The evolution of plant improvement 221 Domestication of wild species

Humans have manipulated the genetic makeup of plants since agriculture began more than 10,000 years ago (Table 3-1). Primitive societies of hunters and gatherers recognized wild species of cereals and harvested them for food. Societies of shifting cultivators gradually domesticated these wild species, creating the basis for sedentary or permanent agricultural systems. These early farmers unconsciously managed the process of domestication over several millennia, selecting and planting the best seeds through many growing cycles. The main attainment of this first phase of crop improvement was to develop domesticated crops more suitable for human cultivation—planting, harvesting, threshing, or shelling—and consumption. Higher germination rates, more uniform growing periods, resistance to shattering, and improved palatability were some of the achievements of this effort. The human selection pressures that accompanied domestication narrowed the genetic base for these crops as farmers selected among the full range of plant types for those that produced more desirable traits (Smale, 1997).

2.2.2 Development of landraces

In the second phase of crop improvement, farmers deliberately selected plant materials suited to local preferences and growing conditions. Many farmers in many locations exerted pressures continuously in numerous directions, resulting in variable crop populations that were adapted to local growing conditions and consumption preferences. These populations, broadly known as landraces, often differ radically from their early ancestors. Although more genetically uniform than these early relatives, landraces are nonetheless characterized by a high degree of genetic diversity within a particular field.

2.2.3 Conventional breeding of modern varieties

The third phase of crop improvement through scientific plant-breeding programs relied on the application of classical Mendelian genetic principles based on the phenotype or physical characteristics of the organism concerned. Conventional breeding, which began about 100 years ago, has been very successful in introducing desirable traits into crop cultivars from domesticated or wild relatives or mutants. The first high-yielding hybrid maize varieties were produced about 50 years ago and the high-yielding, semidwarf varieties of wheat and rice that gave rise to the green revolution were developed less than 50 years ago. The products of this third phase—often called modern varieties—have been widely adopted in intensive agricultural production systems.

As a result of the spread of modern varieties, fields of cereals have become more uniform in plant types with less spontaneous gene exchange. Planned gene migration increased, however, with the worldwide exchange of germplasm among research institutions that was an integral part of the green revolution research paradigm (Pingali and Smale, 2001). Although the nature of crop genetic diversity has changed as a result of the spread of modern varieties, it is neither straightforward nor particularly meaningful to discuss whether genetic diversity has increased or decreased, because a simple count of the varieties in a particular area or measures of genetic distance among varieties may not tell us much about the resilience of crop ecosystems or the availability of crop genetic resources for breeding program (see section 4).

Table 3-1. An agricultural technology timeline

Technology

Genetic interventions

Traditional

About 10,000 BC

About 3,000 BC

Civilizations harvested from natural biological diversity, domesticated crops and animals, began to select plant materials for propagation and animals for breeding. Beer brewing, cheese making, and wine fermentation.

Conventional

Late 19th Century 1930s

1940s to 1960s

1970s

Modern

1980s

1990s

2000s

Identification of principles of inheritance by Gregor Mendel in 1865, laying the foundation for classical breeding methods. Development of commercial hybrid crops.

Use of mutagenesis, tissue culture, plant regeneration. Discovery of transformation and transduction, discovery by Watson and Crick of the structure of DNA in 1953, identification of genes that detach and move (transposons). Advent of gene transfer through recombinant DNA techniques. Use of embryo rescue and protoplast fusion in plant breeding and artificial insemination in animal reproduction.

Insulin as first commercial product from gene transfer. Tissue culture for mass propagation in plants and embryo transfer in animal production.

Extensive genetic fingerprinting of a wide range of organisms, first field trials of genetically engineered plant varieties in 1990 followed by the first commercial release in 1992. Genetically engineered vaccines and hormones and cloning of animals. Bioinformaties, genomics, proteomics, metabolomics_

Source: FAO (2004). 2.2.4 Genomic selection in plant breeding

The latest phase of crop improvement research is based on the identity, location, and function of genes affecting economically important traits and the direct transfer of these genes through transgenesis. Transgenesis permits the introduction of genetic materials from sexually incompatible organisms, greatly expanding the range of genetic variations that can be used in breeding programs. Unlike conventional breeding, transgenesis allows the targeted transfer of the genes responsible for a particular trait, without otherwise changing the genetic makeup of the host plant. This means that a single transgenic innovation can be incorporated into many varieties of a crop, including perhaps even landraces (see Chapter 14). Compared with conventional breeding in which an innovation comes "bundled" within a new variety that typically displaces older varieties, transgenesis allows an innovation to be disseminated through many varieties, preserving desirable qualities from existing varieties and maintaining or, potentially increasing, crop genetic diversity.

On the other hand, the widespread incorporation of a single innovation, such as the Bacillus thuringiensis (Bt) genes that confer insect resistance, into many crops/varieties may constitute a type of genetic narrowing for that particular trait. Furthermore, transgenic crops that confer a distinct advantage over landraces may accelerate the pace at which these traditional crops are abandoned or augmented with the transgenic trait. Regulatory regimes are concerned with the potentially harmful consequences of gene flow from transgenic crops to conventional varieties or landraces. In this context, it is important to recognize that gene flow from conventional varieties to landraces frequently occurs (especially for open-pollinated crops such as maize) and is often consciously exploited by farmers. In the same way, it is likely that farmers would consciously select for transgenic traits that confer an advantage (de Groote et al., 2004) unless biological or legal methods are used to prevent them from doing so. How these offsetting forces will ultimately affect crop genetic diversity depends on the incentives and constraints facing researchers, plant breeders, and farmers. The changing locus of agricultural research from the public to the private sector is a key element in this regard.

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