Principles of Plant Genetics and Breeding. George Acquaah

Those with normal flowers and seed setSpecies in this category produce normal flowers and are capable of sexual reproduction (to varying extents) without artificial intervention (e.g. sugar cane). However, in crop production, the preference is to propagate them sexually. Such species enjoy the advantages of both sexual and asexual reproduction. Hybridization is used to generate recombinants (through meiosis) and introduce new genes into the adapted cultivar, while vegetative propagation is used to indefinitely maintain the advantages of the heterozygosity arising from hybridization.

      2 4 Those with normal flowers but poor seed setSome plant species produce normal‐looking flowers that have poor seed set, or set seed only under certain conditions but not under others. These restrictions on reproduction make it unattractive to use seed as a means of propagation. However, the opportunity for hybridization may be exploited to transfer genes into adapted cultivars.

      3 5 Produce seed by apomixisThe phenomenon of apomixis results in the production of seed without fertilization, as was first discussed in Chapter 6. Over 100 species of perennial grasses have this reproductive mechanism.

      4 6 Non‐flowering speciesThese species may be described as “obligate asexually propagated species” because they have no other choice. Without flowers (or with sterile flowers) those species can only be improved by asexual means. Genetic diversity is not obtained via recombination but by other sources (e.g. mutation).

7.5 Types of clonal propagation

      Clonal propagation can be a natural process or artificially conducted.

       Natural clonal propagationClones are common in nature. Clonal propagation is important for many herbaceous species as well as woody perennial plants throughout the world. The genera Prunus and Populus produce clones by suckering, while Betula, Carpinus, Corylus, Quercus, Salix, and Tilia are all genera with the ability to self‐coppice.

       Artificial clonal propagationSome crops are commercially produced by clonal propagation using various parts (e.g. tubers, corms, bulbs, stolons, etc.). Some species are obligated to this mode of propagation because they have lost their capacity to flower (e.g. leek, some potato cultivars). However, some of these species that are produced clonally as a preference may also have viable sexual reproduction as an option (e.g. potato, strawberry). In fact, some species with flowering capabilities have a long tradition of being reproduced clonally (e.g. apple, roses, ornamental trees, and shrubs). Plants derived from true seeds of those same species often have a long juvenile stage, and take a long time to reach commercially interesting size (orchids, tulips, chrysanthemum, potato). Artificial methods of clonal propagation include cuttings, grafting, and the more sophisticated laboratory technique of tissue culture. For orchids, in vitro clonal propagation is the only commercially viable method of micropropagation.Clonal multiplication of the cultivar is very important in horticulture and silviculture (tree production). The first step in clonal propagation is to identify and select a genetically superior (elite) plant. The part of the plant used as a propagule varies among species and includes stem, roots, bulbs, and stolons as stated previously.

7.6 Importance of clonal propagation in plant breeding

      Clonal propagation has several significant applications in plant breeding.

       Quick production of quality breeding and planting stockIn tree breeding, somatic propagation of trees is advantageous over the use of seeds to raise stock. It is fast and economical. Breeding time is shortened by using micropropagation.

       Early flower inductionClonally propagated plants are known to flower earlier than seed‐produced plants. This helps to speed up the reproductive cycle of the species and hence accelerate the breeding and testing time in a breeding program.

       Germplasm maintenanceGenotypes in clonal banks are maintained by clonal propagation. Clonal testing can be conducted to evaluate the accessions and their interaction with environment in a greenhouse setting.

       Maintenance of genetic uniformityCross‐pollinated species are naturally highly heterozygous, resulting in the progenies from such genotypes not only being true to type, but also missing some of the parental qualities. Clonal propagation is used to maintain the genetic characteristics of species.

       Production of disease‐free plantsVegetative propagation techniques such as grafting the in vitro culture of specific tissues can produce healthy clones from disease‐susceptible species.

       Propagating problematic speciesSome species produce few or no viable seeds, or may have seed dormancy issues, while others have seeds with poor germination capacity. Sometimes, germination is very slow, causing such species to take a long time to produce trees of marketable size. The use of clonal propagation can usually be a more rapid and economical technique than seeds in multiplying such species.

       Multiplication of sexually derived sterile hybridsInterspecific hybridization in plants commonly results in products that are sterile. Clonal propagation may be used to multiply such hybrids.

       Maintenance of genetic gainAfter a breeder makes a cross, the next step is to conduct repeated cycles of clonal reproduction to identify and maintain superior plants. Clonal propagation helps capture and maintain maximum genetic gains.

7.7 Breeding implications of clonal propagation

      There are certain characteristics of clonal propagation that have breeding implications.

       Clonal species with viable seed and high pollen fertility can be improved by crosses.

       Unlike crossing in sexually reproducing species, which often requires additional steps to fix the genetic variability in a genotype for release as a cultivar (except for hybrid cultivars), clonal cultivars can be released immediately following a cross, provided a desirable genotype combination has been achieved.

       When improving species whose economic parts are vegetative products, it is not important for the product of the crossing to be fertile.

       Because of the capacity to multiply from vegetative material (either through methods such as cuttings or micropropagation), the breeder only needs to obtain a single desirable plant to be used as stock.

       Heterosis (hybrid vigor), if it occurs, is fixed in the hybrid product. That is, unlike hybrid cultivars in seed‐producing species, there is no need to reconstitute the hybrid. Once bred, heterozygosity is maintained indefinitely.

       It is more difficult to obtain large quantities of planting material from clones in the short term.

       Plant species that are vegetatively parthenocarpic (e.g. banana) cannot be improved by the method of crossing parents.

       Species such as mango and citrus produce polyembryonic seeds. This reproductive irregularity complicates breeding because clones of the parent are mixed with hybrid progeny.

       Many clonal species are perennial outcrossers and intolerant of inbreeding. These are highly heterozygous.

       Unlike sexual crop breeding in which the genotype of the cultivar is determined at the end of the breeding process (because it changes with inbreeding), the genotype of a clone is fixed and determined at the outset.

       Both general combining ability (GCA) and specific combining ability (SCA), that is performance in crosses, can be fully exploited with appropriate breeding approaches.

7.8 Genetic Issues in Clonal Breeding

       Genetic make‐upAll the progeny from an individual propagated asexually are genetically identical (clones) and uniform. Clones are products of mitosis. Any variation occurring among them is environmental in origin.

       Heterozygosity