Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 68

Since the beginning of the nineteenth century, there has been an explosion of knowledge in plant breeding and its allied disciplines. Discussing each one would simply overwhelm this chapter. Consequently, a sample of the key innovations or discoveries with direct and significant implication on plant breeding will be discussed briefly. Some of these pertain to breeding schemes or methods and other applications that are discussed in detail later in the book and therefore will only be introduced briefly in this chapter.

 Marcus M. Rhoades and D.N. DuvickCytoplasmic male sterility(CMS) was discovered as a breeding technique by Marcus Rhoades in 1933. Duvick was a major player in the discovery of various aspects of this technology. In 1965, he published a summary of work done in this area.

 Nikolai I. VavilovVavilov identified eight areas of the world which he designated centers of diversity of crop species or centers of origin of crops. He distinguished between primary centers, where the crop was first domesticated, and secondary centers, which developed from plants migrating from the primary center. He also established the law of homologous series in heritable variation, showing the existence of parallelism in variability among related species. This law allows plant explorers to predict, within limits, forms that are yet to be described. Germplasm banks explore and collect germplasm from these centers to be classified and preserved for use by researchers.

 E.R. Sears and C.M. RicksSears and Ricks were first to apply their knowledge of cytogenetics to plant breeding of wheat and tomato, respectively. Their efforts showed how researchers could transfer genes and chromosomes from alien species to cultivated crop species. This achievement aided the use of cytogenetics in the evolutionary study of plant species.

 H.J. MullerThe pioneering experiments by Muller (1927) showed that it is possible to alter the effect of genes. Using X‐rays, he demonstrated that the physiology and genetics of an organism could be altered upon exposure to this radiation. Mutagenesis or mutation breeding became possible because of this discovery. In 1928, Stadler described the mutagenic effects of X‐rays on barley.

 Wilhelm JohannsenThe work of Johannsen pioneered the single plant selection method. He was the first to distinguish between genotype and phenotype. Working with the field bean, a self‐pollinated species, he selected extreme individuals in each generation, and observed that improvement only occurred in the first generation (i.e. heritable variation did not extend beyond the first generation). Variation observed in the second and subsequent generations was environmental (not heritable). Repeated selfing, after some time, is unresponsive to selection because of lack of genetic variation. Prolonged selfing leads to an individual with extreme homozygosity. He called such products pure lines. This became the pure line theory in 1903.

 Hardy–WeinbergThe work in 1908 of Hardy, an Englishman, and Weinberg, a German, laid the foundation for modern‐day breeding of cross‐pollinated species. They independently demonstrated that in a large random–mating population, both gene and genotypic frequencies remained unchanged from one generation to the next, in the absence of change agents like mutation, migration, and selection. This later became known as the Hardy‐Weinberg equilibrium or law. This concept is foundational to the breeding strategies employed for breeding cross‐pollinated species.

 Nilsson‐EhleNilsson‐Ehle is credited with being the leader of the first scientific wheat‐breeding program, which was started by the Swedish Seed Association at Svalof. It was there he invented the method of plant breeding called bulk breeding in 1912 to cope with the large number of crosses, generations, and plants involved is his breeding program. His breeding program centered on winter hardiness of wheat. He space‐planted the F1 and bulk‐harvested the F2.

 H.V. Harlan and M.N. PopeHarlan and Pope first applied the backcross breeding scheme to plants in 1922, after observing its success with animal breeding. Unable to observe desired recombinants in the segregating population of a cross between the commercial cultivar, “Manchuria,” a rough‐awned wheat, and a smooth‐awned exotic parent (donor parent), they resorted to a repeated crossing of the F1 to the commercial or adapted parent (recurrent parent).

 C.H. GouldenGoulden developed the single seed decent (rapid generation advance) selection scheme in 1941 as a means of speeding up the attainment of homozygosity. This was later modified by Brim in 1966.

 E.M. East and D.F. JonesThe concept of recurrent selection was independently proposed by Hayes 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.