The most appropriate type of cultivar that can be developed to best fit the needs of a production situation will be determined, in part, by the breeding system and mode of reproduction of the species involved.
A cultivar (or variety) is defined as a group of one or more genotypes with a combination of characters giving it distinctness, uniformity and stability (DUS).
The requirement of distinctness is needed to protect proprietary cultivars and ensure that different organizations are not trying to claim the same cultivar and identify such cultivars as to their proprietary ownership. The requirements of uniformity and stability are there to protect the growers and to ensure that they are being sold something that will grow and exhibit the characteristics described by the breeder.
The further requirement of any new cultivar is perhaps obvious, but nevertheless is a statutory requirement in many countries and referred to as value for cultivation and use (VCU). VCU can be determined by two primary methods and there will always be debate regarding which system is better. In the UK, which organizes statutory trials, VCU is described as follows:
“The quality of the plant variety shall, in comparison with the qualities of other plant varieties in a national list, constitute either generally or as far as production in a specific area is concerned, a clear improvement either as regards crop farming or the use made of harvested crops or of products produced from these crops. The qualities of the plant variety shall for this purpose be taken as a whole, and inferiority in respect of certain characteristics may be offset by other favourable characteristics.”
In a few countries (including the US) any plant breeder can sell seed from a cultivar developed and registered, irrespective of how well adapted it is to a given region or how productive the cultivar is likely to be. The choice of which cultivar to grow is left entirely to farmers and producers. It is common that farmers will allot a small proportion of the farm to plant a new cultivar, and if acceptable, will increase acreage with time. Obviously, unadapted cultivars or those with inferior end-use quality are unlikely to gain in acreage in this way. Similarly, companies (seed or breeding) and organizations rely on their reputation to sell their products. As is obvious, reputations can easily be tarnished by releasing and selling inferior products.
However, it is more common that countries have statutorily organized trialling schemes to determine the VCU of cultivars that are to be released. This testing is usually conducted over two or three years, in a range of environments in which the cultivars are likely to be grown. If breeding lines perform better than cultivars already available in that country, then government authorities will place that cultivar on the National List. Only cultivars that are included on the National List are eligible for propagation in that country. In some countries, newly listed cultivars are also entered into further statutory trials for one or two additional years. Based on performance in these extra years' trials, cultivars may be added to a Recommended Varieties List. This effectively means that the government authority, or testing agency, is recommending that it would be a suitable new cultivar for farmers to consider. The theoretical advantage of statutory VCU testing is that it only allows ‘the very best cultivars’ to be grown and prevents unadapted cultivars from being sold to farmers. The major drawbacks of the regulatory trialling schemes are:
The criteria for judging both DUS and VCU will be strongly determined by the type of species, particularly its mode of reproduction and multiplication for production.
It is essential to have an understanding of the mode(s) of reproduction prior to the onset of a plant breeding programme. The type of reproduction of the species (at least in commerce) will determine the way that breeding and selection processes can be maximized to best effect. There are two general types of plant reproduction, sexual and asexual.
Sexual reproduction involves fusion of male and female gametes that are derived either from two different parents or from a single parent. Sexual reproduction is, of course, reliant on the process of meiosis. This involves megaspores within the ovule of the pistil and the male microspores within the stamen. In a typical diploid species, meiosis involves reduction division of the 2n female cell to form four haploid megaspores, by the process of megasporogenesis. This process in male cells, to form microspores, is called microsporogenesis. Fertilization of the haploid female cell by a haploid male pollen cell results in the formation of a diploid 2n embryo. The endosperm tissue of the seed can result from the union of two haploid nuclei from the female with another from the pollen, and hence ends up as being 3n.
Asexual reproduction is the multiplication by use of plant parts (vegetative propagation, by tubers, by cuttings, in other words by cloning) or by the production of seeds (apomixis) that do not involve the union of male and female gametes. In general, organisms grow by cell division in a process called mitosis. The process of mitosis will result in two cells that are identical in genetic make-up and of the same composition as the parental cell.
