Section 6 Selection methods Chapter 16 Breeding self-pollinated species Chapter 17 Breeding cross-pollinated species Chapter 18 Breeding hybrid cultivars Chapter 19 Breeding clonally propagated species Plant breeders depend on variability for success in their breeding programs. Once assembled or created, breed- ers used selection strategies or methods to discriminate among the variability to identify those with the desired genotypes that can be developed into cultivars. Selection strategies used depend on the modes of reproduction of the species being genetically improved. This section of the book is devoted to discussing the various methods of selection commonly used in plant improvement.
Principles of Plant Genetics and Breeding, Second Edition. Ó 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd. 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. The student should also be able to discuss: 6 The technique/method of backcrossing. 7 The method of multiline breeding.
8 The method of breeding composites. 9 The method of recurrent selection.1 Types of cultivars populations used in plant breeding – inbred pure lines, open-pollinated populations, hybrids, and There are six basic types of cultivars that plant breed- clones. Plant breeders use a variety of methods and ers develop. These cultivars derive from four basic techniques to develop these cultivars.
Principles of Plant Genetics and Breeding, Second Edition. Ó 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.1 Pure-line cultivars more widespread in cross-pollinated species (e., corn, sorghum), because the natural reproductive Pure-line cultivars are developed for species that are mechanisms (e., cross fertilization, cytoplasmic male highly self-pollinated. These cultivars are homoge- sterility) are more readily economically exploitable neous and homozygous in genetic structure, a condi- than in self-pollinated species.
tion attained through a series of self-pollination. These cultivars are often used as parents in the pro- duction of other kinds of cultivars. Pure-line cultivars 16.4 Clonal cultivars have a narrow genetic base. They are desired in Seeds are used to produce most commercial crop regions where uniformity of a product has a high pre- plants.
However, a significant number of species are mium. It should be pointed out, though, that genetic propagated by using plant parts other than seed (veg- uniformity occurs in other types of cultivars besides etative parts such as stems and roots). By using vege- pure lines, for example hybrids and vegetatively prop- tative parts, the cultivar produced consists of plants agated cultivars. with identical genotypes and is homogeneous.
How- ever, the cultivar is genetically highly heterozygous.2 Open-pollinated cultivars Some plant species are sexually reproducing but are propagated clonally (vegetatively) by choice. Such Contrary to pure-lines, open-pollinated cultivars are species are improved through hybridization, so that developed for species that are naturally cross-polli- when hybrid vigor exists it can be fixed (i., the vigor nated. The cultivars are genetically heterogeneous is retained from one generation to another) and then and heterozygous. Two basic types of open-pollinated the improved cultivar propagated asexually.
In seed cultivars are developed. One type is developed by propagated hybrids, hybrid vigor is highest in the F1, improving the general population by recurrent (or but is reduced by 50% in each subsequent generation. repeated) selection or bulking and increasing material In other words, whereas clonally propagated hybrid from selected superior inbred lines. The other type, cultivars may be harvested and used for planting the called a synthetic cultivar, is derived from planned next season’s crop without adverse effects, producers matings involving selected genotypes.
Open polli- of sexually reproducing species using hybrid seed nated cultivars have a broad genetic base. Another must obtain a new supply of seed, as previously important type of cultivar developed for open- indicated. pollinated species is the hybrid cultivar.3 Hybrid cultivars Apomixis is the phenomenon of production of seed Hybrid cultivars are produced by crossing inbred without the benefit of the union of sperm and egg lines that have been evaluated for their ability to pro- cells (i. The seed harvested duce hybrids with superior vigor over and above those are thus genetically identical to the mother plant of the parents used in the cross.
Hence, exploits the phenomenon of hybrid vigor (or hetero- apomictic cultivars have the same benefits of clonally sis) to produce superior yields. Heterosis is usually less propagated ones, as previously discussed. In addition, in crosses involving self-pollinated species than those they have the convenience of vegetative propagation involving cross-pollinated species. Hybrid cultivars through seed (versus propagation through cuttings are homogeneous but highly heterozygous.
Pollina- or vegetative plant parts). Apomixis is common in tion is highly controlled and restricted in hybrid perennial forage grasses. breeding to only the designated pollen source. In the past, physical human intervention was required to 16.6 Multilines enforce this strict pollination requirement, making hybrid seed expensive.
However, with time, various Multilines are developed for self-pollinating species. techniques have been developed to capitalize on natu- These cultivars consist of a mixture of specially devel- ral reproductive control systems (e., male sterility) oped genotypes called isolines (or near isogenic to facilitate hybrid production. Hybrid production is lines) because they differ only in a single gene (or a BREEDING SELF-POLLINATED SPECIES 305 defined set of genes). Isolines are developed primarily hybrid cultivar is the F1 product of a cross of highly for disease control, even though these cultivars, inbred (repeatedly selfed; homozygous) parents.
potentially, could be developed to address other envi- Crossing such pure lines produces highly heterozy- ronmental stresses. Isolines are developed by using gous F1 plants. Because the F1 is the final product the techniques of backcrossing in which the F1 is released as a cultivar, all plants are uniformly hetero- repeatedly crossed to one of the parents (recurrent zygous, and hence homogeneous in appearance. parent) that lacked the gene of interest (e., disease However, the seed harvested from the F1 cultivar is resistance).
