Other breeding programmes

In addition to F1 hybrid breeding, where specific improvements are achieved, plant breeders may wish to bring about more general improvements to existing cultivars, or introduce characteristics such as disease resistance. Programmes are required for crops which self-pollinate (inbreeders), or those which cross-pollinate (outbreeders), and two of these strategies are described.

Pedigree breeding is the most widely used method in plant breeding by both amateurs and professionals. Two plants with different desirable characteristics are crossed to produce an F1 population. These F1 plants of very similar genotype are then crossed (called ‘selfed’) and any offspring with useful characteristics are selected for further selfing to produce a line of desirable plants. After repeated selfing and selection, the characteristics of the new lines are compared with existing cultivars and assessed for improvements. Further field trials will determine a new type’s suitability for submission and possible registration as a new cultivar. If a plant breeder wants to produce a strain adapted to particular conditions (e.g. a cultivar with hardiness), exposure of plants of a selected cultivar to the desired conditions will eliminate unsuitable plants, allowing the hardy plants to set seed. Repetition of this process gradually adapts the whole population. Selection for other characteristics such as earliness may be selected by harvesting seed early.
Disease resistance breeding. A genetic characteristic enables the plant to combat fungal attack. The disease organism may itself develop a corresponding genetic capacity to overcome the plant’s resistance by mutation. The introduction of disease resistance into existing cultivars often requires a backcross breeding programme. This involves a commercial cultivar lacking resistance crossed with a wild plant which exhibits resistance. For example, a lettuce cultivar may lack resistance to downy mildew (Bremia lactucae), or a tomato cultivar lack tomato mosaic virus resistance.

This commercial cultivar is crossed with a resistant wild plant to produce an F1, then an F2. From this F2, plants having both the characteristics of the commercial cultivar and also disease resistance are selected. The process continues with backcrossing of these selected plants with the original commercial parent to produce an F1, from which an F2 is produced. More commercial characteristics may be incorporated by further backcrossing and selection over a number of generations, until all the characteristics of the commercial cultivar are restored, but with the additional disease resistance.