Section 6 Classic methods of plant breeding Chapter 16 Breeding self-pollinated species Chapter 17 Breeding cross-pollinated species Chapter 18 Breeding hybrid cultivars Methods of breeding (or precisely, methods of selection) crops vary according to the natural method of repro- duction of the species. Generally, there are two categories of breeding methods: those for self-pollinated species and those for cross-pollinated species. In practice, there is no hard distinction between the two; breeders crossover and use methods as they find useful. Furthermore, plant breeders may use a combination of several methods in one breeding program, using one procedure at the beginning and switching to another along the way.
It should be mentioned also that the steps described in the various chapters for each selection method are only suggested guidelines. Breeders may modify the steps, regarding the number of plants to select, the number of generations to use, and other aspects of breeding, to suit factors such as budget and the nature of the trait being improved. 16 Breeding self-pollinated species Purpose and expected outcomes As previously discussed, self-pollinated species have a genetic structure that has implication in the choice of methods for their improvement. They are naturally inbred and hence inbreeding to fix genes is one of the goals of a breeding program for self-pollinated species in which variability is generated by crossing.
However, crossing does not precede some breeding methods for self-pollinated species. The purpose of this chapter is to discuss specific methods of selection for improving self-pollinated species. After studying this chapter, the student should be able to discuss the character- istics, application, genetics, advantages, and disadvantages of the following methods of selection: 1 Mass selection. 2 Pure-line selection.
5 Single-seed descent. And to: 6 Describe the technique/method of backcrossing. 7 Discuss the method of multiline breeding. 8 Discuss the method of breeding composites.
9 Discuss the method of recurrent selection. Types of cultivars Pure-line cultivars At the beginning of each project, the breeder should Pure-line cultivars are developed for species that decide on the type of cultivar to breed for release to pro- are highly self-pollinated. These cultivars are homo- ducers. The breeding method used depends on the type geneous and homozygous in genetic structure, a con- of cultivar to be produced.
There are six basic types of dition attained through a series of self-pollinations. cultivars that plant breeders develop. These cultivars These cultivars are often used as parents in the derive from four basic populations used in plant breed- production of other kinds of cultivars. Pure-line cul- ing – inbred pure lines, open-pollinated populations, tivars have a narrow genetic base.
They are desired hybrids, and clones. Plant breeders use a variety of in regions where uniformity of a product has a high methods and techniques to develop these cultivars. BREEDING SELF-POLLINATED SPECIES 283 another), and then the improved cultivar propagated Open-pollinated cultivars asexually. In seed-propagated hybrids, hybrid vigor is Contrary to pure lines, open-pollinated cultivars are highest in the F1, but is reduced by 50% in each sub- developed for species that are naturally cross-pollinated.
In other words, whereas clonally The cultivars are genetically heterogeneous and hetero- propagated hybrid cultivars may be harvested and used zygous. Two basic types of open-pollinated cultivars for planting the next season’s crop without adverse are developed. One type is developed by improving the effects, producers of sexually reproducing species using general population by recurrent (or repeated) selection hybrid seed must obtain a new supply of seed, as previ- or bulking and increasing material from selected super- ously indicated. ior inbred lines.
The other type, called a synthetic cultivar, is derived from planned matings involving Apomictic cultivars selected genotypes. Open-pollinated cultivars have a broad genetic base. Apomixis is the phenomenon of the production of seed without the benefit of the union of sperm and egg cells (i. The seed harvested is hence Hybrid cultivars genetically identical to the mother plant (in much the Hybrid cultivars are produced by crossing inbred lines same way as clonal cultivars).
Hence, apomictic cultivars that have been evaluated for their ability to produce have the same benefits of clonally propagated ones, as hybrids with superior vigor over and above those of the previously discussed. In addition, they have the con- parents used in the cross. Hybrid production exploits venience of vegetative propagation through seed (versus the phenomenon of hybrid vigor (or heterosis) to pro- propagation through cuttings or vegetative plant parts). duce superior yields.
Heterosis is usually less important Apomixis is common in perennial forage grasses. in crosses involving self-pollinated species than in those involving cross-pollinated species. Hybrid cultivars are Multilines homogeneous but highly heterozygous. Pollination is Multilines are developed for self-pollinating species.
highly controlled and restricted in hybrid breeding to These cultivars consist of a mixture of specially devel- only the designated pollen source. In the past, physical oped genotypes called isolines (or near isogenic lines) human intervention was required to enforce this strict because they differ only in a single gene (or a defined set pollination requirement, making hybrid seed expensive. Isolines are developed primarily for disease However, with time, various techniques have been control, even though these cultivars could, potentially, developed to capitalize on natural reproductive control be developed to address other environmental stresses., male sterility) to facilitate hybrid produc- Isolines are developed by using the techniques of back- tion. Hybrid production is more widespread in cross- crossing in which the F1 is repeatedly crossed to one of pollinated species (e., corn, sorghum), because the the parents (recurrent parent) that lacked the gene of natural reproductive mechanisms (e., cross-fertilization, interest (e.
cytoplasmic male sterility) are more readily economically exploitable than in self-pollinated species. Genetic structure of cultivars Clonal cultivars and its implications Seeds are used to produce most commercial crop plants. The products of plant breeding that are released to However, a significant number of species are propag- farmers for use in production vary in genetic structure ated by using plant parts other than seed (vegetative and consequently the phenotypic uniformity of the parts such as stems and roots). By using vegetative parts, product.
