eChapters

Introduction

• Thus "genetic enhancement" or "pre-breeding" refers to the transfer or introgression of genes or gene combinations from unadapted sources into breeding materials (FAO, 1996).

• Genetic enhancement plays an important role in utilizing unadapted and unutilized germplasm collections and creating vast genetic variability for development of productive cultivars / hybrids.

• Objective of Genetic Enhancement is broadening the genetic base and increases genetic diversity

• Gene pool – Harlan and de Wet (1971).

• Primary gene pool (GP-1): Members of this gene pool are probably in the same "species" (in conventional biological usage) and can intermate freely.

• Secondary gene pool (GP-2): Members of this pool are probably normally classified as different species than the crop species under consideration (the primary gene pool).

• Tertiary gene pool (GP-3): Members of this gene pool are more distantly related to the members of the primary gene pool. The primary and tertiary gene pools can be intermated, but gene transfer between them is impossible without the use of "rather extreme or radical measures such as embryo rescue (or embryo culture, a form of plant organ culture) induced polyploidy (chromosome doubling) bridging crosses (e.g., with members of the secondary gene pool).

Allard (1960) defines germplasm as “the sum total of the hereditary  materials in  species” The word germplasm in context of plants is now referred to as plant genetic resources i.e. PGR.PGR is a component of the world’s biological diversity. The term biological diversity is a vast and often undervalued resource. Biological diversity includes every form of life, from the smallest microbe to the largest beast. Biodiversity is the variety and variability of all plants, animals and microorganisms and the ecological complexes of which they are part. The earth’s biodiversity – its ecosystems, species and genes – are the product of over 3000 million years of evolution. Throughout this time, small changes have accumulated in populations, resulting in a multitude of living forms closely adapted to the physical conditions they face and to each other. They supply all our food, much of our raw materials and energy and many of our medicines. Intact eco-systems also play a central role in the functioning of biosphere. Plants are of crucial importance in stabilizing climate, protecting watersheds on soils and maintaining the chemical balance of the earth. When key species are lost, vital ecological services are disrupted (Zedan, 1995).

Apomixis is an asexual method of reproduction through the seed. Apomictic reproduction is widespread and found in over 300 species of plants in 30 families. Interest in apomixis as a reproductive mechanism has increased as its potential and role are being recognized in genome preservation, polyploid build up, propagation of derivatives of wide crosses, permanent fixation of hybrid vigour, evolution, speciation and plant breeding .

Winkler (1906) defined apomixis as “the substitution for sexual reproduction of another asexual reproductive process that does not involve nuclear or cellular fusion.” One of the chief barriers to an understanding of the apomictic phenomenon has been the highly complex terminology as well as the differences of opinion among specialists as to the correct usage of these terms, probably because the phenomenon by itself is complex and in many ways quite different from that of amphimixis. Convincing evidence for the presence of apomixis can be obtained by tracing the steps that are involved in embryonic development. It should be amply clear that as the prerequisite for apomixis, fertilization does not take place at all in any form. Thus there is no normal development of embryo from the egg cell. However, the embryo may develop from an unfertilized egg cell other than the egg cell within the embryo sac.

Apomixis is widely distributed among higher plants. More than 300 species belonging to 35 families are apomictic. It is most common in Gramineae, Compositae, Rosaceae and Rutaceae. Among the major cereals like maize, wheat and pearl millet have apomictic relatives.

Historically, Indian scientists have recognized the importance of conserving plant biodiversity with particular reference to indigenous     and naturalized crops and their wild relatives. Indian scientists have recognized the importance of conserving plant biodiversity as early as 1935 by the ‘Crops and Soil Wing’ of the then Board of Agriculture and Animal Husbandry. Late Dr. B. P. Pal, in his classic paper ‘Search for New genes’ published in 1937 emphasized the importance of germplasm augmentation and utilization in crop improvement programmes. The need was reiterated in a meeting of the Indian Society of Genetics and Plant Breeding in 1941, which inter alia discussed the subject of economic crops. Dr. B. P. Pal, working at the then Imperial (now Indian) Agricultural Research Institute approached the then Imperial (now Indian) Council of Agricultural Research (ICAR) to set-up a unit for assembly of global germplasm under phytosanitary conditions in India. The ICAR scheme for plant introduction started functioning in 1946 under the leadership of late Dr. Harbhajan Singh as the first ‘Operational Scientist’. The scheme was further expanded and strengthened as ‘Plant Introduction and Exploration Organization’ in the Botany Division in 1956 and later established as a separate Division of Plant Introduction in IARI in 1961 headed by Dr. Harbhajan Singh.

