Research

Gregor Mendel and Plant Breeding

Introduction to Experiments in Plant Hybridisation
    Plant characteristics
    Hybrids
    Why was it important?
    Mendel's Laws
The structure of DNA
Modern plant breeding
    Different techniques
    Choice of characteristics
Mendel and the history of genetics
Further reading

Introduction

In 1900 Gregor Mendel's work on genetics was presented by William Bateson in the Journal of the Royal Horticultural Society. Bateson coined the term 'genetics' to describe the new science.

Mendel was born in 1822 in Austria as Johann Mendel. He joined a monastery of St Thomas at Brünn, known as 'The Königkloster', where he was given the name 'Brother Gregor'.

It is reported that he read Darwin's work on the Origin of Species and commented "Nature cannot continue to produce new species in this way (that is by self pollination of pure bred stock) there must be something more behind." His paper, which was the introduction to modern genetics was the culmination of a number of years work in the grounds of the monastery. He died in 1884, before he could see his work accepted by the scientific community.

His work had a broad scope, in that it concerned the changes which were passed on between parents and progeny over many generations. His paper described the occurrence of traits in hybrid parents and progeny.

Mendel's paper - Experiments in Plant Hybridisation

Most of Mendel's experiments were carried out on garden peas (Pisum sativum). He clearly defined the requirements for the plants in his experiments.

They were to possess constant differentiating characters. This meant that they could be distinguished by their characteristics, which remained the same when identical plants were bred together.
During the flowering period the hybrids of such plants had to be protected from all other pollen, or be easily capable of such protection. Peas were useful due to their floral structure and their differentiating characteristics.
A further requirement was that the hybrids and offspring should be able to produce viable seeds themselves.
These plants were also easy to culture, and had a short period of growth.

Plant characteristics

Mendel's experiments involved the separate examination of seven different characteristics of peas. There were two forms of each characteristic. These were:

round or wrinkled ripe seeds
yellow or green endosperm (flesh)
grey-brown or white seed coat
Grey-brown seeds were seen to be associated with reddish purple flowers, and white seeds were associated with white flowers.
smooth/inflated or wrinkled seed pods
green or yellow unripe pods
position of flowers axial (along the stems) or terminal (at the end of the stems)
length of stem long (six to seven feet) or short (three quarters to one and a half feet)
Hybrid vigour was a term coined after the results of the experiment on length of stems, where progeny were found to be taller than their parents, indicating increased vigour.

The plants were true breeding at the start of experiments in that their progeny were identical to the parents. This was important to establish that any changes in the progeny were due to the crosses made, rather than any latent or random variation.

Each of the two different forms were used both as the seed parent and the pollen parent. This is known as reciprocal crossing. The two different crosses were found to produce identical progeny.

First generation

Hybrids were not found to be intermediate between parents but resembled one parent. This parent was described as having the "dominant" character in that it was always seen in the progeny. The terms dominant and recessive were coined to describe forms where one is always expressed rather than the other, when the two are bred. The first of the forms in the above list of experiments were found be dominant, and were always expressed in the hybrid progeny. The second forms were seen to be recessive. These forms were given the name allele.

Diagram of the possible progeny from a hybrid - their genetic make-up and appearance

In a gene for colour in peas, there are two forms or alleles, A and a. When the hybrid progeny are inbred, AA and Aa forms appear yellow in colour, only aa forms are green.

Diagram representing Gregor Mendel's first experiments

Second generation

When the first generation hybrids were inbred, three quarters of the progeny were found to have the dominant character, and one quarter the recessive character. This relationship was found to hold true for all the characteristics tested.

Third generation

The second generation of hybrids were then inbred. Those of the recessive form bred true and produced identical progeny. Of the remainder which had the dominant form, one third bred true for the dominant character. The other two thirds displayed the dominant and recessive characters in the proportions three quarters to one quarter and so showed the same proportions as the hybrid form. Therefore the final proportions of plants were one quarter true-breeding for the dominant character, half hybrids, and one quarter true breeding for the recessive character. The dominant character was thus seen in two cases; in a pure strain, and in a hybrid with a dominant allele. The proportion of each was calculated from the progeny they produced when inbred.

Appearance of characteristics in successive generations

Collection of seeds from the progeny and re-planting was carried through successive generations. It was seen that hybrid plants are inclined to revert to the parental forms. This feature can be seen when seed from purchased hybrid plants is repeatedly collected and sown in the garden.

Why was it important?

The idea of transfer of characteristics from parents to progeny had been seen by others before Mendel, but his work was unique in showing the mathematical relationship between them. Mendelism was concerned with the simple inheritance of major genes which control identifiable character differences. The characteristics which he studied were each controlled by one gene. It was the discovery of a pattern in the characteristics of the progeny which allowed Mendel to suggest that there were units of heredity. An alternative theory at the time was blending inheritance which proposed that "any offspring inherited half its characteristics from its parents, a quarter from its grandparents, an eighth from its great-grandparents, etc, until it made a complete unit." However this did not take account of what characteristics were shown in any one individual.

Mendel formulated three laws from his results.

