Friday 16 September 2016

BREEDING METHOD FOR VEGETATIVELY PROPAGATED CROPS: SOMATIC MUTATIONS. EXAMPLES; SUGAR CANE AND POTATO



                                                                  ABSTRACT
Many agricultural crops of economic importance are vegetatively propagated. This work was done to elaborate more on vegetatively propagated crops, importance and developing new cultivars of vegetative propagated crops. Vegetative propagation aids to fix favorable combinations of important traits, very specific chemical compositions, superior genetic variance interactions and high levels of heterozygosity. Breeding vegetative propagated crops involve few genetic crossing and genetic combination by sexual reproduction. Vegetatively propagated crops are best suited for maintaining hybrid vigour because once a desired hybrid has been produced; there are fewer chances of losing it. Breeding by mutation enables to get improved characters in plant like tolerance to environment pressure, plant shape, changes in oil content, disease resistant in plant and protein quality.
                                                            INTRODUCTION
Vegetative propagation is an important part of agriculture, it refers to a process by which new organisms arise without production of seeds or spores. Vegetative propagated crops are very important horticulture crops, because it makes possible to fix and multiple favorable genetic combinations, provide food and cash crops.Vegetative or clonal propagation is an asexual reproduction in which successive mitosis of specialized vegetative propagules (e.g. bulb, corn, tubers, cuttings, buds and apomictic seed) develop new plants and results in a clonal population.Clone is defined as a genetically uniform material derived from a single individual that is vegetative propagated either in vitro or invivo. In species cultivated for vegetative products, as potato (Solanium tuberosom), cassava( Manihot esculenta), and sugar cane (Saccharum officinarum), the flowering and fertility are reduced and less useful in a new clone. Vegetative propagated crops are among the most important food crops. The food and agricultural organization ranks potato as the fourth crop after wheat, maize and rice, cassava is ranked sixth after barley. Potato is known as the world’s major staple crop, it has protein with the highest biological quality, sugar cane, a member of the grass family (Poaceae) is widely cultivated and propagated, providing around 70% of the world’s sugar. Sugar cane yields the highest number of calories per unit area of cultivated crops, is the process of changing the traits of plants in order to produce desired characteristics.Vegetatively propagated crops are bred by intentionally crossing of closely or distantly related individual to produce new crop varieties or lines with desirable traits. Breeding of vegetative crops have successfully improved quality, yield, tolerance of crops to environmental pressure. Breeding helps in producing crops that are resistant to viruses, fungi and bacteria and helps in longer storage period for the harvested crop.Species cultivated for fruit or reproductive product, like strawberry (Fragoria x ananassa), fruit yield and quality are very important traits, as most of these crops combine sexual and asexual reproduction, and there is wide genetic variation among species and populations, depending upon the relative importance of sexual and asexual reproduction (Eckert, 2002).
WHAT IS VEGETATIVE PROPAGATION?
Plant propagation is the method of increasing the number of plants of a particular cultivar or species. Vegetative propagation also known as asexual or clonel vegetation is a plant propagation method that produce new plants from vegetative parts (leaves, stems and roots) of the original plant. It is a process by which new organisms arise without production of seeds or spores. It can occur naturally or be induced by horticulturists.
TYPES OF VEGETATIVE PROPAGATION
i)        Natural vegetative propagation. ii)      Artificial vegetative propagation.
·         Natural Vegetative Propagation
Natural vegetative propagation occurs when an axillary bud grows into a lateral shoot and forms its own roots (adventitious root). Plant structures involved in natural vegetative propagation include Bulbs, Rhizomes, Stolons and tubers. Bulbs: They form lateral buds from the base of the mother bulb producing smaller bulbs or bulbels in coming years e.g.Onion. 
Rhizomes: They are root like stems that grows horizontally under the round. New roots and shoot form at the nodes with shoots growing upwards to form new plantlet. Examples includes iris and root ginger. 
Stolons: They are also known as runners. They are horizontal stems that grows above the ground e.g. strawberries. Tiny plantlets are seen along stolons, and form roots where they touch the ground. The new plant are independent when they break from the parent plant. 
Tubers: They are swollen part of an underground stem that store food so a plant can lie dormant. E.g.  Potatoes. Auxiliary bud form over the surface of the tuber and produce shoots that grow into a new plant.
·         Artificial Vegetative Propagation
These are induced form of vegetative propagation. It involves taking a piece of one parent plant and causing it to regenerate itself into a new plant. They methods are commonly: cuttings, grafting and budding, tissue culture and vegetative apoximixis. Cutting: A cutting is a piece from a plant that has been cut off from a mother plant and then caused to grow in a whole plant. Often the stems piece are treated with hormones(Auxins) to encourage new roots to form before planting. 
Tissue culture: In these  method, tiny fragments of plants are treated with plant hormones in a sterile growing medium. The hormones stimulate the growth of a callus, from which a new seedling can grow. Large numbers of identical seedlings are produced using this method. 
Grafting and budding: This method involves joining a stem piece or a single bud onto the stem of a plant that has roots. The stem piece is called the scion and the plant with root is called a root stock. This method is used to produce fruit trees sometimes with more than one variety of the same fruit species growing from the same
Vegetative Apomixes: This is a method that does not involve fertilization. It includes (apospory and diplospora) in flowering plants the unfertilized seeds are used or plantlets that grow instead of flower all use this method e.g. Dandelion and some citrus plants.
ADVANTAGES OF VEGETATIVE PROPAGATION 
The main advantage of vegetative propagation is that new plants contain the genetic material of only one parent, they are essential clones of the parent plant. Quality plant and ensure consistency of a variety crop by commercial growers. Vegetative propagation helps plants by passing the immature seedling phase and reach mature phase earlier. It saves time, energy, and money for commercial production of plant.
DISADVANTAGES OF VEGETATIVE PROPAGATION
·      The main disadvantages are the potential to impact on biodiversity of a species.
·         If a particular plant clone is susceptible to certain disease, there is a potential to lose entire crops. E.g. is seen in the impact of Kiwi fruit disease PSA. The only way to protect against the disadvantages is to establish a genetic bank of seeds or plants (germ plasma collection).  


