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Backcross breeding is defined as system of breeding whereby recurrent backcrosses are made to one of the parents of a hybrid, accompanied by selection for a specific character or characters. It is basically applicable to self-pollinated crops and is precise way improving varieties that excel in large number of desirable attributes but lack in a few characteristics. This method makes use of a series of backcrosses to the variety to be improved during which character for which improvement is sought is maintained by selection. At the end of the backcrossing, the gene to be transferred remains in heterozygous condition and selfing after the last backcross produces homozygosity for the gene under transfer. As a result, the improved variety will remain like recurrent variety in terms of yield, adaptability, quality features, etc. will be superior for the character which was planned to be transferred from the donor parent.

The application of backcross breeding in plants was first proposed by H.V. Harlan and M.N. Pope in 1922. In principle, backcross breeding does not improve the genotype of the product, except for the substituted gene(s).

Single seed descent (SSD) method of breeding is applicable to self-pollinated crops. The method of single seed descent (SSD) was born out of a need to speed up the breeding programme by rapidly inbreeding a population prior to initiating individual plant selection and evaluation while reducing a loss of genotypes during the segregating generations. SSD procedure is based on advancing generations through single seed from each plant in each generation. This separates the inbreeding phase from the selection phase and the population reaches F6 generation faster. This procedure carries forward maximum number of F2 plants to a stage near homozygosity, i.e. F6. It was first proposed by C. H. Goulden in 1941 when he obtained the F6 generation in two years by reducing the number of generations grown from a plant to one or two while conducting multiple plantings per year using greenhouse and off-season planting. H. W. Johnson and R. L. Bernard described the procedure of harvesting a single seed per plant for soybean in 1962. However, it was C.A. Brim who, in 1966, provided a formal description of the procedure of single seed descent, calling it a modified pedigree method.

Bulk population method of breeding is defined as the growing of genetically diverse populations of self-pollinated crops in a bulk plot with or without mass selection, followed by single plant selection. It is applicable to self-pollinated crops and not applied to cross-pollinated crops. Bulk population breeding is a strategy of crop improvement in which natural selection effect is solicited more directly in the early generations of the procedure by delaying stringent artificial selection until later generations. The Swede, H. Nilsson-Ehle, developed the procedure. H.V Harlan and colleagues provided additional theoretical foundation for this method through their work in barley breeding in 1940s. As proposed by Harlan and colleagues, the bulk method entails yield testing of the F2 bulk progenies from crosses and discarding whole crosses based on yield performance. In other words, the primary objective is to stratify crosses for selection of parents based on yield values. The current application of the bulk method has a different objective.

Pedigree method of plant breeding is defined as a method of breeding in which individual plants are selected in the segregating generations from a cross on the basis of their desirability judged individually and on the basis of a pedigree record. This has been the most widely used method of plant breeding in all the self-pollinated crops and still continues to be so. This method has been first described by H. H. Lowe in 1927. This is the most widely used method to handle segregating generations following crosses between two parents to develop new cultivars combining desirable traits from both the parents. In this method, superior types are selected in successive segregating generations and a record is maintained of all parent-progeny relationships. The records help the breeder to advance only progeny lines with plants that exhibit genes for the desired traits. Selection begins in F2 generation where individual plants are selected which in judgment of breeder will produce the best progeny in F5/F6 when the selection is finally concluded. In F3 and F4 generations, many loci will have become homozygous and family characteristics begin to appear. By F5/F6 generation, most families are expected to be homozygous and uniform and hence selection within such families is no longer effective. 

A line homozygous at all loci, ordinarily obtained by successive self-fertilizations in plant breeding is called a pure line. This concept was proposed by Danish biologist, W.L. Johannsen in 1903 while conducting a series of selection experiments for seed weight in common bean, which is strictly self-pollinated species. This is applied in self-pollinated crops, like wheat, rice, pulses, several vegetable crops, namely, tomato, chilli, eggplant, okra, cucurbits, etc. This is also applied to develop inbred parental lines in maize and several other cross-pollinated crops. However, ultimately these parental lines are used in development of new hybrids. This could be summarized as follows:

• Lines that are genetically different may be successfully isolated from within a population of mixed genetic types.
• Any variation that occurs within a pure line is not heritable but due to environmental factors only. Consequently, as Johansen’s bean study showed, further selection within the line is not effective.

