Classical plant breeding

See Agriculture for related topics and context.

See Plant breeding for an introduction.

Classical plant breeding uses deliberate interbreeding (crossing) of closely or distantly related species to produce new crops with desirable properties. Plants are crossed to introduce traits/genes from a particular variety 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 be tested for yield 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. Germplasm resources from genebanks have invaluable for classical breeding.

Classical breeding relies heavily on the naturally occuring plant life-cycle and homologous recombination to generate genetic diversity and to eliminate undesirable traits. It may also makes use of a variety of artificial laboratory procedures to overcome obstacles to introduction of useful traits from wild species that do not usually exchange genes with the domesticated line. These approaches include in vitro techniques such as protoplast fusion, embryo rescue or mutagenisis (see below) to generate genetic alterations and produce transgenic plants that would not exist in nature.

The Yecoro wheat (right) cultivar is sensitive to salinity, plants resulting from a hybrid cross with cultivar W4910 (left) show greater tolerance to high salinity

Traits that breeders' have tried to incorporate into crop plants in the last 100 years include:

1. Increased quality and yield of the crop
2. Increased tolerance of environmental pressures (salinity, metal-ions, extreme temperature, drought)
3. Resistance to viruses, fungi and bacteria
4. Changes to plant morphology, such as dwarfing traits
5. Differences in fruit color
6. Decreased production of antinutrient chemicals
7. Increased tolerance to insect pests
8. Increased tolerance of herbicides

Before World War II

Intraspecific hybridization within a plant species was demonstrated by Charles Darwin and Gregor Mendel, and was further developed by geneticists and plant breeders. In the early 20th century, plant breeders realized that 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.

In 1908, George Harrison Shull described heterosis, also known as hybrid vigor. Heterosis 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 lead 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.

Heterosis made breeders aware of the broad practical value of many genes carried in plant chromosomes even when the identity and trait specified by the paticular genes is unknown - that is that diverse plant Germplasm is generally valuable to the breeder.

By the 1920s, statistical methods were 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, enabling the production of hybrids and removing the need for detasseling maize plants.

The scientific use of Transgenic plants in farming gained impetus in the 1930s when a transgenic wheat variety named Hope bred by E. S. McFadden with a transgene originating in a wild grass saved American wheat growers from devastating stem rust outbreaks.

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.

Success stories like Hope and hybrid-vigor made it clear that genetic divesity present in wild-species was of great potential value to plant breeders, and eventially lead to the establisment of Germplasm collections consisting of seed-banks devoted to preservation of potentially useful uncharacterised traits for posterity.

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 other wise 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. The cereal triticale is a wheat and rye hybrid. The first generation created from the cross was sterile, so the cell division inhibitor colchicine was used to double the number of chromosomes in the cell. Cells with an uneven number of chromosomes are sterile.

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 (Oryza 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 EMS and DMSO, radiation and transposons are used to generate mutants with desirable traits to be bred with other cultivars. 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 also found uses.

When a desirable trait has been bred into a species, a number of crosses to the favoured parent are made to make the new plant as similar as the 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.

It should be noted that 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 accidentally re-introduced into varieties of potato through plant breeding.

Issues and concerns

Modern plant breeding, whether classical or through genetic engineering, comes with issues of concern, particularly with regard to food crops.

Surveys of changes in American foods 1950-1999 have suggested there may be decreases in nutitional quality of many garden crops over this time period, possibly because of breeding for higher yield

This is not a new issue though. Recent studies have revealed that at the begining of agriculture, a gene was lost from wheat that mobilizes nutrients from leaves, causing better yields at the expense of protein content. It has long been known that among the many varieties of wheat used in modern times, there is an inverse relationship between yield and protein content.. There is also increasing emphasis on breeding crops for nutritional improvement .

The debate surrounding plant breeding genetic modification of plants is huge, encompassing the ecological impact of genetically modified plants and the safety of genetically modified food. It extends also to the issue of Food security because of the strong link between increases in crop output and matching of food supply to growing food demand caused by population growth and economic growth. Agencies such as the International Food Policy Research Institute (IFPRI) have highlighted the mis-match between amount of agricultural R&D and food security in the developing world .

Plant breeders' rights is also a major and controversial issue. Efforts to strengthen breeders' rights, for example, by lengthening periods of variety protection, are ongoing. Today, production of new varieties is dominated by commercial plant breeders, who seek to protect their work and collect royalties through national and international agreements based in intellectual property rights.

The range of related issues is complex. In the simplest terms, critics of crop-breeding argue that, through a combination of technical and economic pressures, commercial breeders are reducing biodiversity and significantly constraining individuals (such as farmers) from developing and trading seed on a regional level.

But seed breeding is a specialised economic activity that most farmers do not have the time to pursue, and better seed provides a simple means of technology transfer that provides an economic benefit to the farmer. Expansion of a commercialized seed industry is historically associated with substantial economic gains in that sector as illustrated by hybrid maize in the USA, and more recently, the Indian cotton seed industry . Critics of excessive precautionary regulation argue that costly regulatory burdens and delayes imposed on new seed-breeding technologies restrict investment in much modern agricultural technology to organisations having substantial financial assets, which limits the effectiveness of public research efforts in developing countries.

General Bibliography

• Borojevic, S. 1990. Principles and Methods of Plant Breeding. Elserier, Amsterdam. ISBN 0-444-98832-7
• Chrispeels, M.J.,and Sadava, D.E. 2003 Editors. Plants, Genes, and Crop Biotechnology. 2nd Edition. Jones and Bartlett/American Society of Plant Biologists ISBN 0-7637-1586-7
• Gepts, P. (2002). A Comparison between Crop Domestication, Classical Plant Breeding, and Genetic Engineering. Crop Science 42:1780–1790
• Origins of Agriculture and Crop Domestication - The Harlan Symposium
• Fedoroff, N. V. and Brown, N. M. 2004 Mendel in the Kitchen: A Scientist's View of Genetically Modified Food. National Academy Press. ISBN 0-3090-9205-1
• McCouch, S. 2004. Diversifying Selection in Plant Breeding. PLoS Biol 2(10): e347.
• news@nature.com. 1999 Are non-GM crops safe?
• Sun, C. et al. 1998. From indica and japonica splitting in common wild rice DNA to the origin and evolution of Asian cultivated rice. Agricultural Archaeology 1998:21-29

External links

• Making genetically engineered plants
• Adoption of Genetically Engineered Crops in the U.S.(1996-2006) ERS USDA
• ISAAA Briefs 34-2005: Global Status of Commercialized Biotech/GM Crops: 2005
• Biotech Crops Reduce Pesticide Use, Greenhouse Gas Emissions Planting of these crops generates additional US$27.5 billion in global farm income 2005
• 2006 Update of Impacts on US Agriculture of Biotechnology-Derived Crops Planted in 2005]