BREEDING METHODS FOR VEGETATIVE PROPAGATED CROPS
            Plant Breeding?
Plant breeding is the genetic improvement of plants in order to create desired plant types that are better suited for cultivation, give quality yields, tolerate environment pressure and are disease resistant. Vegetative propagated crops which are the most horticultural crops used for food and cash crops are bred to improve human and economic live of a nation. Most of our major crops are derived from the domesticated varieties. Due to advancement in genetics, molecular biology and tissue culture plant breeding is being carried out by using molecular genetics tools.The possibility of combining sexual and asexual reproduction systems in almost all vegetatively propagated horticultural crops is the key strategy to develop new cultivars, because hybridization is the best method of increasing genetic diversity and combine traits in an individual (Hancock, 2004).In vegetative propagation breeding, the identification of one genotype with superior hybrid combination is sufficient because vegetative propagation fixes its heterozygosity and non- additive – interaction and allow its multiplication, even of sterile genotypes. An adequate breeding strategies and population size enable fully exploitation of general and specific combination ability that result in high hybrid vigour (Mckey et al., 2010).Therefore allowing a reasonable breeding strategy that will exploit as much as possible the gene pool diversity.

 METHODS INVOLVED IN THE BREEDING OF VEGETATIVE PROPAGATED CROPS
·         Domestication : This is not majorly involved in vegetative propagated crops breeding, but is involved in the process. It is the process of cultivating vegetative crops and keeping them under human care and management.
·         Collection of germ plasm: Germ plasm is the sum total of all genes present in a crop. The entire collection of vegetative propagated crops having all the diverse alleles for all genes in a given crop is known as Germ plasm collection. The germ plasm of any crop species consist of the following types of materials:
1)      Cultivated improved varieties
2)      All the wild species related to the crop species
3)      Improved varieties that are no more cultivated
4)      Old local varieties.
The collection of germ plasm from different sources id an essential first step in any breeding work, and germ plasm is usually stored at a low temperature, and it is cone within country or from other countries.
·         Evaluation and selection of parents: The parents are evaluated to identify plants with desirable combination of characters.
·         Plant Evaluation: This is the process of introducing plants of germ plasms either from a foreign country or introducing plants or germ plasm from one region to other region of the same country. After plant introduction, an adaptation period is followed. Plant introduction is done for the purpose of genetically improvement of economical crops and also for studying the origin, distribution, classification and evolution of plants.

