George Acquaah - Principles of Plant Genetics and Breeding

Cell fusion or specifically protoplast (excluding cell wall) fusion is a technique used by breeders to effect in vitro hybridization in situations where normal hybridization is challenging. It can be used to overcome barriers to fertilization associated with interspecific crossing. The first successful application of this techniques occurred in 1975.

Hybridization may be used as a means of generating variation for selection in a breeding program. It may also be done to create the end product of a breeding program. The discovery of the phenomenon of heterosis laid the foundation for the hybrid seed technology. Breeders spend resources to design and develop special genotypes to be used as parents in producing hybrid seeds. Hybrid seed is expensive to produce and hence costs more than non‐hybrid seed. In the 1990s, the Genetic use restriction technology( GURT), colloquially, terminator technology, was introduced as a means of deterring the unlawful use of hybrid seed. This technology causes second generation seed from a hybrid crop to be reproductively sterile (i.e. a farmer cannot harvest a crop by saving seed from the current year's crop to plant the next season's crop). Allied techniques that drive the hybrid seed industry include male sterility and self incompatibility, techniques used to manage pollination and fertility in the hybrid breeding industry.

Whereas fertility is desired in a seed‐bearing cultivar, sometimes, seedless fruits are preferred by consumers. The observation that triploidy (or odd chromosome number set) results in hybrid sterility led to the application of this knowledge as a breeding technique. Crossing a diploid (2 n ) with a tetraploid (4 n ) yields a triploid (3 n ) which is sterile and hence produces no seed.

Evolution is driven by mutations that arise spontaneously in the population. Since the discovery in 1928 by H. Muller of the mutagenetic effects of X‐rays on the fruit fly, the application of mutagens (physical and chemical) have been exploited by plant breeders to induce new variation. Mutation breeding is a recognized scheme of plant breeding that has yielded numerous successful commercial cultivars, in addition to being a source of variation.

The advent of the recombinant DNA technology in 1985 revolutionized the field of biology and enabled researchers to directly manipulate an organism directly at the DNA level. The most astonishing capacity of this technology is the ability of researchers to move DNA around without regard to genetic boundaries. Simply put, DNA (or gene) from an animal may be transferred into a plant. The DNA technology also allows researchers to isolate and clone genes and pieces of DNA for various purposes. This precise gene transfer is advantageous in plant improvement. Mutagenesis can now be targeted and precise instead of random as in the use of mutagens in conventional applications.

A new category of cultivars, genetically modified (GM) cultivars, have been developed using recombinant DNA technology. DNA technologies and techniques are exploding at a terrific rate, with new ones being regularly added while existing ones are refined and made more efficient and cost effective. One of the most useful applications of DNA technology in plant breeding is in molecular markers.

Plant Variety Protection ActEnacted in 1970 and amended in 1994, the Plant Variety Protection Act gave intellectual property rights to innovators who developed new crop varieties of sexually reproducing species and tuber‐propagated species. The commercial seed industry is thriving because companies can reap benefits from their investments in the often expensive cultivar development ventures.

First commercial GM cropThe FlavrSavr tomato was the first commercially approved and grown GM cultivar for human consumption. It was developed in 1992 by the biotech company Calgene, using the antisense gene technology to downregulate the production of the enzyme polygalacturonase that degrades pectin in fruit cell walls, resulting in fruit softening. FlavrSavr tomato hence ripens slowly and stays fresher on the shelf for a longer time. In 1995, Bt corn, engineered to resist the European corn borer, was produced by Pioneer Hi bred company, while RR(Roundup ready) soybean, a Monsanto product, was introduced in 1996.

Selection or the discrimination among variability is the most fundamental of techniques used by plant breeders throughout the ages. In some cases, individual plants are the units of selection; in other cases, a large number of plants are chosen and advanced in the breeding program. With time, various strategies (breeding schemes) have been developed for selection in breeding programs.

Breeding schemes are discussed in detail in Chapters 17– 20. They are distinguished by the nature and source of the population used to initiate the breeding program, as well as the nature of the product. The most basic of these schemes is mass selection; others are recurrent selection, pedigree selection, and bulk population strategy.

Marker technique is essentially selection by proxy. Selection is generally conducted by visually discriminating among variability, in the hope that the variation on hand is caused by differences in genotype and not by variation in the environment. Markers are phenotypes that are linked to genotypes (or precisely genes of interest). Markers are discussed in detail in Chapter 21. They are useful in facilitating the selection process and making it more efficient and cost effective. Molecular (DNA‐based) markers have superseded morphological markers in scale of use in plant breeding. Marker‐assisted selection (MAS) is used to facilitate plant breeding (see Chapter 24).

Gene mapping entails a graphic representation of the arrangement of a gene or a DNA sequence on a chromosome. It can be used to locate and identify the gene (or group of genes) that conditions a trait of interest. It depends on availability of markers. The availability of molecular markers has greatly facilitated gene mapping. Further, genomic DNA sequencing produces the most complete maps for species. Now, QTL (quantitative trait loci) mapping is becoming more widespread. Modern plant breeding is greatly facilitated by genetic maps.

Genomic selection methodology and genome wide techniques are helping to facilitate the improvement of quantitative traits.

An organism's complete set of DNA is called its genome. The concept of genomics began with the successful sequencing of the genomes of a virus and a mitochondrion by Fred Sanger and his colleagues starting in the 1970s. Previously, researchers were limited to understanding plant structure and function piecemeal (gene‐by‐gene). With the advances in technology, whole genomes of certain species have been sequenced, thereby making all the genes they contain accessible to researchers. Because of the cost of such undertakings, whole genome sequences have so far been limited to the so‐called model organisms, including Arabidopsis, rice, and corn. Through comparative genome analysis, researchers seek to establish correspondence between genes or other genomic features in different organisms, without the need to have whole genome maps of all organisms. In sum, the goal of plant genomics is to understand the genetic and molecular basis of all the relevant biological processes that pertain to a plant species, so that they can be exploited more effectively and efficiently for improving the species. Genomics is hence important in modern plant breeding efforts. Two of the major tools employed in genomics research are microarrays and bioinformatics.