'') over hundreds of years, resulting in dozens of today's agricultural crops.
Cabbage,
kale,
broccoli, and
cauliflower are all
cultivars of this plant. One major technique of plant breeding is
selection, the process of selectively propagating plants with desirable characteristics and eliminating or "culling" those with less desirable characteristics. Another technique is the deliberate interbreeding (crossing) of closely or distantly related individuals to produce new crop varieties or lines with desirable properties. Plants are crossbred to introduce
traits/
genes from one variety or line into a new genetic background. For example, a
mildew-resistant
pea may be crossed with a high-yielding but susceptible pea, the goal of the cross being to introduce mildew resistance without losing the high-yield characteristics. Progeny from the cross would then be crossed with the high-yielding parent to ensure that the progeny were most like the high-yielding parent, (
backcrossing). The progeny from that cross would then be tested for yield (selection, as described above) and mildew resistance and high-yielding resistant plants would be further developed. Plants may also be crossed with themselves to produce
inbred varieties for breeding.
Pollinators may be excluded through the use of
pollination bags. Classical breeding relies largely on
homologous recombination between chromosomes to generate
genetic diversity. The classical plant breeder may also make use of a number of
in vitro techniques such as
protoplast fusion,
embryo rescue or
mutagenesis (see below) to generate diversity and produce hybrid plants that would not exist in
nature. Traits that breeders have tried to incorporate into crop plants include: • Improved
quality, such as increased nutrition, improved flavor, or greater beauty • Increased
yield of the crop • Increased
tolerance of environmental pressures (
salinity, extreme
temperature,
drought) • Resistance to
viruses,
fungi and
bacteria • Increased tolerance to
insect pests • Increased tolerance of
herbicides • Longer storage period for the harvested crop
Before World War II Gartons Agricultural Plant Breeders in
England was established in 1880, which became a public company in 1898, by John Garton, who was one of the first to commercialize new varieties of agricultural crops created through cross-pollination. The firm's first introduction was the , an
oat variety. In the early 20th century, plant breeders realized that
Gregor Mendel's findings on the
non-random nature of inheritance could be applied to
seedling populations produced through deliberate
pollinations to predict the frequencies of different types. Wheat hybrids were bred to increase the crop production of
Italy during the so-called "
Battle for Grain" (1925–1940).
Heterosis was explained by
George Harrison Shull. It describes the tendency of the progeny of a specific cross to outperform both parents. The detection of the usefulness of heterosis for plant breeding has led to the development of inbred lines that reveal a heterotic yield advantage when they are crossed.
Maize was the first species where heterosis was widely used to produce hybrids.
Statistical methods were also developed to analyze gene action and distinguish heritable variation from variation caused by environment. In 1933 another important breeding technique,
cytoplasmic male sterility (CMS), developed in maize, was described by
Marcus Morton Rhoades. CMS is a maternally inherited trait that makes the plant produce sterile
pollen. This enables the production of hybrids without the need for labor-intensive
detasseling. These early breeding techniques resulted in large yield increase in the
United States in the early 20th century. Similar yield increases were not produced elsewhere until after
World War II, the
Green Revolution increased crop production in the developing world in the 1960s.
After World War II Following
World War II a number of techniques were developed that allowed plant breeders to hybridize distantly related species, and artificially induce genetic diversity. When distantly related species are crossed, plant breeders make use of a number of
plant tissue culture techniques to produce progeny from otherwise fruitless mating. Interspecific and intergeneric hybrids are produced from a cross of related species or genera that do not normally
sexually reproduce with each other. These crosses are referred to as
Wide crosses. For example, the
cereal triticale is a
wheat and
rye hybrid. The cells in the plants derived from the first generation created from the cross contained an uneven number of chromosomes and as a result was sterile. The
cell division inhibitor
colchicine was used to double the number of
chromosomes in the
cell and thus allow the production of a fertile line. Failure to produce a hybrid may be due to pre- or post-
fertilization incompatibility. If fertilization is possible between two species or genera, the hybrid
embryo may abort before maturation. If this does occur the embryo resulting from an interspecific or intergeneric cross can sometimes be rescued and cultured to produce a whole plant. Such a method is referred to as
embryo rescue. This technique has been used to produce
new rice for Africa, an interspecific cross of Asian rice
Oryza sativa and
African rice O. glaberrima. Hybrids may also be produced by a technique called
protoplast fusion. In this case protoplasts are fused, usually in an electric field. Viable recombinants can be regenerated in culture. Chemical
mutagens like
ethyl methanesulfonate (EMS) and
dimethyl sulfate (DMS),
radiation, and
transposons are used for
mutagenesis. Mutagenesis is the generation of mutants. The breeder hopes for desirable traits to be bred with other
cultivars – a process known as
mutation breeding. Classical plant breeders also generate genetic diversity within a species by exploiting a process called
somaclonal variation, which occurs in plants produced from tissue culture, particularly plants derived from
callus. Induced
polyploidy, and the addition or removal of chromosomes using a technique called
chromosome engineering may also be used. When a desirable trait has been bred into a species, a number of crosses to the favored parent are made to make the new plant as similar to the favored parent as possible. Returning to the example of the mildew resistant pea being crossed with a high-
yielding but susceptible pea, to make the mildew resistant progeny of the cross most like the high-yielding parent, the progeny will be crossed back to that parent for several generations (See
backcrossing). This process removes most of the genetic contribution of the mildew resistant parent. Classical breeding is therefore a cyclical process. With classical breeding techniques, the breeder does not know exactly what genes have been introduced to the new cultivars. Some
scientists therefore argue that plants produced by classical breeding methods should undergo the same safety testing regime as
genetically modified plants. There have been instances where plants bred using classical techniques have been unsuitable for human consumption, for example the
poison solanine was unintentionally increased to unacceptable levels in certain varieties of
potato through plant breeding. New
potato varieties are often screened for
solanine levels before reaching the marketplace. Even with the very latest in
biotech-assisted conventional breeding, incorporation of a
trait takes an average of seven
generations for
clonally propagated crops, nine for
self-fertilising, and seventeen for
cross-pollinating. == Modern plant breeding ==