ECOL3453 - Crop Ecology

Semester 2 2004-2005

Lecture 7 - Plant Breeding

The breeder works with a pool of locally adapted parental materials, supplemented from elsewhere to maintain an adequate genetic base.

The breeder's decisions include:-

• which parents to use
• how to combine them
• what to select for
• how to select
• when to select
• what to discard
• what to keep

Varieties produced by the breeder are subjected to official trials (developed countries), recommendations are made but it is the farmers' decisions whether to adopt or not.

Local adaptation

Only plants that flower and fruit under the conditions at the breeding station, or those that can be induced to do so, can be used in a cross.

Genetic base

The genetic variability in a crop always exceeds what one breeder can effectively handle. The parental material will include:-

Recombination

Enhanced adaptation follows from the isolation and selection of new genotypes, the products of genetic recombination, which are better adapted than their parents. Recombination is therefore a crucial phase of any plant breeding programme. The breeder sets up crosses to generate recombinants.

Selection

This is both natural (environment in breeders' plots) and human.
The problems in human selection are that the products of each years breeding are normally in the thousands. Generally, selection in the early years of a cross, when numbers are large, must perforce be on a visual basis. Subsequently, when numbers are reduced, actual measurements can be made - yields measured in small plots, disease resistance tests run, quality of the product assessed, etc.

Populations

There are four basic plant breeding populations determined by mating and propagation systems:-

• inbred lines (IBL) - seed propagated inbreeders
• open pollinated populations (OPP) - seed propagated outbreeders with high heterozygosity
• hybrids (HYB) - constructed by crossing more or less inbred lines, variability is low but heterozygosity is high
• clones (CLO)

The population type is a dominant feature of any plant breeding programme.

Mutation induction

Certain chemicals and ionizing radiation cause mutations. These agents potential assistance to plant breeding has not realised its early promise but they may have potential in some circumstances:-

• if a genetically plausible specific mutant seems possible - even if yet unknown
• if produced the mutant is easily detectable
• if the production of a mutant is easy compared to other problems

Mutation-induction is more useful in expanding the genetic base than solving specific problems.

Conservation of Genetic Resources

Long-term progress in plant breeding depends on the provision of adequate genetic variability in the form of diverse parents that can be included in the genetic base of breeding programmes. However, successful plant breeding replaces old varieties with new ones and, the more the success the fewer are the new varieties, which are often also inter-related. Thus, local variability in crops tends to decline, a trend that has been accelerating since the 1930s. One consequence is a locally narrow genetic base, apparent in slow breeding progress and disease crises. Another is the world-wide reduction in the variability available to breeders.

The International Biological Programme (1964-1974) saw collaboration of scientists from around the world investigating "The Biological Basis of Productivity and Human Welfare". One section of the programme dealt with "plant genetic resources".

This was justified on the basis of:-

• the accelerated rate of displacement of primitive crop varieties by locally selected or introduced cultivars
• rapidly growing interest in the primitive cultivars and the immense range of variation they contain, and in the wild relatives of economic species, many as yet scarcely explored and exploited.

This diversity needs to be protected. In addition to protecting habitats, germplasm may be stored as:-

• living plants in situ or ex situ
• seed banks
• stored pollen
• tissue or meristem culture
• DNA

All of which are costly and, without further research, some species can only be stored using one or two techniques. E.g. seeds of many species are recalcitrant and can not be stored in seed banks. It could however be money well spent.

Worldwide there were 10 long term gene banks in 1974 by 1996 this number had grown to >1300 with over 6 million acessions. (More information).