5)      Hybridization: This is the mating or crossing of two plants or lines of dissimilar genotype. The objective of hybridization is to create genetic variation. When two dissimilar genotypical palnts are crossed, the genes from both the parents are brought together in F1 generation. Segregation and recombination produce many new gene combination in F2 and further generation. The aim of hybridization may be transfer of one or few qualitative characters, improvement of one or more quantitative traits or use of the F1 as a hybrid variety.
STEPS INVOLVED IN HYBRIDIZATION  
A)    Selection of parents from available material possessing desired characters.
B)    Selfing of plants to obtain homozygosity in desired traits.
C)    Emasculation In this the anthers are removed before they mature and have shed their pollen.
D)    Bagging , tagging and labeling of males as well as females to be used in crossing.
E)     Crossing , in which the pollen from bagged males are spread on the bagged female plant.

HYBRIDIZATION IN VEGETATIVE PROPAGATED CROPS
We have two types of hybridization
1)      Interspecific.
2)      Inter – generic.
In interspecific hyvridization: The plants of two different species belonging to the same genus are crossed together. It is also known as intra- generic hybridization. Disease, insect, drought and frost resistant varieties in wheat, tomato, sugarcane e.t.c. have been evolved by this method.
Gene Introgression to Cultivated Germ Plasm
Gene introgression also known as introgressive hybridization in genetics is the movement of a gene from one species into the gene pool of another by the repeated backcrossing of an inter specific hybrid with one of its parent species.
Introgression in Potato
Solanium rybinii is a wild diploid species of potato and is resistant to frost and virus infection. Another species Solanium tuberosum is cultivated and tetraploid species. The potato probably has the wildest genetic diversity among related wild species than any other cultivated plant, with the vast majority (74%) of the diploid species, and the rest are triploid, tetraploid, pentaploid and hexaploid (Hawkes, 1990).
In most Solanium species, the germ plasm can not all be directly used for breeding due to a combination of diploid level and endoplasm balance number (EBN) imcompatibility (Hewkes and Jackson, 1992).
The majority of the diploid wild species can be directly crossed with dihaploids (2n = 2x= 24) of cultivated potato. Dihaploid occur as result of parthenogenesis (haploid pollination technique) or another culture (Hermsen, 1994).
Dihaploid x wild species can be crossed with cultivated potato through unilateral sexual poly ploidization (4x – 2x crosses) using 2n gametes. Also, two dihaploid x wild species hybrids that produce 2n pollen and 2n eggs can be crossed in a bilateral sexual polyploidization (2x – 2x crosses) to create novel tetraploids (Hermsen, 1994; Hanneman, 1999). The wild parents are selected for useful traits as tuberization, tuber quality traits, tolerance to environmental stresses and resistance to diseases and sexual fertility and 2n gamete production before crossing with dihaploids.
Importance of Gene Introgression to Cultivated Germ Plasm
The gene introgression was use for the introgression of late blight (Phytophthora infestans) resistance from the wild diploid species S. microdontum (2n = 4x = 24e 2EBN) to cultivated potato (2n = 4x = 48e 4EBN).  A total of 175 clones from six accessions of Solonium microdontum were evaluated against the use of genotype.
A Z matting type of P. infestons and 27 highly resistant ones were selected (Oouches et al., 2001).
 The quantitative trait loci conferring late blight resistance in S. microdontum could be followed through poliploidization and identified microsatellite used in a marker assisted selection program to introgress the resistance gene from a wild species to cultivated potato (Bisognin et al., 2005). Besides , gene introgression, interspecific hybridization was also involved in the origin of a new horticultural crop. Solanium rybinii , a wild diploid species of potato and is resistant to frost and viral infection and Solanium tuberosum are not crossed directly because S. tuberosum is tetraploid and the ploidy levels are different. So Solanium rybinii is first raised to tetraploid level by auto- polyploidy one then crosses with Solanium tuberosum to introduce the characters of the wild species.
  
                                                INTER – GENERIC HYBRIDIZATION
This is a crossing beteen plants belonging to two different genera. Inter generic hybridization is been observed when sugar cane and sorghum are crossed. Sugar cane takes about 9 months to ripe and so no other crop can be grown. Sorghum is a short duration crop (3 to 4 months). So early maturing sugar cane varieties have been evolved by crossing with sorghum and since sorghum has less sugar content, the crossing results to a low sugar content hybrid, but by repeated back crossing of the hybrid with sugar cane early maturing varieties having normal sugar contents have been evolved.

