Principles of Plant Genetics and Breeding. George Acquaah
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development of his enduring conventions for naming living organisms, the universally accepted binomial nomenclature, also called Linnaean taxonomy or the scientific classification of organisms. The binomial nomenclature classifies nature within a hierarchy, assigning a two‐part name to an individual, a genus, and a species (specific epithet). His work was published in his most noted publication Species Plantarum. There are specific rules and guidelines for writing scientific names, which are in Latin, the genus beginning with a capital letter while the species is not; being non‐English, the name is italicized (or underlined), for example, Zea mays (corn). Further, the genus can stand alone, but not the species (e.g. Zea, Zea sp., or Z. mays).
Charles DarwinCharles Robert Darwin was an English naturalist, with one of the most recognizable names of all time, because of his work that led to one of the most enduring theories ever, the theory of evolution. He proposed what is sometimes called the unifying theory of life sciences that all species of life have evolved over time from a common ancestor. The process of evolution is extremely slow, requiring thousands and even millions of years to bring about the gradual changes which incrementally result in the divergence or diversity of life that we see. The primary mechanism of evolution, he reckoned, is natural selection, the arbiter in deciding which individuals survive to contribute to the subsequent generations (survival of the fittest). Genetic mutations are the ultimate source of variation, but natural selection decides which modifications are advantageous and contribute to the survival of individuals. The survival or extinction of an organism depends on its ability to adapt to its changing environment. He published his seminal work in his 1859 book On the origin of species.For all intents and purposes, modern plant breeding is evolution happening in real time. Instead of thousands and millions of years to bring about a new variety, plant breeders achieve their goal in about 10 years, depending upon the method used, among other factors. Random mutations may be used to create variation, but other more efficient methods are preferred today. Once generated, breeders use artificial selection (not natural selection) to discriminate among the variability to decide which individual plants to advance to the next step in the breeding program.
Gregor MendelBorn in 1822, Gregor Mendel, an Augustinian monk, is known for his scientific research that led to the foundations of modern transmission genetics. Of German ethnicity, his nationality was Austrian‐Hungarian. Even though several researchers in his time and prior to that time had conducted research or made observations similar to what he did, it was Mendel who was credited with being first to provide empirical evidence about the nature of heredity, the underpinnings of traits, and how genes that condition them are transmitted from parents to offspring. He made his ground‐breaking findings from making and studying Pisum (pea) hybrids. His paper “Experiments with hybrid plants” was published in 1866 to reveal what became known as the laws of Mendel – the laws of dominance, segregation, and independent assortment, which are the foundations of modern genetics. In fact, Mendel is often referred to as the father of modern genetics. In addition to the laws he established, Mendel also made two other significant contributions to the field of genetics – the development of pure lines, and good record keeping for use in statistical analysis that led to his discoveries (he counted plant variants).
Luther BurbankAn American botanist and horticulturalist, Burbank is known to have developed numerous varieties of fruits, flowers, grains, grasses, and vegetables. One of his most remarkable creations is the Russet Burbank potato, a russet‐colored skin that is used worldwide today. This natural variant was isolated and propagated by Burbank.
It is significant to note that some of the most widely used plant breeding methods of selection were developed prior to the nineteenth century, preceding Mendel! These methods include mass selection, pedigree selection, and bulk breeding.
2.5 Later pioneers and trailblazers
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