George Acquaah - Principles of Plant Genetics and Breeding

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.

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 and heterosisMany species 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.

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

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.

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

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).

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