The Law of Uniformity stated that when plants with a trait which differed in two ways were crossed, the progeny were uniform and resembled one parent.

The Law of Segregation of Alleles stated that alleles separate and recombine in the progeny. An allele is a form in which a gene may occur, such as yellow or green peas. Different alleles give rise to different expressions of a character. This explained the manner in which characteristics were passed on from the parents to their progeny. It explained why characteristics in an individual were not a blend from its ancestors, and why characteristics could disappear and re-appear in subsequent generations.

The Law of Independent Assortment stated that alleles of different genes sorted independently from one another. The relation of each pair of different characters in hybrids was independent of the other differences in the two original parental stocks.

Development of Mendel's Laws

A number of refinements to Mendel's Laws were later shown.
Not all traits are dominant or recessive. Some are intermediate between the two parents.

The Law of Independent Assortment also does not always hold true. Some characteristics are "linked" and appear together in the progeny.

Some characteristics are not controlled solely by one gene. Most characters of economic importance are quantitative and controlled by many genes. Therefore the impact of Mendelism on breeding practice was slow, being mainly based on empirical methods. In fact plant breeding was successfully carried out before Mendelian genetics was described.

Conclusion

Mendel's experiments played an important part in determining how characteristics are passed from parents to offspring. In particular, the mathematical relationship which he derived for inherited characteristics indicated that genes were discrete units which could be studied separately. It allowed the prediction of possible new hybrids which would be produced from breeding.

His work was influential in demonstrating the existence of units of inheritance. It was part of a wider search for the structure of those units, which was discovered in the chemical DNA described below.

DNA and transfer of genetic information

DNA or deoxyribonucleic acid is the chemical present in cells which defines the inherited characteristics of an individual. It does this by regulating the synthesis of proteins in an organism, which in turn defines what form it takes and how it appears.

Double helix structure of DNA

Diagram of the structure of DNA

Constituents of DNA

DNA is a long chain molecule formed from phosphate, pentose sugars, and nitrogen purine bases. The sequence of bases determines the genetic code and this gives rise to the differences between organisms and individuals. As a molecule of DNA is very long, it is measured in thousands of bases or kilobases (Kb). There are 4700Kb in a tiny organism like the bacterium E.coli. In larger animals and plants the number of kilobases increases so much that the DNA molecule splits itself into separate pieces called chromosomes.

DNA structure
The DNA molecule structure is a double helix, which is a double spiral about a common central axis. The phosphate and sugar molecules form the spine of the spiral, while the bases are positioned at intervals along it. The two identical strands are weakly bound together. During reproduction these strands separate and are copied. Mutations occur when the molecule is not copied exactly or differences arise spontaneously.

Chromosomes
The genetic blueprint of an organism is laid out in chromosomes. These chromosomes are present in pairs. The number of pairs varies from organism to organism. In humans there are 23 pairs, in peas 7. In most species the number is constant for the species, but in plants there can be extra copies of pairs (polyploidy) which may result in more vigorous or larger plants.

Genes
Genetic information governing traits is carried by genes, each of which is a segment of a chromosome. A gene is a unit of heredity.

Alleles
Mendel showed that different forms of a gene, or alleles, are possible for a given trait - e.g. an allele for wrinkled seed pods and an allele for smooth seed pods of a gene for seed pod texture. These alleles are either dominant or recessive in that where there is a combination of two different alleles, the dominant one will be expressed over the recessive one. A catalogue of an organism’s alleles is its genotype. The way it is expressed is its phenotype.

Modern plant breeding

Breeding is the selection of a novel or desirable cultivar from plants with variable characteristics. Variability provides the means by which adaptation to differing environments can occur, and also the means by which plant breeders can select and use cultivars in breeding programmes. Variability arises in nature in a number of ways, such as by mutation, by spontaneous hybridisation, and changes in chromosome form and number. It can also be promoted artificially to aid the production of new useful cultivars.

Variation
Natural sports are the consequences of gene and chromosome mutations. A plant with a new characteristic arises spontaneously in this way. This plant can be selected for use in a breeding program. Brassica oleracea is a species which, by the use of selection for unusual characteristics has given rise to cabbage, Brussel sprouts, kale and cauliflower.

Crossing
Diversity can also be achieved from crossing cultivars of the same species. The plants which result from these crosses have new characteristics which are the consequences of recombinations of alleles. Some crops have been altered by selecting variants within a single species, such as peas.

Types of crosses
Different types of crosses are used in plant breeding. Interspecific hybridisation, or breeding between different species has been used where they each possess desirable attributes. Introgression is the hybridisation between related species or subspecies and subsequent backcrossing with one of the parents, and has been widely used in the development of crop plant cultivars. Strawberry Fragaria x ananassa is a hybrid of F. chiloensis and F. virginiana.

This type of breeding is easily achieved between some groups of related species. Orchids hybridise very easily. However amongst others the crosses are either unsuccessful or require specialised techniques to obtain the progeny. This is due to incompatibility between the pollen tube and the pistil, a low rate of fertilization or inhibited development of the seed. In some cases the progeny are sterile.

