Breeding of Vegetable, Spice and Tuber crops (2+1)

History of selection

• Crop improvement refers to the genetic alteration of plants to satisfy human needs. In prehistory, human forebears in various parts of the world brought into cultivation a few hundred species from the hundreds of thousands available.

• In the process, they transformed elements of these species into crops though genetic alterations that involved conscious and unconscious selection, the differential reproduction of variants.

• Through a long history of trial and error, a relatively few plant species have become the mainstay of agriculture and thus the world's food supply.

• This process of domestication involved the identification of certain useful wild species combined with a process of selection that brought about changes in appearance, quality, and productivity.

• The exact details of the process that altered the major crops is not fully understood, but it is clear that the genetic changes were enormous in many cases.

• In fact, some crop plants have been so changed that for many of them, maize, for example, their origins are obscure, with no extant close wild relatives.

• The selection of naturally occurring variants is the basis of crop improvement.

• Over thousands of years this technique resulted in the development of modern basic crops.

• The discovery of techniques for asexual (vegetative) propagation, such as by using natural offshoots, rooting stem cuttings, or various grafting techniques, made it possible to "fix" genetic variants.

• This was the technique used for many tree fruits, enabling identical plants to be cultivated in orchards. Naturally produced seedlings derived from intercrosses of these selected plants were then available for selection again. Many present-day fruit crops are similar to those cultivated in antiquity, and some ancient selections are still cultivated—dates, for example.

• As humans carried improved crops to new locations, opportunities opened to increase genetic variation from natural intercrosses with new wild populations.

• The changes that occur can be dramatic over time, as seen in the proliferation of breeds of animals and especially the wide range of changes brought about by fanciers of dogs, chickens, and pigeons. The observation of these changes influenced the thinking of Charles Darwin to suggest that natural selection, the survival of the fittest, could lead to enormous genetic changes if carried out over a long enough time, and could lead to the origin of new species.

• In the eighteenth and nineteenth centuries, a conscious attempt was made to predict the performance of plants that could be expected from one seed generation to the next.

• The concept that ancestry was important in crop improvement led to refinement in the selection process, brought about by keeping records and the assessment of lineage. Furthermore, it became obvious that variation could be managed by controlling the mating process, an extension of what had long been known in animal breeding.

• This new type of selection, termed pedigree selection, was found to increase the efficiency of the process. Progeny testing (evaluating the genetic worth by progeny performance) increased efficiency of this process.

• The origins of commercial plant breeding began in the second half of the nineteenth century among seed producers. It involved controlled crosses (hybridization) between selections to control genetic recombination, followed by selection of improved types.
• This is still the basis of traditional plant breeding. Interestingly much of this early type of plant breeding was carried out without a clear understanding of the genetic mechanism involved in inheritance.

• Until the famous experiments with the garden pea by Gregor Mendel (1822–1884), a Catholic priest in Brünn, a Moravian town then in the Austro-Hungarian Empire, the basic theory of inheritance involved the concept of blending.

• Mendel unraveled the basic concept of inheritance and clearly showed that characters in the pea were due to elements, later called genes, that remained unaltered as they were inherited.

• Many characters in peas, such as tallness and dwarfness, were shown to be controlled by a pair of genes, of which one member was not always expressed (the concept of dominance and recessiveness).

• Mendel demonstrated that the gametes of the pea contained one member of the gene pairs that controlled characters and that recombined randomly at fertilization.

• Mendel's paper was published in 1866, but it had no impact until the paper was "rediscovered" in 1900, when it created a sensation.

• It was soon obvious that the differences in appearance among plants (phenotypes) could be explained by the interaction of various genes (genotypes) as well as interaction with the environment.