Seeds are effectively classified according to the source of pollen that is responsible for the fertilization. In the case of self-pollination, the seeds are a result of fertilization of female egg cells by pollen from the same plant. Cross-pollination occurs when female egg cells are fertilized by pollen from a different plant, usually one that is genetically different. As a result plant species are usually classified into self-pollinating and cross-pollinating species. This is of course a gross generalization. There are species which are effectively 100% self-pollinating, those that are 100% cross-pollinating, but there exists a whole range of species that cross-pollinate or self-pollinate to varying degrees. From the top 122 crop plants grown worldwide, 32 are mainly self-pollinating species, 70 are predominantly cross-pollinating, and the remaining 20 are cross-pollinating but do show a degree of tolerance to successive rounds of inbreeding.
The method of pollination will be an important factor in determining the type of cultivar that can, or will, be most adapted to cultivation. For example, most species that can be readily used in F1 hybrid production are generally cross-pollinating but need to be tolerant of inbreeding by selfing. This is because the hybrids are effectively the cross-pollinated progeny between two inbred parents.
Self-pollinating species:
Cross-pollinating species tend to be intolerant to inbreeding, principally because they carry many deleterious recessive alleles (these exist in the populations since they can be tolerated in heterozygous form). Generally, cross-pollinating species:
Particularly important are the outcrossing mechanisms. Cross-pollinating species often have distance barriers, time barriers or other mechanisms that limit, reduce or prevent self-pollination. Plants may be monoecious, where separate male and females flowers are located on different parts of the plant (e.g. maize) or indeed dioecious, where male and female flowers occur on different plants. Cross-pollination is also favoured in many cases where male pollen is shed at a time when the female stigma on the same plant is not receptive.
Other, more clearly defined sets of mechanisms are those termed as self-incompatibility. Self-incompatibility occurs when a plant with fully functional male and female parts will not produce mature seed by self-pollination. There is a set of mechanisms that have naturally evolved to prevent self-pollination and hence to increase cross-pollination within plant species, thus promoting heterozygosity. Adaptation to environmental conditions is greater if wider ranges of genotypes are produced in a progeny (i.e. the progeny shows greater genetic variation). Thus the chances of survival of at least some of the progeny will be enhanced, and conversely the chance of extinction will be reduced.
There are a number of mechanisms than can determine self-incompatibility in higher plants:
In several species (e.g. Brassica spp.) self-incompatibility can be overcome by bud-pollination, where pollen is applied to receptive stigmas of plants before the flowers open, as the self-incompatibility mechanism is not functional at this growth stage. Self-incompatibility is rarely complete, and usually a small proportion of selfed seed can be produced under certain circumstances. For example, it has been found that environmental stress factors (particularly caused by applying salt solution to developing flowers) tends to increase the proportion of self-seed produced.
Asexual reproduction in plants produces offspring that are genetically identical to the mother plant, and plants that are produced this way are called clones. Asexual reproduction can occur by two mechanisms: reproduction through plant parts that are not true botanical seeds and reproduction through apomixis.
A number of different plant parts can be responsible for asexual reproduction. For example, the following are some of the possible organs that are reproductive propagules of plants:
Asexual production of plant seeds can occur in obligate and facultative apomicts. In obligate apomicts the seed that is formed is asexually produced, while in facultative apomicts most seeds are asexually produced, although sexual reproduction can occur.
Apomixis can arise by a number of mechanisms that differ according to which plant cells are responsible for producing an embryo (i.e. androgenesis from the sperm nucleus of a pollen grain; apospory, from somatic ovary cells; diplospory, from 2n megaspore mother cells; parthenogenesis from an egg cell without fertilization; and semigamy from sperm and egg cells independently without fusion). Apomixis can occur spontaneously, although in many cases pollination must occur (pseudogamy) if viable apomictic seeds are to be formed. Although the role of pseudogamy is not understood in most cases, pollination appears to stimulate embryo or endosperm development.