Wild species
The use of wild forms of crop plants in breeding is also beneficial to maintain or increase genetic variability. The centre of diversity of a plant is the region where the greatest variability in its genotype is found, and the most useful area to search for suitable wild types for use in breeding programmes. Resistance to Fusarium disease has been incorporated into some cultivars of domestic tomato from a wild species.

Artificial variation
Greater variability in a plant group can be achieved by inducing mutations artificially using ionising radiations such as X rays, gamma rays, UV radiation, or chemical mutagens. The outcome of such treatments are not predictable and many useless forms may arise. Desirable altered plants can then be propagated to produce new varieties. Some new forms of food and ornamental plants have been produced in this way, such as differently coloured Chrysanthemum, Weigela, Caryopteris and Forsythia.

Polyploidy
In some plants there are multiple sets of chromosomes. This is termed polyploidy. It can arise due to doubling of the chromosome number of a species or a hybrid between races of the same species. Alternatively it can occur during the hybridisation between diverged species followed by doubling of the chromosome number, thus combining two distinct genomes. Polyploidy is useful in plant breeding because it can maintain the fertility of some interspecific hybrids where they would normally be sterile. In some cases the plants are larger or more vigorous. Where this is the case polyploidy is induced by the application of certain chemicals such as colchicine. Wheat is an example of a polyploid, thought to be an interspecific hybrid between three natural species. Apples with three sets of chromosomes have also been bred for the large size of their fruit.

Somatic hybridisation
Somatic hybridisation is a technique which uses cells to artificially hybridise plants which will not do so naturally. Parts of the cells of the parent plants are fused and a new hybrid grown from these cells. For example the cross between two genera Brassica napus and Sinapis alba or white mustard conferred resistance to Alternaria disease but reqired somatic hybridisation to achieve.

DNA techniques
A number of techniques have been developed to distinguish and reproduce sections of DNA so that useful genes can be studied and made use of in crop development. Marker techniques use the property of gene linkage to locate useful genes and to identify desirable individuals, such as those which are resistant to disease. Gene cloning involves the exact duplication of desirable gene sequences so that the progeny are genetically identical. This is useful to obtain crops with consistent productivity, such as genetically identical disease resistant bananas for plantations.

Genetic modification
Genetic transformation or genetic modification is a relatively new technique which could be made use of in novel plant breeding. It involves selecting a particular desirable gene sequence and inserting it directly into the cells of a plant where it is incorporated into its DNA. In transgenic plants genes may be incorporated which are derived from quite different plant groups or from different organisms.

Choice of characteristics
Information on the potential for hybridisation comes from the organism, the environment in which they exist or interactions between these two. Features of the organism include the physical features, the microscopic features of internal anatomy, and the cell structure features, such as the number, size and shape of chromosomes. This might be determined by molecular evidence or chemical analyses. Environmental factors include the plants’ geographical distribution or habitats. The interactions could be a particular pollinator for a plant, or information from previous hybridisations.

There are a number of plant traits which are of merit in breeding programmes. They include for example genetic variability, hybrid vigour, stature, cold or heat tolerance, drought tolerance, salt tolerance, resistance to disease, resistance to specific insecticides, fungicides and herbicides, blocking flower senescence, or exploiting infertility.

Mendel's paper and events in genetics

The turn of the nineteenth century was a time of advancement in the understanding of the transfer of characteristics from plants and animals to their progeny. Significant events around the time of Mendel's work are given below.

1859:	The well-known work On the Origins of Species by Means of Natural Selection was published by Charles Darwin, discussing the evolution of different, better adapted individuals in nature.
1862:	The continuity of germplasm, or reproductive tissues, was questioned by Weissman who asked "Can the results of acquired parental experience be passed to the offspring?".
1866:	Mendel presented his paper Experiments in Plant Hybridisation which was first published by the Natural History Society of Brünn. It concerned the analysis of hereditary differences in plants.
1869:	Nucleic acids were isolated by Friedrich Miescher. These are the chemical type which make up the genetic code in organisms.
1888:	Chromosomes were described by Wilhelm von Waldeyer.
1889:	The concept that characteristics may be passed from one generation to another in the form of single units, rather than a complete package, was suggested by Hugo de Vries.
1900:	The term "mutation" was coined by de Vries in his paper The Law of Inherited Characteristics.
1900:	Mendel’s paper was re-presented to The Royal Horticultural Society by Bateson.
1909:	The term "gene" was coined by Wilhelm Johannsen. He demonstrated the relationship between phenotype, the characteristics which can be seen, and genotype, the inherited characteristics.
1920:	"Centres of origin" for a range of plant groups were proposed by Vavilov, where those groups originated and where the highest level of genetic variability of the species could be found.
1944:	It was established that DNA, a nucleic acid, rather than proteins are responsible for the transfer of genetic information from parents to progeny by Oswald Avery and co-workers.
1953:	The structure of DNA was determined by Francis Crick and James Watson.

Since then the rate of progress in genetics has increased rapidly, becoming an expanding area of research and industry.

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