Mass selection is a form of selection in which individual plants are selected phenotypically and the next generation propagated from the aggregate of their seed. It is applicable to both self and cross-pollinated crops. The obvious first step in the improvement of self-pollinated variety is the elimination of undesirable types. This may be achieved by simply roguing them as the variety grows in the field. Sometimes, this is called as negative selection. Another approach is to tag desirable types and to harvest only those at maturity. This is called as positive selection. Either way, results are similar. The best plants in the variety are identified and bulked. A refinement of mass selection is to harvest the best plants separately and to grow them as pure lines for comparison. Only those pure lines that are similar and superior are bulked as an improvement of an established variety. 

Broadly and conceptually breeding methods can be classified into four groups. These are:

• Selection breeding
• Combination breeding
• Hybrid breeding
• Special breeding techniques

There are fluid transitions and similarities between the individual groups of breeding methods. Selection not only takes place in the group termed selection breeding but also in other groups. In breeding varieties by combination, new variation is generated by crossing genetically different parents followed by application of various selection methods and ultimately development of a new recombinant type pure-line. In hybrid breeding, inbred lines of different genetic constitution are produced by selfing, pair crossing and other methods and from these best lines are selected and new hybrids are developed. In addition, breeding methods can also be classified according to other criteria. For example, breeding methods have also been classified according to the manner of propagation of the varieties. These are:

Crop Domestication

Domestication can be loosely defined as the process of bringing of a wild species under the management of man. Domestication is the process by which genetic changes (or shifts) in wild plants are brought about through a selection process imposed by humans. Because of the role of humans, the process results in characteristics that are beneficial to humans but some that would be disadvantageous for plants in their natural habitats. It is an evolutionary process in which selection (both natural (frost tolerance) and artificial (large attractive fruits)) operates to change plants genetically, morphologically, and physiologically. 

To start with let the self-incompatibility be defined.

• Self-incompatibility is the mechanism by which plant can prevent selffertilization.
• Self-incompatibility is defined as inability of a plant producing functional gamete to achieve self-fertilization.
• Self-incompatibility is a genetically based mechanism which controls mating within angiosperm species by imposing a ‘barrier’ between pollination and fertilization.
• The term incompatibility is used to describe the failure of pollen tube to penetrate the full length of the style and to effect fertilization.

Male sterility is a condition in plants whereby the anthers or pollen are nonfunctional. The condition may manifest most commonly as absence of pollen or extreme scarcity of pollen, severe malformation or absence of flowers or stamens, or failure of pollen to dehisce. Just like self-incompatibility, male sterility enforces cross-pollination. Similarly, it can be exploited as a tool to eliminate the need for emasculation and for hybrid seed production on commercial scale in field and vegetable crops. There are three basic kinds of male sterility based on the origin of the abnormality related to functional pollen grain.

Hybrid vigor or heterosis is opposite and complementary to inbreeding depression (reduction in fitness as a direct result of inbreeding). In theory, the heterosis observed after crossing is expected to be equal to the depression upon inbreeding, considering a large number of crosses between lines derived from a single base population. In practice, plant breeders are interested in heterosis expressed by specific crosses between selected parents, or between populations that have no known common recent origin. Furthermore, because heterosis is subject to the interactions between genotype and environment, it is desirable to describe the heterosis of a particular hybrid line for a specific trait at a specific location or under specified environmental conditions.

Inbreeding

Inbreeding means the mating together of individuals that are related to each other by ancestry. This may be defined as any system of mating that will lead to an increase in homozygosity. Selfing is the most intense form of inbreeding which provides the most rapid approach to homozygosity. When selfing is not possible, full-sib and half-sib mating are used to do inbreeding. The progeny of selfed plant is symbolized by Sj and the selfed progeny of the Sj by S₂ and so on. Occasionally symbol  is used to designate selfed seed. The effects of inbreeding are as follows:

Random Mating

Random mating occurs when each female gamete has an equal chance of being fertilized by any male gamete of the same plant or with any other plant of the population and, furthermore, there is an equal chance for seed production. It is not possible to achieve true random mating in plant breeding because selection is involved. Consequently, it is more realistic to describe the system of mating as random mating with selection. Whereas true random mating does not change gene frequencies, existing variability in the population or genetic correlation between close relatives, random mating with selection changes gene frequencies and the mean of the population, with little or no effect on homozygosity, population variance, or genetic correlation between close relatives in a large population.