Furthermore, the nature of the product has the cultivar produced consists of plants with identical implications in how it is maintained by the producers, genotypes and is homogeneous. However, the cultivar regarding the next season’s planting. is genetically highly heterozygous. Some plant species sexually reproduce but are propagated clonally (vegeta- Homozygous and homogeneous cultivars tively) by choice.
Such species are improved through hybridization, so that when hybrid vigor exists it can be A cultivar may be genetically homozygous and hence fixed (i., the vigor is retained from one generation to produce a homogeneous phenotype or product. 284 CHAPTER 16 Self-pollinated species are naturally inbred and tend to will allow the farmer to save seed for planting. Com- be homozygous. Breeding strategies in these species are posite cultivars are suited to production in developing geared toward producing cultivars that are homozygous.
countries, while synthetic cultivars are common in forage The products of economic importance are uniform. production all over the world. Furthermore, the farmer may save seed from the current season’s crop (where legal and applicable) for planting Homozygous and heterogeneous cultivars the next season’s crop, without loss of cultivar per- formance, regarding yield and product quality. This Examples of such a breeding product are the mixed attribute is especially desirable to producers in many landrace types that are developed by producers.
The developing countries where the general tradition is to component genotypes are homozygous, but there is save seed from the current season for planting the such a large amount of diverse genotypes included that next season. However, in developed economies with the overall cultivar is not uniform. well-established commercial seed production systems, intellectual property rights prohibit the reuse of com- mercial seed for planting the next season’s crop, thus Clonal cultivar requiring seasonal purchase of seed by the farmer from Clones, by definition, produce offspring that are not seed companies. only identical to each other but also to the parent.
Clones may be very heterozygous but whatever advan- Heterozygous and homogeneous cultivars tage heterozygosity confers is locked in for as long as propagation is clonally conducted. The offspring of a The method of breeding of certain crops leaves the clonal population are homogeneous. Once the geno- cultivar genetically heterozygous yet phenotypically type has been manipulated and altered in a desirable homogeneous. One such method is hybrid cultivar way, for example through sexual means (since some production, a method widely used for the production species are flowering, but are vegetatively propagated of especially outcrossing species such as corn.
The and not through seed), the changes are fixed for as heterozygous genetic structure stems from the fact long as clones are used for propagation. Flowering that a hybrid cultivar is the F1 product of a cross of species such as cassava and sugarcane may be genetically highly inbred (repeatedly selfed, homozygous) parents. improved through sex-based methods, and thereafter Crossing such pure lines produces highly heterozygous commercially clonally propagated. Because the F1 is the final product released as a cultivar, all plants are uniformly heterozygous and hence homogeneous in appearance.
However, the seed har- Types of self-pollinated cultivars vested from the F1 cultivar is F2 seed, consequently pro- ducing maximum heterozygosity and heterogeneity In terms of genetic structure, there are two types of self- upon planting. The implication for the farmer is that the pollinated cultivars: current season’s seed cannot be saved for planting the next season’s crop for obvious reasons. The farmer who grows hybrid cultivars must purchase fresh seed from 1 Those derived from a single plant. the seed company for planting each season.
Whereas this 2 Those derived from a mixture of plants. works well in developed economies, hybrids generally do not fit well into the farming systems of developing Single-plant selection may or may not be preceded by a countries where farmers save seed from the current planned cross but often it is the case. Cultivars derived season for planting the next season’s crop. Nonetheless, from single plants are homozygous and homogeneous.
the use of hybrid seed is gradually infiltrating crop pro- However, cultivars derived from plant mixtures may duction in developing countries. appear homogeneous but, because the individual plants have different genotypes, and because some outcrossing (albeit small) occurs in most selfing species, heterozy- Heterozygous and heterogeneous cultivars gosity would arise later in the population. The methods Other approaches of breeding produce heterozygous of breeding self-pollinated species may be divided into and homogeneous (relatively) cultivars, for example, two broad groups – those preceded by hybridization synthetic and composite breeding. These approaches and those not preceded by hybridization.
BREEDING SELF-POLLINATED SPECIES 285 Common plant breeding notations the next season, the symbolism will be as follows: Plant breeders use shorthand to facilitate the documen- tation of their breeding programs. Some symbols are System I: the planted seed produces an standard genetic notations, while others were developed F4 line. Unfortunately there is no one comprehen- System II: the planted seed produces an sive and universal system in use, making it necessary, F2 derived line in F4 or an F2:4 line. especially with the breeding symbols, for the breeder to always provide some definitions to describe the specific Case 2.
The breeder harvests a single F4 and plants F5 steps in a breeding method employed in the breeding seed in a row. System I: the planted row produces an F5 line. Symbols for basic crosses System II: the planted row constitutes 1 F. The symbol F (for filial) denotes the progeny of a an F4 derived line in F5 or an F4:5 line.
cross between two parents. If the parents are Similarly the S notation may be treated likewise. homozygous, the F1 generation will be homogeneous. Taking case 1 for example: Crossing of two F1 plants (or selfing an F1) yields an F2 plant (F1 × F1 = F2).
Planting seed from the F2 System I: S1 line.