HETEROSIS AND INBREEDING DEPRESSION
Heterosis also known as hybrid vigour is the increased size, yield, general vegetative luxuriance, resistance to diseases and to insects e.t.c. observed in the F1 generation of certain crosses as compared to the parents. A.F Sholl, 1914 attributes vigour to the “effects of a changed nucleus and relatively unchanged cytoplasm upon each other.
Dominance and over dominance forms the genetic bases of heterosis. In dominance hypothesis, the superiority of hybrid are attributed to the suppressing of undesirable recessive alleles from one parent by dominant alleles from the other. It attributes the poor performance of inbreed strains to the loss of genetic diversity, with  the strains becoming purely homozygous at many loci. These hypothesis was expressed in 1908 by the geneticist Charles Davenport.
In over dominance hypothesis of heterosis, certain combinations of alleles that can be obtained by crossing two inbreed strains are advantageous in the heterozygote. The over dominance hypothesis attributes the poor performance of inbreed strains to a high percentage of these harmful recessives. It was first developed independently by Edward M. East (1908) and George Shull (1908).
Inbreed vigour has been exploited in commercial crops such as maize, sorghum, barja, rice, sugar- beet, tomato, petunia etc and it is lost by inbreeding. Vegetatively propagated crops are best suited for maintaining hybrid vigour because once a desired hybrid has been produced there are fewer chances of losing it.

HYBRID VIGOUR IN PLANT
Crosses between inbreeds from different heterotic groups result in vigorous f1 hybrids with significantly more heterosis than f1 hybrids from inbreeds within the same heterotic group. Heterotic groups are created by plant breeders to classify inbreed lines, and can be progressively improved by reciprocal recurrent selection.

INBREEDING DEPRESSION
This is the reduced biological fitness in a given population as a result of inbreeding or breeding of related individuals. Inbreeding depression is often as a result of s population bottleneck. The degree of inbreeding depression varies considerably from one species to another. Vegetative propagated crops such as onion show little or no inbreeding depression. Species like maize and barja have moderate inbreeding depression.
MUTATION BREEDING IN VEGETATIVE PROPAGATED CROPS
Mutation are sudden unpredictable heritable changes without any intermediate stage in characteristics of organism. It is the permanent and relatively rare change in the nucleotide sequence of the concerned gene or chromosome structure and chromosome number.
Mutation was first discovered by Wright in 1791 in male lamb with short legs, and it was later discovered and studied in Denothera by Hugo deuries in 1900 Morgan in Drosoptilla (White eye mutant) in 1910. The term mutation was coined by De vries.
Mutation occurring naturally are called spontaneous mutations. They are both germinal and somatic. Useful somatic mutation can be incorporated in crop improvement only in vegetative propagated plants e.g. seedless grape, Naval Orange, Bhaskara banana. Vegetative propagation is also useful in maintaining germinal variation through sexual reproduction e.g. apple, mango, potato, sugar cane.
Thus, spontaneous mutations are the source of all the genetic variations occurring in all living things today.
INDUCED MUTATION
The rate of spontaneous mutation is very low. Therefore , rate of mutation is accelerated by means of certain agents called mutagens. Mutagens are of two types;
* Physical
* Chemical
Chemical mutagens: They are mutagens from chemical substances such as ethylmethane sulphonate (EMS) sodium azide that induce mutation, maleic hydrazide and Dimethyl Nitrosoamine.
Physical mutagens: They are different kinds of radiation like x rays, gamma rays, ultra violet rays, etc . They induce changes in DNA and chromosomes which produce mutations. Mutations produced in response to mutagens are known as induced mutations.


