Principles of Plant Genetics and Breeding. George Acquaah

and Garber in 1919, and East and Jones in 1920. Hayes and Garber also proposed the method of synthetic breeding in 1919.

       F.H. HullHull coined the term recurrent selection in 1945. His work included recurrent selection for combining ability.

       F.E. Comstock, H.F. Robinson, and P.H. HarveyThese breeders proposed the method of reciprocal recurrent selection in 1949.

       C.M. DonaldAn Australian biologist, Donald proposed the ideotype breeding concept as a way of managing plant breeding programs by modeling plant architecture. Breeding based on a plant model (archetype) meant that breeders paid more attention to their breeding goals and strategies. They could introgress exotic germplasm and expand genetic diversity in their program, following judicious strategies. Even though it did not attain prominence in plant breeding, notable applications were made by Wayne Adams (the major graduate advisor of the author of this book) at Michigan State University, and by Rasmussen at the University of Minnesota.

       H.H. FlorFlor proposed the gene‐for‐gene hypothesis in 1956 to postulate that both host and parasite genetics were significant in determining whether or not a disease resistance reaction would be observed. The expression of resistance by the host was dominant while the expression of avirulence by the parasite was dominant. In other words, there was a single gene in the host that interacted with a single gene for the parasite.

       G.H. ShullGeorge Shull coined the term “heterosis” for the phenomenon of hybrid vigor. His research on crossing corn, an open pollinated species, led to the observation of hybrid vigor. This observation had also been made by East and Yates and other researchers, but it was Shull who gave the correct interpretation of heterosis in 1908. Hybrid vigor is the reason why hybrid seed is a huge commercial success.

       W.J. BealBeal was one of the pioneers in the development of hybrid corn. He is also noted for the oldest and continuously operated botanical garden (The W.J. Beal Botanical Garden) in the US, located at Michigan State University. His noted publications include the The New Botany, Grasses of North America, and History of Michigan Agricultural College. In 1879, Beal started one of the longest running experiments in botany, designed to determine how long seed can remain viable. The experiment, which includes periodic retrieval and germination testing of the buried seeds is scheduled to be completed in 2100.

       Ronald FisherThough not a plant breeder, this biologist made major contributions to the field of statistics and genetics. He introduced the concept of randomization and the analysis of variance procedure that are indispensable to plant breeding research and evaluation. The concept of likelihood (maximum likelihood) is his original idea. His contributions to quantitative genetics aided breeders in the understanding and manipulation of quantitative traits.

       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.

2.6 History of plant breeding technologies/techniques

      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.

      2.6.1 Technologies/techniques associated with creation of variation

      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

      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.

       Hybridization

      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 F₂ is 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.