F2 seed, consequently producing maximum heterozy- gosity and heterogeneity upon planting. The implica- tion for the farmer is that the current season’s seed 16.2 Genetic structure of cultivars and its cannot be saved for planting the next season’s crop implications for obvious reasons. The farmer who grows hybrid cultivars must purchase fresh seed from the seed com- The products of plant breeding that are released to pany for planting each season. Whereas this works farmers for use in production vary in genetic structure well in developed economies, hybrids generally do and, consequently, the phenotypic uniformity of the not fit well into the farming systems of developing product.
Furthermore, the nature of the product has countries where farmers save seed from the current implications in how it is maintained by the producers season for planting the next season’s crop. Nonethe- regarding the next season’s planting. less, the use of hybrid seed is gradually infiltrating crop production in developing countries.1 Homozygous and homogeneous cultivars 16.3 Heterozygous and heterogeneous cultivars A cultivar may be genetically homozygous and, hence, produce a homogeneous phenotype or product. Self- Other approaches of breeding produce heterozygous pollinated species are naturally inbred and tend to be and homogeneous (relatively) cultivars, for example, homozygous.
Breeding strategies in these species are synthetic and composite breeding. These approaches geared toward producing cultivars that are homozy- allow the farmer to save seed for planting. The products of economic importance are uni- cultivars are suited to production in developing coun- form. Furthermore, the farmer may save seed from tries, while synthetic cultivars are common in forage the current season’s crop (where legal and applicable) production all over the world.
for planting the next season’s crop, without loss of cultivar performance, regarding yield and product 16.4 Homozygous and heterogeneous cultivars quality. This attribute is especially desirable to pro- ducers in many developing countries where the gen- An example of such a breeding product is the mixed eral tradition is to save seed from the current season landrace types that are developed by producers. The for planting the next season. However, in developed component genotypes are homozygous but there is economies with well-established commercial seed such a large amount of diverse genotypes included production systems, intellectual property rights pro- that the overall cultivar is not uniform.
hibit the re-use of commercial seed for planting the next season’s crop, thus requiring seasonal purchase 16.5 Clonal cultivar of seed by the farmer from seed companies. Clones, by definition, produce offspring that are not only identical to each other but also the parent.2 Heterozygous and homogeneous cultivars Clones may be very heterozygous but whatever The method of breeding of certain crops leaves the advantage heterozygosity confers is locked in for as cultivar genetically heterozygous yet phenotypically long as propagation is clonally conducted. The off- homogeneous. One such method is hybrid cultivar spring of a clonal population is homogeneous.
Once production, a method widely used for production of, the genotype has been manipulated and altered in a especially, outcrossing species such as corn. The het- desirable way, for example through sexual means erozygous genetic structure stems from the fact that a (since some species are flowering but are not 306 CHAPTER 16 propagated through seed but vegetatively) the Crossing of two F1 plants (or selfing an F1) yields an changes are fixed for as long as clones are used for F2 plants (F1 F1 ¼ F2). Planting seed from the F2 propagation. Flowering species such as cassava and plants will yield an F2 population, the most diverse sugarcane may be genetically improved through sex- generation following a cross, in which plant breeders based methods, and thereafter commercially clonally often begin selection.
Selfing F2 plants produce F3 propagated. plants, and so on. It should be noted that the seed is one generation ahead of the plant, that is, an F2 plant bears F3 seed.3 Types of self-pollinated cultivars In terms of genetic structure, there are two types of self-pollinated cultivars: The symbol is the notation for selfing. (i) Those derived from a single plant.
S (ii) Those derived from a mixture of plants. The S notation is also used with numeric subscripts. Single plant selection may or may not be preceded In one usage So ¼ F1; another system indicates by a planned cross but often it is the case. derived from single plants are homozygous and homogeneous.
However, cultivars derived from plant mixtures may appear homogeneous but, because the 16.2 Symbols for inbred lines individual plants have different genotypes, and because some outcrossing (albeit small) occurs in Inbred lines are described by two systems. System I most selfing species, heterozygosity would arise later describes an inbred line based on the generation of in the population. The methods of breeding self- plants that is being currently grown. System II pollinated species may be divided into two broad describes both the generation of the plant from which groups – those preceded by hybridization and those the line originated as well as the generation of plants not proceeded by hybridization.
being currently grown. The following examples are used to distinguish between the two systems. Example 1: The base population is F2.4 Common plant breeding notations selects an F2 plant from the population and plants the F3 seeds in the next season. Plant breeders use shorthand to facilitate the docu- System I: The planted seed produces an F3 line.
mentation of their breeding programs.