This is Volume 3: Fruits, which is focused on advances in breeding strategies for the improvement of individual fruit crops. Each chapter comprehensively reviews the modern literature on the subject and reflects the authors' own experience. Here at Walmart.
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Genetic modifications : unintentional effects at each phase
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Tell us if something is incorrect. Comparison of schematic rice plants representing typical traditional varieties, modern semidwarf cultivars, and the proposed new plant type. Figure 3. Experimental plots showing the contrast between lines with and without the allele conferring submergence tolerance of the Sub1A gene. Plots without the tolerance gene have been almost completely eliminated after 15 days of flooding. This article references 70 other publications. Food Chem. View Author Information. Fax: Cite this: J.
ACS AuthorChoice. Article Views Altmetric -. Citations PDF 4 MB. Abstract High Resolution Image. Plant breeding can be considered a coevolutionary process between humans and edible plants. People caused changes in the plants that were used for agriculture and, in turn, those new plant types allowed changes in human populations to take place.
Civilization could not exist without agriculture, and agriculture could not sustain the civilized world without modern crop varieties. In industrialized countries, only a small portion of the population is engaged in agriculture. The vast majority of people rely on a tacit social pact for their survival, which assures that someone will provide food in exchange for some service or good. This pact is so basic to modern life that people take for granted that food is available in the nearest supermarket. However, agriculture failure could cause a disruption of this pact, leaving people in a situation of food insecurity.
Thus, protecting agriculture means warranting the foundation pact of modern civilization.
The core of plant breeding is the selection of better types among variants, in terms of yield and quality of edible parts; ease of cultivation, harvest, and processing; tolerance to environmental stresses; and resistance against pests. Each of these aspects of agronomic or food value can be dissected in many specific traits, each presenting its own range of variation. Manipulating a single trait, disregarding all others, is relatively straightforward; however, this is unlikely to result in a useful variety. The challenge of plant breeding resides in improving all of the traits of interest simultaneously, a task made more difficult by the genetic correlations between different traits, which may be due to genes with pleiotropic effects , to physical linkage between genes in the chromosomes, or to population genetic structure.
Whether this assumption is reasonable or not is a matter of debate. The objective of this paper is to discuss plant breeding methods as an evolving technology, considering the increasing levels of knowledge of the underlying mechanisms and the control of the process of generating and selecting superior plant types.
In this context, three main eras of plant breeding can be identified: i plant breeding based on the selection of observed variants, disregarding their origin; ii generation and selection of expanded variation by controlled mating; and iii monitoring the inheritance of within-genome variation and selection of specific recombinants. The fourth stage of plant breeding, which is not discussed in this paper, can be considered the creation and introduction of novel variation into genomes through genetic engineering.
The varieties resulting from the methods presented in this paper can be considered a reference against which transgenic plants are compared with regard to their food safety.
Table of contents
Plant Breeding Based on Observed Variation. The most primitive form of plant breeding was the selection of naturally occurring variants in the wild and, later, in cultivated fields. Genetic variation was continuously submitted to the selection pressure of food gathering or planting—harvesting cycles. In some cases, this process resulted in deep changes in plant phenotypes, as exemplified by the derivation of maize from teosinte. For a given gene, mutations are rare events, but considering the large numbers of plants in a field and of genes in a plant, mutations are quite frequent events in a population.
However, some of these mutations may result in more favorable phenotypes either in terms of cultivation or in terms of food quality. Some of those mutants were rescued by ancient farmers, who protected them against competition and established with those otherwise disabled plants a relationship of symbiosis. Unlike wild habitats, cultivated fields were environments in which those mutations conferred a selective advantage, thus becoming the predominant type through human selection. The accumulation of this type of mutation is the major cause of the domestication syndrome, a set of characteristics that made many cultivated species irreversibly dependent on humans for their survival.
The molecular variability in domesticated plants tends to be smaller than in related wild species, as a consequence of the founder effect during domestication. By strongly selecting for the rare mutant plants adapted to cultivation, early farmers dropped most of the variation present in the wild populations from which cultivated forms arose. It is now clear that many valuable genes, especially those related to resistance to pests, were left out of the cultivated gene pool. Landraces are populations of plants that have been cultivated for many generations in a certain region, being shaped by biotic and abiotic stresses, crop management, seed handling, and eating preferences.
