to the same or closely related plant species. If they belong to different (related) plant species, all kinds of techniques may be required to obtain hybrid progeny.

 The parents, obviously, should together supply the critical genes needed to accomplish the breeding objective.

 One parent is usually designated as female. Whereas some breeding methods may not require this designation, breeders usually select one parent to be a female and the other a male (pollen source). This is especially so when hybridizing self‐pollinated species. Whenever genetic markers are available (e.g. white flowers, white seeds), the female exhibits the recessive morphological trait. In some cases, selected parents of cross‐pollinated species may be isolated and allowed to randomly cross‐pollinate each other.

 The female parent usually needs some special preparation. In complete flowers (having both male and female organs), the flowers of the parent selected to be female is prepared for hybridization by removing the anthers, a tedious procedure called emasculation (discussed next). Emasculation is eliminated in some crossing programs by taking advantage of male sterility (renders pollen sterile) when it occurs in the species.

 Pollen is often physically or manually transferred. Artificial hybridization often includes artificial pollination, whereby the breeder physically deposits pollen from the male parent unto the female stigma. However, when hybridization is conducted on large scale (e.g. commercial hybrid seed development), hand pollination is rarely a feasible option.

Figure 6.1 Crossing in yam. The use of magnifying glasses helps with the crossing process.

Source: Photo is courtesy of Asrat Asefaw, International Institute of Tropical Agriculture, Ibadan, Nigeria.