George Acquaah - Principles of Plant Genetics and Breeding

C.C. CockerhamHis contribution to the role of statistics in plant breeding was summarized in his seminal paper of 1961. It connected statistics to genetics by shedding light on sources of variation and variance components, and covariance among relatives in genetic analysis. There are other names that are associated with this effort, including Mather and Jinks, and Eberhardt and Comstock.

Murashige and SkoogTissue culture technology is vital to plant breeding. Many applications such as embryo rescue, anther culture, micropropagation, in vitro selection, and somaclonal variation depend on tissue culture. The development in 1962 of the Murashige‐Skoog media (MS media). Modern methods of genetic engineering depend on tissue culture systems for key steps such as transformation and regeneration.

Watson and CrickThe understanding of heredity that underlies the ability of plant breeders to effectively manipulate plants at the molecular level to develop new cultivars, depends on the seminal work of Watson and Crick. Their discovery of the double helical structure of the DNA molecule laid the foundation for the understanding of the chemical basis of heredity.

Norman BorlaugIn the modern era of agriculture, Norman Borlaug deserves mention, not so much for his contribution to science as much as application of scientific principles to address world food and hunger, according to a methodology driven by his personal philosophy. This philosophy, dubbed the “Borlaug Hypothesis” by some economists, proposes to increase the productivity of agriculture on the best farmland to help curb deforestation by reducing demand for new farmland. His signature accomplishment for which his name is synonymous, and for which he received the prestigious Nobel Prize (for Peace) in 1970, the first agriculturalist to be so recognized, was the Green Revolution. While the award signified an acknowledgment of the positive impact of this work, the Green Revolution received criticism from a broad spectrum of sources. Undeterred by his detractors, Borlaug continued his advocacy for the poor and those plagued by perpetual hunger, working hard till his death 2009 to alleviate world hunger.

Herb Boyer, Stanley Cohen, and Paul BergIn 1973, Herb Boyer, Stanley Cohen, and Paul Berg led the way into the brave new world of genetic manipulation in which DNA from one organism could be transferred into another, by achieving the feat with bacteria. Called recombinant DNA technology, the researchers successfully transferred foreign DNA into a bacterium cell. This began the era of genetic engineering. Currently, this is one of the major technologies in modern plant breeding, albeit controversial.

Modern plant breeding is an art and a science. The two key activities in plant breeding are the creation (or assembling) of variation, and discriminating (selecting) among the available variability to identify and advance individuals that meet the breeding objectives. Consequently, advances in plant breeding technologies and techniques focus on facilitating and making these two distinct activities more efficient and cost effective.

Plant breeders depend on variation for plant improvement. Variation may be natural in origin, or it may be artificially generated in a variety of ways. Through the years, breeders have used various technologies and techniques in the quest for desired variation.

Artificial pollination, the deliberate transfer by humans of pollen from the flower (anther) of one plant to the flower (stigma) of another plant is an ancient practice, as previously noted. Babylonians and Assyrians were known to have conducted it on date palms. These ancient cultures did this without the benefit of the knowledge of the underlying science of pollination and fertilization. These ancient efforts were not geared toward creating variation; they were primarily for fertilization for fruit production. Science‐based artificial pollination started after the discovery of sex in plants by Camerarius and the ensuing work of Koelreuter. Artificial pollination (controlled pollination) is used in a variety of ways in modern plant breeding. Naturally cross‐pollinating species can be artificially self‐pollinated to create variability for selection, or to generate special parental breeding stock for experimentation or development of new cultivars. Experiments in heredity (e.g. Mendel's) depend on controlled pollination. These applications are discussed in detail elsewhere in this book.

One of the commonly used techniques in modern plant breeding to create variation is hybridization (crossing) of genetically different plants. It is commonly used to generate the initial population in which selection is practiced in a breeding program. The F 2is the most variable generation in which selection is often initiated. Breeders working in the field often have crossing blocks where controlled hybridization is conducted. Depending on the species and breeding objective, pollination may be done manually, or with the aid of natural agents (wind, insects). Whereas hybridization for the creation of variation may entail just two parents, there are various sophisticated hybridization schemes in modern plant breeding in which a number of parents are included (e.g. diallele crosses). Hybridization is commonly conducted with parents that are crossable or genetically compatible. However, there are occasions in plant breeding where it is desirable or even necessary to seek to introduce genes into the breeding program from genetically distant sources. Wild germplasm is considered a rich source of genes for modern crop improvement. The term “wide cross” is used to refer to hybridization that involves plant materials from outside the pool for cultivated species. Some wide crosses involve two species (interspecific cross), or even genera (intergeneric cross). The more distant the parents used in hybridization, the higher the incidence of genetic complications pertaining to meiosis, and the lesser the chances of success. Breeders use certain techniques and technologies to boost the success of wide crosses (discussed next).

Tissue culture entails growing plants or parts of plants in vitro under an aseptic environment. It has various applications in modern plant breeding. Regarding the generation of variation, the specific application of tissue culture is in rescuing embryos produced from wide crosses. Due to genetic incompatibility arising from the genetic distance between parents in wide crosses, the hybrid embryo often does not develop adequately to produce a viable seed. The technique of embryo cultureenables breeders to aseptically extract the immature embryo and culture it into a full grown plant that can bear seed.

To circumvent a major barrier to interspecific crossing, breeders use the chromosome doubling technique to double the chromosomes in the hybrid created (which is reproductively sterile due to meiotic incompatibility) in order to provide paring partners for successful meiosis and restoration of fertility. Chromosome doubling is achieved through the application of the chemical colchicine.

The bridge cross is another technique developed to facilitate wide crossing. This technique provides an indirect way of crossing two parents that differ in ploidy level (different number of chromosomes) through a transitional or intermediate cross. This intermediate cross is reproductively sterile and is subjected to chromosome doubling to restore fertility.