Self incompatibility in plants is a mechanism preventing the seed set by self pollination, and is probably way in which out crossing is enforced. Other factors may cause selfed seed may not to set seed due to embryo abortion, but SI is prezygotic and prevents embryo formation. SI is, therefore prevention of fusion of fertile male and female gametes after fertilization. Koelreuter first time in 18^th century reported S.I in Verbascum phoeniceum plants. In self-incompatible pollen grains fail to germinate on the stigma of the flower that produced them. Self incompatibility has been reported in more than 300 plant species belonging to 70 family of angiosperm. It is important out breeding device (cross pollination) for normal fruit set. It is maintained high degree of heterozygosity and can take place at any stage between pollination and fertilization.

Marshall and Brown (1975) have considered the subject of optimal sampling strategies for use in the genetic conservation of crop plants. The theme of this paper has been relatively narrow. These authors have defined a strategy for populations under imminent threat of extinction applicable to particularly the traditional landraces of some major crops such as wheat, rice and barley on account of danger of replacement of the landraces on a very broad scale due to the spread of modern cultivars. According to Brown and Marshall (1995) the emphasis in collecting of crop genetic resources has  changed radically over the years. Instead of crisis-driven broad scale crop specific programmes, the collection of plant genetic resources now involves 3 important elements as given below :

The diversity collection and making it available for future use is one of the most important activities related to management to plant germplasm resources. The collection of plant germplasm from their natural habitat is broadly known as plant germplasm exploration. The main reasons for collecting germplasm of given gene pool in a given geographical location are as follows (Engels et al., 1995)

The commercial exploitation of heterosis started only after the rediscovery of Mendel’s law of inheritance and consequent understanding of the theoretical basis of hybrid vigour. Heterosis is considered as a major revolution in classical plant breeding era. Utilization of heterosis has substantial influence in genetics to provide hybrid vigour, consequently high yielding hybrids in different crops and plants. Yield increase from heterotic hybrids due to the expression of heterosis is up to 30% superior to conventional varieties. The term heterosis was first used by Shull (1914) and represents the increase or decrease in fitness and vigour of the F₁ hybrid over the mean or mid parent values, produced by crossing two genetically different individuals for one or more traits. Generally, positive heterosis is desirable but in some cases like maturity duration and presence of toxic substances, negative heterosis is condiderable as desirable. Heterosis can be estimated in three different ways. When the heterosis is estimated over mid parent, it is called as relative heterosis or mid parent heterosis. Estimation of heterosis over better parent out of the two parents involved in the cross is referred as “Heterobeltiosis”, the term which was coined by Fonseca and Patterson (1968). “Standard Heterosis”, often called as useful heterosis which was coined by Meredith and Bridge (1972) and denotes the superiority of F₁ over the standard commercial check.

The most important root and tuber crops cultivated on a worldwide scale are potato (Solanum tuberosum), cassava (Manihot esculenta), sweet potato (Ipomoea batatas), yams (Dioscorea spp.) and taro (Colocasia esculenta), but there are many others that are of regional, national or local importance, in a total of over a dozen dicot and monocot families. Most of these originated in tropical or subtropical areas and are mainly used as sources of carbohydrates. Many minor root and tuber crops, such as turmeric (Curcuma longa) and arrowroot (Maranta arundinacea, Tacca leontopedaloides), are also used in folk medicine and as spices (Sastrapradja et al., 1981). All these crops are vegetatively propagated. There are also vegetatively (or clonally) propagated crops that are not roots or tubers, e.g. bananas and sugarcane. A selection of the more important crops is given in Table 13.1 (Huaman et al., 1995).