The response to selection (R) or selection advance or genetic gain is the difference between the mean phenotypic value of the offspring of the selected parents and the whole of the parental generation before selection. The response to selection is simply the change of population mean between generations following selection. Similarly, the selection differential (S) is the mean phenotypic value of the individuals selected as parents expressed as a deviation from the population mean (i.e., from the mean phenotypic value of all the individuals in the parental generation before selection). Response to selection is related to heritability by the following equation:

While dealing with polygenic traits and the continuous variation in plant breeding programs, it is important to know as to what extent the phenotype is decided by the genotype and to what extent by the environment. It is well known fact that what we observe i.e. phenotype is the result of genotype, environment and interaction between the genotype and the environment. The total variance is phenotypic variance or the variance of phenotypic values and various components of variance are as shown in Table 6.1.

Quantitative traits are governed by many genes or polygenes genes with small individual effects. They are often described by their gene action rather than by the number of genes by which they are governed. There are four types of gene action: additive, dominance, epistasis, and overdominance. Because gene effects do not always fall into clear-cut categories, and it should be pointed out that gene action is conceptually the same for major genes as well as minor genes, the essential difference being that the action of a minor gene is small and significantly influenced by the environment. A general way of distinguishing between these types of gene action based on interaction among alleles is as follows:

Qualitative characters are those characters which show discontinuous or discrete differences and are counted into different classes and there is no need to measure them. Typically speaking, characters studied by Mendel in garden pea whose results were published in 1966 as Mendel’s Law were the typical characters that fitted into this class of characters in those days. To recall, these characters were (i) tall vs dwarf plants, (ii) round vs wrinkled seeds, (iii) yellow vs green pods, (iv) violet vs white flowers, (v) flat pods vs constricted pods, (vi), green pods vs yellow pods and (vii) axial flowers vs terminal flowers. Mendel postulated “Factors” responsible for these differences and he could explain his results of F₂ in ratio of 3:1 while dealing with one trait at a time and 9:3:3:1 while dealing with two traits simultaneously and proposed (i) law of segregation and (ii) principles of independent assortment to explain these ratios. Later on the term “factor” was called as gene.

Gametogenesis

Male Gametogenesis: Male gametogenesis is the formation of male gametes and it occurs in the anthers. The end products of meiosis in higher plants are four microspores. Two mitotic divisions occur in microspores. The first division produces a tube nucleus and a generative nucleus. The second mitotic division occurs only in the generative nucleus resulting into production of two sperm cells (Fig. 3.1). The microspore is haploid (n) and develops into pollen grain in the anther, and the pollen grain further develops into a pollen tube on the stigma and in the style. The pollen tube, when it reaches the embryo sac is the fully developed male gametophyte and is trinucleate, two sperm cells and one tube nucleus.

Systematic scientific efforts to improve genetic potential of crops in India were initiated in 1905 at the Imperial/Indian Agricultural Research Institute. In 1929, with the establishment of the Indian Council of Agricultural Research (ICAR), several associated developments took place in the first half of the 20th century leading to establishment of several crop-based institutes (sugarcane, cotton, jute, rice, oilseeds, tobacco, potato) and multidisciplinary institutes such as the Indian Agricultural Research Institute (IARI), Central Arid Zone Research Institute (CAZRI), and Central Plantation Crops Research Institute (CPCRI). Additionally, a Project on Intensification of Regional Research in Cotton, Oilseeds and Millets (PIRRCOM) was implemented during 1950s with establishment of several research stations in different regions.

Several definitions of plant breeding have been put forward. All of these have a core component of the definition that is “the art and science of improving the heredity of plants for the benefit of the humankind” as mentioned by famous plant breeding teacher and a popular plant breeding text book author, J. M. Poehlman. N. I. Vavilov has defined plant breeding as “evolution directed by the will of the man”. The most modern definition of plant breeding by Bernardo (2002) is “plant breeding is the science, art, and business of improving plants for human benefit”. A few more definitions on these lines given by several other famous plant breeders are as follows.