Principles of Plant Genetics and Breeding. George Acquaah
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in detail in Chapter 22. Inducing variability via mutagenesis is challenging for two key reasons. Being rare events, a large population of M1V2 is needed to have a good chance of observing desired mutants. Obtaining a large number of vegetative propagules is difficult. Also, mutations occur in individual cells. Without the benefit of meiosis, the mutated clonal material develops chimeras. Using adventitious buds as starting material reduces the chance of chimeras. A mutation in the epidermal cell (usually there is one) would result in an adventitious shoot that originated from a single mutant cell. This technique is not universally applicable.
7.9.5 Breeding implications, advantages, and limitations of clonal propagation
There several advantages and limitations of breeding clonally propagated species:
Advantages
Sterility is not a factor in clonal propagation because seed is not involved.
Because clonal plants are homogeneous, the commercial product is uniform.
Micropropagation can be used to rapidly multiply planting material.
Heterozygosity and heterosis are fixed in clonal populations.
Disadvantages
Clonal propagules are often bulky to handle (e.g. stems, bulbs).
Clones are susceptible to devastation by an epidemic. Because all plants in the clonal population are identical, they are susceptible to the same strain of pathogen.
Clonal propagules are difficult to store for a long time, because they are generally fresh and succulent materials.
7.10 Natural propagation
Some crops rely on clonal propagation: tubers, corms, cuttings, bulbs, stolons, etc. Such crop species may have lost the capacity to flower (leek, some potato cultivars). Their progeny is genetically identical to the plant from which it was derived (except if the primordial cell contained some mutation [chimerism]). Normally, such species may also have sexual reproduction as a natural option. Potato, for example, may form berries with true seeds, and strawberry produces fruits with seeds. Such seeds produce genetically heterogeneous progeny because of segregation, since most clonally reproducing species have a high level of heterozygosity. Plants from natural clonal tissues are usually vigorous and can produce flowers and fruit in the same or next season. Plants derived from true seeds of those same species often have a long juvenile stage, and take long to reach commercially interesting size (orchids, tulips, chrysanthemum, potato). The same is true for species that naturally do not reproduce clonally, but as crops which have been reproduced that way for a long time. Examples are apple, rose, and ornamental trees and shrubs, which are reproduced by grafting or cutting.
7.11 In vitro culture
In vitro culture or tissue culture of cells, tissues, organs, and protoplasts is used as a technique by plant breeders and growers for propagation. It is critical in some modern plant breeding approaches, specifically biotechnology, in which genetic alterations are conducted under aseptic conditions. The cell is the fundamental unit of structure and function of a plant, containing all the genetic information. Tissues and even single cells can be nurtured to develop into full plants. In biotechnology, it is critical to be able to nurture a single cell into a full plant in order to apply some of the sophisticated techniques such as gene transfer or transformation. The technique of tissue culture may be used to assist plant breeders who realize wide crosses to be able to nurture young embryos into full plants. Plant germplasm of vegetatively propagated species may be maintained in germplasm banks using the tissue culture technique (see Box 7.1).
David Okeh Igwe, George Nkem Ude, and George Acquaah
Department of Natural Sciences, Bowie State University, Bowie, Maryland, USA
Introduction
Bananas and plantains (Musa spp.) are perennial crops with rapid growth rate. They belong to the family Musaceae and are cultivated all year round in the tropics and sub‐tropics. Considered to be the favorite fruit crops of the world, they also are globally distributed in more than 120 countries, with a total production of approximately 106–140 million tonnes per year (Molina and Kudagamage 2002; FAOSTAT 2014). Bananas and plantains are the highest export fruit crops (FAO 2011) and are ranked fourth‐most important in sub‐Saharan Africa (SSA) after other vital crops including cassava, maize, and yam (FAO 2009). They are rich sources of carbohydrates, vitamin C, potassium, and sodium (International Banana Association 2007) in addition to their abundant uses in medicines, industrial raw materials, and other domestic applications (Abiodun‐Solanke and Falade 2010). The different ploidy constitutions of Musa species were derived from Musa acuminata (AA) and Musa balbisiana (BB) and are categorized into different groups including diploids (AA, AB, and BB), triploids (AAA, AAB, ABB, and BBB), and tetraploids (AAAA AAAB, AABB, and ABBB) (INIBAP 2003; Pollefeys et al. 2004). Also, East African bananas (mainly dessert types) (AA, AAB, AAA, ABB, and AB), and the African plantains (AAB) are grown mainly in Central and West Africa, while the East African Highland Banana (AAA), are for cooking and beer brewing (Karamura et al. 1998). Variations are evident in the phenotypic expressions of different ploidy constitutions including triploid plantain (ABB), diploid banana (AA), and triploid banana (AAA) (Figure B7.1) (IITA 2018). Phenotypically, plantains are known to be taller than banana trees (Figure B7.2). Bananas, especially the dessert ones (M. Cavendish, AAA) have thinner fleshy texture, and when in ripened form, more compact fruits than those of plantains, and change to yellow color (Figure B7.3). They contain sugar, have a sweeter taste, and are usually consumed raw. Plantains on the other hand, have thicker, fleshy skin due to high starchy composition, and turn yellow first and then black when ripened. In addition, plantain fruits have rough surfaces and more vitamins A and C and potassium, and can be cooked before consumption unlike banana fruits (Valmayor et al. 2000). The identified cultivated banana and plantain fruits vary with wild relatives by exhibiting seedless condition and parthenocarpy (development of fruit without seed or pollination and fertilization).
Figure B7.1 Variations in phenotypic expressions of different ploidy constitutions of plantain and banana fruits. (a) = triploid plantain (ABB); (b) = diploid banana (AA); (c) = triploid banana (AAA) (IITA+banana&fr = tightropetb&imgurl=http%3A%2F).