Reasons from examples

• In California in 1951 the Barley Yellow Dwarf Virus (BYDV), transmitted by aphids, caused a severe yield reduction. In 1959 a nation-wide outbreak occurred. Resistance was found in Ethiopian material in the world collection - but only one gene. Since viruses tend to have a high mutation rate, a single gene provides little protection. It was later found that 21% of barleys from Ethiopia were resistant to BYDV and it is believed that this was as a result of natural hybridisation of barley in that country. The California barley crop is alone worth in excess of US$160 million per year.
• The two principal cultivated African species of yams, Dioscorea rotundata and D. cayenensis, had an enormous diversity at the village level. This diversity has been reduced by "better" cultivars, the Asiatic D. alata, cassava, rice and maize. Often driven by International Aid Organisations trying to reduce dependence on a single staple and to provide a more balanced diet.
• Wild coffee in the Ethiopian rainforest was the traditional source of seed, and a continuing source of variation. Like other parts of the world, the area under forest is shrinking rapidly.
• Ginkgo biloba has survived in China whilst all of its close relatives have perished. Ginkgo is extraordinarily resistant to herbivory and disease. Amongst the unique chemicals found in G. biloba are bilobol, ginkgolic acid, bilobalide and the ginkgolides. Could these be the basis for the next generation of plant protection chemicals? The herbal extracts from this plant are increasingly popular and it may be used to treat heart disease and Alzheimer's disease.

International Collaboration

The mandate of the International Plant Genetic Resources Institute (IPGRI) is to advance the conservation and use of plant genetic resources for the benefit of present and future generations. IPGRI is the successor to the International Board for Plant Genetic Resources (IBPGR) which was formed in 1974. IPGRI is one of the centres of the CGIAR ( the Consultative Group on International Agricultural Research) system.

The Convention on Biological Diversity which came out of the United Nations Conference on Environment and Development (the Earth Summit) held in Rio de Janeiro, Brazil in June 1992, has been ratified by 171 countries. The United States of America has not and American companies are implicated in Biopiracy - the patenting of plants, genes, and other biological products that are indigenous to another, normally a third-world, country.

As of 1 January 1998, the other UN member countries that had not ratified the convention were Afghanistan, Andorra, Angola, Azerbaijan, Bosnia and Herzegovina, Brunei Darussalam, Iraq, Kuwait, Liberia, Libyan Arab Jamahiriya, Malta, Palau, Sao Tome and Principe, Saudi Arabia, Somalia, Thailand, The former Yugoslav Republic of Macedonia, United Arab Emirates, and Yugoslavia.

The Global Plan of Action for The Conservation and Sustainable Utilization of Plant Genetic Resources for Food and Agriculture and The Leipzig Declaration were adopted at The Fourth International Technical Conference on Plant Genetic Resources in Leipzig, Germany, 17 to 23 June 1996, by the 150 countries and 54 inter-governmental and non-governmental organizations that attended.

Genetic engineering

Students who have not taken a course in Molecular Biology should check the links here.

Molecular Breeding

Techniques have been developed that allow us to transfer genes between organisms that could not be achieved by traditional plant breeding, e.g.

• insect to plant - the luciferase enzyme from a fire fly to tobacco
• bacterium to plant - the insecticidal endotoxin from Bacillus thuringiensis  
• virus to plant - viral coat proteins that make plants immune to viruses

Is this desirable? Is it safe? Is it sustainable?
Possible side effects, transfer of genes to wild relatives/weeds, development of worse pesticide resistance problems.

Links

What is genetic engineering?
The role of microorganisms in genetic engineering
HORT 250 - Biotechnology in Agriculture at Purdue University
A Guide to Biotechnology in Crop Production at North Carolina State University
Old Crops in New Bottles? Issues on Genetic Engineering - Royal Agricultural Society of England.
Regulation of genetically engineered organisms and products in the USA
Biotechnology Information Center (BIC) at the National Agricultural Library of USDA-ARS.
Greenpeace latest on genetic engineering of food crops.
Shaping Genes: Ethics, Law and Science of Using New Genetic Technology in Medicine and Agriculture. A book by Darryl R. J. Macer of the Eubios Ethics Institute.
Careers in Biotechnology

Surrogate Transformation

Transform an endophyte rather than the host plant.

Acremonium spp. asexual (anamorphic) form of Epichloë spp. (Clay, 1992, Tsai et al., 1992, Murray et al., 1992).