Base-broadening: introgression and incorporation.

Many crop plant collections were built-up and maintained in various places over the last 60-70 years of the 20th century. They have provided some interesting scientific data but have been but little exploited for plant breeding. The evidence of the need for base-broadening of a crop plant lies in historical information, pedigrees and poor progress under selection. Two means of using a collection may, broadly, be distinguished: introgression implies the introduction of a character by backcrossing a single gene or a small group of polygenes; incorporation implies the large-scale introduction of genetic variability by mass crossing followed by mass selection under some approximation to random mating. The former has been widely used for monogenic disease resistances, which have generally failed. The latter has been successfully exploited in potatoes and sugarcane, and is being ever more widely adopted as a longer-term strategic measure for the improvement of other crops. In principle, it is applicable to all crops, and a great many (e.g. all the inbred cereals) would benefit if so treated. Incorporation, if correctly exploited, generates substantial reserves of adapted parental materials for incorporation in plant breeding programmes. It may itself generate new crop varieties but is not primarily intended to do so: its function is to widen the genetic base.