Late Selection and Early Selection

Late selection is used on self-pollinated crops. This term means that selection is delayed until several generations of self-pollination have occurred. The selection is then made on lines that are close to being homozygous. This has two advantages. First, the effects of heterosis (i.e., hybrid vigour) are eliminated. These effects can be very misleading and can result in individuals being selected for the wrong reasons. Second, some of the polygenes controlling horizontal resistance may be recessive. This means that they are expressed only in the homozygous state. There would be no selection pressure for their accumulation if selection occurred on heterozygous populations. This comment applies to other selection criteria also, such as yield and quality of crop product.

The obvious advantage of late selection is that it is a very efficient screening process. The obvious disadvantage of late selection is that it results in a long breeding cycle, usually of two years. This breeding cycle can be shortened with techniques such as single seed descent and hydroponics.

Open-pollinated crops will tolerate early selection. That is, they will tolerate the selection of heterozygous individuals. This is exactly what happened with the maizes of tropical Africa when exposed to Puccinia polysora. The obvious advantage of early selection is that it results in a short breeding cycle, and a breeding program that produces results quickly.

One of the advantages of dealing with vegetatively propagated crops is that this problem does not arise. Vegetative propagation produces an instant cultivar that is heterozygous but 'breeding true'.

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