Principles of Plant Genetics and Breeding. George Acquaah

is considered a rich source of genes for modern crop improvement. The term “wide cross” is used to refer to hybridization that involves plant materials from outside the pool for cultivated species. Some wide crosses involve two species (interspecific cross), or even genera (intergeneric cross). The more distant the parents used in hybridization, the higher the incidence of genetic complications pertaining to meiosis, and the lesser the chances of success. Breeders use certain techniques and technologies to boost the success of wide crosses (discussed next).

       Tissue culture/embryo culture

      Tissue culture entails growing plants or parts of plants in vitro under an aseptic environment. It has various applications in modern plant breeding. Regarding the generation of variation, the specific application of tissue culture is in rescuing embryos produced from wide crosses. Due to genetic incompatibility arising from the genetic distance between parents in wide crosses, the hybrid embryo often does not develop adequately to produce a viable seed. The technique of embryo culture enables breeders to aseptically extract the immature embryo and culture it into a full grown plant that can bear seed.

       Chromosome doubling

To circumvent a major barrier to interspecific crossing, breeders use the chromosome doubling technique to double the chromosomes in the hybrid created (which is reproductively sterile due to meiotic incompatibility) in order to provide paring partners for successful meiosis and restoration of fertility. Chromosome doubling is achieved through the application of the chemical colchicine.

       Bridge cross

      The bridge cross is another technique developed to facilitate wide crossing. This technique provides an indirect way of crossing two parents that differ in ploidy level (different number of chromosomes) through a transitional or intermediate cross. This intermediate cross is reproductively sterile and is subjected to chromosome doubling to restore fertility.

       Protoplast fusion

      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.

       Hybrid seed technology/technique

      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.

       Seedlessness technique

      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 (2n) with a tetraploid (4n) yields a triploid (3n) which is sterile and hence produces no seed.

       Mutagenesis

      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.

       rDNA technology

      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.

       Important modern milestones associated with the creation of variation

       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.

      2.6.2 Technologies/techniques for selection

      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.

       Selection (breeding) schemes

      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.

       Molecular marker technology

      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