Principles of Plant Genetics and Breeding. George Acquaah

that are asexually propagated are highly heterozygous. They are highly heterotic. Consequently, they are susceptible to inbreeding depression. For those species that can be hybridized without problems, any advantage of asexual propagation is that heterosis, where it occurs, is fixed for as long as the cultivar is propagated asexually.

       PloidyMany known species that are asexually propagated are interspecific hybrids or have high ploidy.

       ChimerismClones are stable over many generations of multiplication. The only source of natural variation, albeit rare, is somatic mutation in the bud. Plant breeders may generate variability by the method of mutagenesis. Whether natural or artificial, somatic mutations are characterized by tissue mosaicism, a phenomenon called chimerism.A chimera or chimeric change occurs when an individual consists of two or more genetically different types of cells. Though heritable changes, these mosaics can only be maintained by vegetative propagation (not transferable to progenies by sexual means).

      There are four basic types of chimeras.

      1 SectorialThis chimera is observed in a growing shoot as two different tissues located side‐by‐side. The effect of this modification is that the stem develops with two distinct tissues on each half.

      2 PericlinalThis type of chimera consists of two thin layers of different genetic makeup, one over the other.

      3 MericlinalWhen an outer layer of different genetic tissue does not completely extend over the layer below, the chimera is mericlinal.

      4 Graft chimerasUnlike the first three chimeras that have genetic origin, a graft chimera is a non‐heritable mixture of tissues that may occur after grafting is made.

      Additional information on chimeras is found in Chapter 12 (12.8). Whereas they are undesirable in crop plants, chimeras may be successfully exploited in horticulture.

7.9 Breeding approaches used in clonal species

      Several breeding approaches are used in the breeding of clonally propagated species.

      7.9.1 Planned introduction

      Just like seed, vegetative material (whole plants or parts) may be introduced into a new production area for evaluation and adaptation to the new area. Seedlings or cuttings may be introduced. However, the technology of tissue culture allows a large variety of small samples to be introduced in sterile condition. These disease‐free samples are easier to process through plant quarantine.

      7.9.2 Clonal selection

      Clonal selection has two primary goals: to maintain disease‐free and genetically pure clones, and the development of new cultivars.

       Purifying an infected cultivar

      Clonal cultivars may become infected by pathogens, some of which may be systemic (e.g. viruses). Two general approaches may be used to purify a cultivar to restore it to its disease‐free original genetic purity.

      1 Screening for disease‐free materialPlant materials may be visually inspected for the presence of pathogens. However, because some pathogens may be latent, a variety of serological and histological techniques are used to detect the presence of specific pathogens. Called indexing, these techniques can detect latent viruses (viral indexing) as well as other pathogens. A negative test may not always be proof of the absence of pathogens. It could be that the particular assay is not effective. The clean clonal material is then used as starting material for multiplication for propagation.

      2 Elimination of pathogenA positive test from indexing indicates the presence of a pathogen. Should this be the only source of planting material, the breeder has no choice but to eliminate the pathogen from the plant tissue by one of several methods.Tissue cultureEven when the pathogen is systemic, it is known that tissue from the terminal growing points is often pathogen‐free. Tissue from these points may be ascetically removed and cultured under tissue culture conditions to produce disease‐free plantlets. Through micropropagation, numerous disease‐free plants can be obtained.Heat treatmentThis may be short‐ or long‐duration. Short‐duration heat treatment is administered to the plant material for about 30 minutes to 4 hours at 43–57 °C. This could be in the form of hot air treatment or by soaking the material in hot water. This works well for fungal, bacterial, and nematode infections. For viruses, a longer treatment of about several weeks (two to four weeks) is used. Potted plants are held at 37 °C in a controlled environment for the duration of the treatment. Cuttings from the treated plants may be used as scions in grafts, or rooted into a seedling.Chemical treatmentThis surface sterilization treatment is suitable for elimination of pathogens that are external to the plant material (e.g. in tubers).Use of apomictic seedViral infections are generally not transmitted through seed in cultivars that are capable of apomixis (e.g. citrus).

       Clonal selection for cultivar development

      This procedure is effective if variability exists in the natural clonal population.

Year 1	Assemble clonal population. Plant and expose to diseases of interest. Select resistant clones with other superior traits and harvest individually.
Year 2	Grow progenies of selected clones and evaluate as in year 1. Select superior clones.
Year 3	Conduct preliminary yield trials. Select superior clones.
Year 4–6	Conduct advanced yield trials at multiple locations for cultivar release.

      7.9.3 Crossing with clonal selection

      This procedure is applicable to species that are capable of producing seed in appreciable quantities. Because heterosis can be fixed in clonal populations, the breeder may conduct combining ability analysis to determine the best combiners to be used in crosses. The steps below are not applicable to trees in which much longer times are needed.

Year 1	Cross selected parents. Harvest F1 seed.
Year 2	Plant and evaluate F1s. Select vigorous and healthy plants.
Year 3	Space plant clonal progeny rows of selected plants. Select about 100 to 200 superior plant progenies.
Year 4	Conduct preliminary yield trials.
Year 5–7	Conduct advanced yield trials for cultivar release.

      Other techniques that are applicable include backcrossing to transfer specific traits and wide crossing. The challenges with backcrossing are several. As previously indicated, clonal species are very heterozygous and prone to inbreeding depression. Backcrossing to one parent (the recurrent parent) provides opportunity for homozygosity and consequently inbreeding depression. To prevent this, breeders may cross the backcross to another clone instead of the recurrent parent, followed by selection to identify superior plants. The process is repeated as needed.

      7.9.4 Mutation breeding

      The