It may seem obvious that modes of reproduction determine the type of cultivar that is produced for exploitation. Cultivar types include pure-lines, hybrids, clones, open-pollinated populations, composite-crosses, synthetics and multilines. Obviously it would be difficult, if not impossible to develop a pure-line cultivar of a crop species like potato (Solanum tuberosum) as it is mainly reproduced vegetatively, and has many deleterious (or lethal) recessive alleles. Similarly, pea (Pisum sativum) is almost an obligate self-pollinator and so it would be difficult to develop hybrid pea, if nothing else seed production is likely to be expensive. A brief description of the different types of cultivar is presented below.
Pure-line cultivars are homozygous, or near-homozygous, lines. Pure-line cultivars can be produced most readily in naturally self-pollinating species (e.g. wheat, barley, pea and soybeans). But they can also be produced from species that we tend to consider as cross-pollinating ones (e.g. pure-line maize, gynoecious cucumber and onion). There is no universally agreed definition of what constitutes a pure-line cultivar, but it is generally accepted that it is normally one in which the line is homozygous for the vast majority of its loci (usually 90% or more).
The most common method used to develop pure-line cultivars from inbreeding species is to artificially hybridize two chosen (usually) homozygous parental lines, allow the heterozygous first filial generation (F1) to self to obtain F2 seed, and continue to allow selfing in future generations, up to a point where the line is considered to be ‘commercially true breeding’, maybe the F6 or F7. At the same time, it has been common to carry out recurrent phenotypic selection on the segregating population over each generation. As described in Chapter 9, the use of double haploid lines is gaining popularity in a number of crop species as a fast method to gain homozygosity.
Open-pollinated cultivars are heterogeneous populations comprised of different plants which are genetically non-identical. The component plants tend to have a high degree of heterozygosity. Open-pollinated cultivars are almost exclusively from cross-pollinating (out-breeding) species. These are populations that have been selected to a standard that allows for variation in many traits, but which shows ‘sufficient’ stability of expression in the characters of interest. Stability of these traits can be used to pass the DUS requirements necessary for cultivar release. Examples of open-pollinated cultivars would include onions, rye, herbage grass, non-hybrid sweetcorn, sugar beet and oil palm.
In developing open-pollinated cultivars, the initial hybridization (the point at which the genetic diversity and variation is created) is usually between two open-pollinated populations. In this case segregation is apparent at the F1 generation. Desirable populations are identified and improved by increasing the frequency of desirable phenotypes within them.
Hybrid cultivars (single cross, three-way cross and double cross hybrids) have different levels of homogeneity but, importantly, are highly heterozygous. An F1 hybrid (single cross) cannot be reproduced from seed collected from them because the progeny would then effectively be an F2 and would segregate, thus resulting in a different and non-uniform population. However, the other types of hybrid (three-way cross and double cross) are produced and grown, and although they give segregating populations of plants, they do so in a predictable way and at a predictable level.
Hybrid breeding is perhaps the most complex of the breeding methods. The process of cultivar development involves at least two stages. The first stage is to select desirable inbred lines from chosen out-pollinated populations. These inbred selections are then used in test crosses to allow their comparison and assessment in relation to their general or specific combining ability. Superior parents are selected, and these are then hybridized to produce seed of the hybrid cultivar. The parent lines are then maintained and used to continually reproduce the F1 hybrids. Despite its complexity, hybrid breeding has been the method of choice in many out-breeding species because, through the exploitation of heterosis, hybrid cultivars often yield more than other types of cultivars.
Clonal cultivars are genetically uniform but tend to be highly heterozygous. Uniformity of plant types is maintained through vegetative rather than sexual reproduction. Cultivars are vegetatively propagated by asexual reproduction (cloning) including cuttings, tubers, bulbs, rhizomes and grafts (e.g. potatoes, bananas, peaches, apples, cassava, sugarcane, strawberries, blueberries and chrysanthemums). A cultivar can also be classified as a clone if it is propagated through obligate apomixis (e.g. buffelgrass).
Clonal varietal development begins by either sexual hybridization of two parents (often clones) or the selfing of one of them to generate genetic variability through controlled crosses but using the normal process of sexual reproduction. Most of the parental lines will be highly heterozygous and segregation will be observed even in the first generation. Desirable recombinants are selected from between the various clones. Breeding lines are maintained and multiplied through vegetative reproduction, and hence the genetic constitution of each selection remains ‘fixed’.