Polyploidy, the condition in which a normal diploid cell or organism acquires one or more additional sets of chromosomes. In other words, the polyploid cell or organism has three or more times the haploid chromosome number. Polyploidy arises as the result of total nondisjunction of chromosomes during mitosis or meiosis. The somatic chromosome number of any species, whether diploid or polyploidy is designated as 2n. The somatic cells of plants are diploid that they posses two complete set of chromosomes, one received from seed parent and another from pollen parent. The gametic chromosome number is said to be haploid and denoted by n. Haploid is defined as cell with gametic chromosome number. While monoploid has the basic chromosome number and represented by x.

Classification of Polyploidy

Polyploids are classified in to two groups, euploidy and aneuploidy. Euploidy have chromosome number which is an exact multiple of basic number. Euploidy includes monoploids, diploids and polyploids. In aneuplod individual with other than exact multiple of basic chromosome number or change in chromosome number which posses one or few chromosomes of the genome.

In 1868, Johann Friedrich Miescher, a young Swish medical student got excited with an acidic substance that he isolated from pus cells obtained from bandages used to dress human wounds. To obtain large quantities of pus for his experiment, Miescher began purchasing used bandages from surgical clinics in Tuebingen, Germany. He separated pus cells from bandages and treated the cells with pepsin, a proteolytic enzyme that he isolated from stomach of pigs. After pepsin treatment, he recovered an acidic substance that was named as “nuclein” by him. This nuclein was unusual in the sense that it was rich in nitrogen and phosphorous, two elements known at that time to coexist only in certain types of fat. Miescher’s paper on this investigation was published in 1871. At that time, importance of Miescher’s nuclein was not fully appreciated. Even the existence of polynucleotide chains, the key component of the acidic material in Miescher’s nuclein was not documented until 1940s. The role of nucleic acid in storing and transmitting genetic information remained un-established until 1944 and the double-helix structure of DNA was not discovered until 1953 by James Watson and Francis Crick. Even in 1953, many geneticists remained skeptical to accept the role of nucleic acid as the genetic information carrier rather than proteins as nucleic acids exhibited less structural variability than the proteins (Snustad et al., 1997).

Marker Assisted Selection (MAS)

Marker Assisted Selection [MAS] is defining as the indirect selection for a desired plant phenotype based on the banding pattern of linked molecular (DNA) markers. It is based to infer the presence of a gene from the presence of a marker which is tightly linked to the gene of interest and high heritability of the gene of interest. Marker assisted selection (MAS) is also known by other names as marker aided selection and marker assisted breeding (MAB).

After harvesting and threshing, the seed usually arrives at the cleaning plant with trash, green leaves, weeds, other crop seeds and insects, etc. If the seed contains leaves, etc. and other high moisture materials, it cannot be accurately cleaned and safely stored until most of the foreign material has been removed. The process of removing the unwanted materials from a seed lot is known as seed conditioning. Seed conditioning is an essential component of seed technology to ensure that the farmers get the seed of high quality.

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:

Seed enhancement is the process or technology to further improve seed performances under very specific regimes and with certain planting equipment. Three major seed enhancement technologies are as follows (Copeland and McDonald, 1995). 
        Seed hydration 
        Biological seed treatments 
        Seed coatings

Regulator Genes

Bacterial cells are capable of producing many kinds of enzymes in relatively large quantities but the kinds and amount of enzymes actually produced at any given time are usually those which are required at that particular time in response to the prevailing environment. Inducible enzymes are ordinarily absent or present in very small quantities until a specific compound appears. This compound is called as inducer. A typical example of this is production of b galactosidase (an enzyme that splits lactose into galactose and glucose, Fig. 44.1) once the lactose is introduced into the medium on which E. coli cells are grown. Here lactose acts as inducer for the enzyme b galactosidase.

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.