They are dynamic genetic entities: continuously changing as a consequence of intentional and unintentional selection, seed mixture, and pollen exchange. Landraces are shaped by a balance between stabilizing selection , which keeps the identity of the landrace in a given region, and mild directional selection , leading to slow adjustments to environmental changes. In some cases, quick changes can take place, especially when the landrace is taken to a different region or when new materials are cultivated in close proximity with the original landrace. Landraces can still nowadays derive from modern cultivars, if certified seed production is discontinued and farmer-saved seeds are planted recurrently, without care for isolation against seed or pollen contamination.
The major characteristics of landraces are 12 i high levels of genetic diversity within populations, characterized by a limited range of variation between individuals, with distinctive traits that make the landrace identifiable; ii adaptation to soil and climate conditions typical of the region, combined with resistance to common pests; iii edible parts that are valued by local people, normally shaping and being shaped by the local cuisine; and iv modest but stable yield, conferring food security to the local community under normal environmental variation.
Intuitive farmer selection has the virtue of shaping varieties for the actual and specific environment of use and for the local food preferences, serving well the case of subsistence agriculture, where most of the production is locally consumed. However, when farmers select for one trait, genetic correlations may result in undesirable changes in other traits.
For example, cereal landraces are normally tall plants, prone to lodging and presenting low harvest index , probably as a result of human selection for large edible parts panicles, ears, spikes. Nevertheless, for their wealth in genetic variability and adaptability to different environments, landraces are the most valuable genetic resources for long-term plant breeding programs and also prime targets for germplasm collections.
New systems of germplasm conservation have been built on social networks connecting people interested in the subject as a hobby e. Those networks take advantage of modern communication tools to replicate on a global scale what used to happen through personal contact in traditional communities.
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Notwithstanding, a large part of the variability that once existed in cultivated fields of annual plants may have been irreversibly lost during the introduction of modern, high-yielding cultivars. In this sense, the same modern cultivars that saved millions from starving may have wiped out varieties that were the result of centuries of local intuitive selection by farmers and a valuable resource for future genetic improvement.
The earliest method of plant breeding based on an elementary knowledge of the laws of inheritance has been the selection of plants within landraces, based on the assumption that the progenies of the best individuals are expected to be superior to the progeny of a random sample of the population. This method was formally proposed by Louis de Vilmorin in , although there are mentions of the use of its principles by some farmers earlier in the 19th century.
From this point on, within-field heterogeneity was considered to be undesirable and both plant breeding and agronomy developed methods to achieve maximum spatial homogeneity e. In self-pollinating species, such as rice and wheat, landraces can be thought of as a mixture of pure lines, including some heterozygous individuals derived from a low frequency of cross-pollination.
In this type of population, selecting single plants and deriving inbred progenies invariably result in some lines that outperform the original landrace for a given growing condition. However, this superiority comes at a cost, because pure lines are normally less stable than diverse populations in the face of stresses, especially diseases, and have no capacity for long-term adaptation, because it is monomorphic for most genes. In the case of open-pollinated species, such as maize, landraces are populations of random mating individuals, approximating the Hardy—Weinberg equilibrium HWE , with some deviations due to mild selection.
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Mass selection and recombination of the top-performing portion of the population result in a gradual increase in the frequency of favorable alleles. Successive generations of selection on maize landraces resulted in improved open-pollinated varieties, which were the basis of corn production until the advent of hybrid maize. Plant Breeding Based on Controlled Mating. Despite the great spontaneous diversity that can be found in the landraces, simply applying selection on preexisting diversity is an eroding process that eventually comes to a limit.
The true creative power of plant breeding resides in promoting recombination for shuffling favorable alleles. One could conceivably start a commercial breeding program from a dozen well-adapted founding parents, with a clear focus on a specific target environment and evaluating large segregating progenies. Injection of novel variability might become necessary in the case of a significant change of the target environment, such as the emergence of new pests for which the founder materials had no resistance. Given the myriad of possible genotypes resulting from crossing diverse parents, the limitation for genetic gains becomes the capacity of the breeding program to evaluate a large number of plants, derived from a large number of crosses.
For this reason, plant breeding is frequently dubbed a numbers game, and large competitive programs in commodities invest heavily in high-throughput methods for seed handling, planting, evaluating, and harvesting.
Al-Khayri, Jameel M.
As genetic gains accumulate, the bar is gradually raised, and increasingly higher investments are required to keep a steady rate of genetic progress. The limit of this escalation is the financial viability of returns in the seed market and associated business. The main methods developed for efficient use of resources in breeding programs are discussed next.
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