Intercrossing a set number of defined parent lines generates a synthetic variety, and so it can be maintained by reconstruction of the population from the parents. In the simplest case a ‘first generation, two-parent synthetic’ is very similar to an F1 hybrid. Synthetic lines can be derived using parents that are clonally propagated or inbred lines, although the latter instance is not common. From the initial open pollination, synthetic cultivars are labelled as a series of generations/categories (Syn.1, Syn.2,…, Syn.n) according to the number of open-pollinated generations that have been grown since the synthetic line was first generated. So, the first generation of a synthetic variety is classified as Syn.1; if this population is then open pollinated, the next generation is classified as Syn.2.
The use of synthetic cultivars has been most successful in cases where crop species show partial self-incompatibility (e.g. alfalfa). Examples of other crops where synthetic varieties have been released include canola (B. rapa cultivar types), rye, pearl millet, broom grass and orchard grass.
Multiline cultivars are mixtures or blends of a number of different cultivars or breeding lines. By definition it is accepted that each genotype in the mixture will be represented by at least 5% of the total seed lot. Many multilines are the result of developing near-isogenic lines and mixing these to form a population of lines. These cultivars are usually self-pollinating species. A multiline is therefore not the same as a synthetic, where the aim is to maintain heterozygosity by inter-crossing between the parent lines. Multilines became popular because of the wish to increase disease resistance but reduce the selective pressure on the pathogen to evolve/mutate in order to overcome the biological resistance that had been bred into the lines. For example, near-isogenic lines of barley, which differ in terms of possessing qualitative disease resistance genes, could be mixed to make a multiline. The main thought is to affect the epidemiology of the pathogen such that it would be less likely to evolve virulence to all resistance genes in the mixture.
Composite-cross cultivars are populations derived by intercrossing two or more cultivars or breeding lines. These cultivar types have all tended to be inbreeding species (e.g. barley or lima beans). After the initial hybridizations have been carried out, the composite-cross population is multiplied in a chosen environment such that the most adapted segregants will predominate and those less adapted to these conditions will occur at lower frequencies. A composite-cross population cultivar is therefore continually changing and can be considered (in a very loose sense) similar to the old land races. These cultivars are difficult to commercialize as they cannot be maintained with the same composition as the originally released cultivar.
Plant species are categorized into annuals and perennials. World crop plants are fairly evenly distributed between annuals (approximately 70 species) and perennials (approximately 50 species). All major self-pollinating crop species are annuals, while the greatest majority of cross-pollinating crops are perennials. Perennials pose greater difficulty in breeding than most annuals. For example, most perennials do not become reproductive within the first years of growth from seed, and also most perennials are clonally propagated, which can cause additional difficulties in maintaining disease-free parental lines and breeding material. On the other hand, winter annuals also present difficulties as they require vernalization or a chilling treatment before moving from vegetative to reproductive growth, and so can increase the time necessary for developing new cultivars.
Female and male sterility has been identified in many crop species, and both genetic and cytoplasmic sterility has been identified in various species. Such sterility can, of course, cause problems to breeders and limit the choice of parental cross combinations that are possible. But it also can be exploited when, for example, developing hybrid cultivars.
1. Complete the following table by assigning a YES or NO to each of the 16 cells.
| Pure-line cultivar | Open-pollinated cultivar | F1 Hybrid cultivar | Clonal cultivar | |
| Are these cultivars composed of only one single genotype? | ||||
| Is heterosis a major yield factor in resulting cultivars? | ||||
| Are resulting cultivars propagated by means of botanical seeds? | ||||
| Can the seed, or plant parts, of a cultivar be used for its own propagation? |
2. Inbreeding and out-breeding species tend to have different characteristics. Explain factors that would determine whether a given species should be classified as inbreeding or out-breeding.
3. A number of different cultivar types are available in agriculture. Outline the major features of the following cultivar types:
4. List two inbreeding crop species and two out-breeding crop species that have been exploited as:
5. Describe the major features of the following types of apomixis:
6. List five different plant parts that can be used for asexual reproduction.