SOMATIC MUTATIONS IN IMPROVEMENT OF VEGETATIVELY PROPAGATED CROPS
When a mutation (plastidial, genic, chromosomal, genomic) arises in a cell within a shoot apical of a bud. The mutated cell propagates mitotically and produces a mutated sector (sectorial or mericlinal chimera) (D’ AMATO, 1977).
During further vegetative growth of the initial bud one or more wholly mutated buds (solid mutant)  producing mutated plants (buds) may develop into an independent plant through various naturally occurring modes of vegetative peopagation (tubers, tubercles, tuberous, roots, bulbs, pseudo bulbs, cormers, stolons, rhizomes, root crowns, stem points, inflorescence, bulbils, adventitious embryo and apomictic seed) obviously sports can be propagated via true camphimictic) seed when the original plant reproduces sexually. In most cases sports differ from the parent plant for a monogenic character. In some other cases, two or more genes or gene complexes are involved (Whitham and Slobodckikofe, 1981; Shamel and Pomeroy, 1936).
SELECTION AMONGST SOMACLONAL VARIATION
Genetic variation present among plant cells during tissue culture is called somaclonal variation. It is used for the genetic variation present in plants regenerated from a single culture. This variation has been used to develop several useful varieties.
Some of the somaclonal variations are stable and useful e.g. resistance to disease and pest, stress tolerance, male sterility , early maturation, better yield, better quality e.t.c.
Thus , somaclonal variation have produced wheat tolerance to rust and high temperature, potato tolerance to phytophthora infestions (Late blight of potato) e.t.c. other useful variations include high protein content of potato, short durations sugar cane and increase of shelf life of tomato.
ACHIEVEMENT OF MUTATION BREEDING
·         The first commercial success with induced mutations was reported in 1934 with the release of a new tobacco ciltiva,  “Chlorina” through x ray radiation.
·         Many varieties of barley contain artificial mutated genes which contributed to reduction in height, increase in yield, in sensitivity to day length  and resistance to mildew disease. Shorbati sonara and pusa lerne are two amber grain colour mutants of wheat produced from the red grained sonara 64 and lerma Rojo 64A. Induced mutations have also become recently important in developing parents, useful in hybridization programmes. 45 cultivars were been developed by the year 1982. Either by direct radiation or by crossing with induced mutants.
·         Many crop plants are propagated vegetatively e.g. Potato, Tapioca and sugar cane. In these genetic improvement is carried out using sexual reproduction but the maintenance of the improvement varieties is by cloning.
·         Spontaneous mutation is somatic cells of a vegetatively propagated plant are commonly called SPORT. Such desirable sports occurring in well adapted, asexually reproducing plants may result in quick improvements such as the colour sports in many apple and superior shrub typed in coffee plants.
·          The characters improved through mutation breeding include tolerance to environment pressure, plant shape, changes in oil content, disease resistant and protein quality.


GENETIC ENGINEERING AND BIOTECHNOLOGY IN PLANT BREEDING
Genetic engineering also known as recombinant DNA  technology is aimed at introducing one or more genes into an organism that normally does not possess them. This requires isolation of fragment of DNA corresponding to a desirable character, hooking it to a vector (such as the plasmid in a bacterium,agrobacterium tumifaciens) and transferring it to a cell.
Genetic transformation is also possible through co- cultivation, electro poration (applying high electric potential for a few micro seconds to change the porosity of protoplast to take up DNA) and by micro- injection of DNA into the cell by fine needles.
Successful genetic engineering involves identification of the desired genes, their transfer to the cells of a target crop plant, their integration and expression.
SELECTION
Selection is one of the oldest methods for crop improvement. It can either be artificial or natural. Natural selection acts as a sieve in favour of the well adapted strains and varieties. Local varieties of crops are the result of natural selection. Selection pressures have arising differences between species and sub species.
Artificial selection involves picking out of the plants having desired characters from a mixed population where the individuals differ in characters. There are various methods of artificial selection:
A)    Mass selection: It is practiced in those plants which are cross pollinated like Zea, Brassica. Plants are selected based on the phenotypic expression from the mixed population of a crop, then the seeds are obtained, the seeds obtained are used for the raising of next crop, this selection is continued till the plants show uniformity in the desired characters.
B)    Pure- line selection: This method is practiced in self pollinated crops such as wheat, barley, rice, legume. It also made based on the phenotypic expression. It involves testing the progeny of single individual plant separately. This method of selection from a single individual is continued till a true breeding type is obtained. Thus a breeder by pure line selected renders a particular type more or less homozygous and the progeny has a uniform population, but it lacks variability.
C)    Clonal selection: This method is practiced in vegetatively propagated crops such as banana, potato, onion, citrus e.t.c. Clones are plants propagated vegetatively from a single individual. The genotypic constitution of plants propagated in this way is not likely to change. Superior clones are the ones selected on the basis of their phenotypic characters. Selections are done between clones and never within clones, as all the individuals of a clone have the same genetic constitution.
POLYPLOIDY BREEDING
Polyploidy breeding in vegetative propagated crops involves the number or sets of chromosome or genomes in a cell of an organism. Vegetative propagated crops having polyploidy conditions are potato, sugar cane, banana, tobacco, e.t.c. This condition arises due to failure of chromosomes to separate at the time of anaphase either due to non- disjunction or due to non- formation of spindle. This condition can be artificially induced by application of colchicines depending upon the number of genomes present in a polyploidy. It is known as: Triploid (3n), Tetraploid (4n), Pentaploid(5n), Hexaploid (6n), Heptaploid(7n), Octaploid(8n) e.t.c.
The polyploids with odd numbers of genomes present in an triploids, pentaploids are sexually sterile because the odd chromosomes do not form synapsis and they are therefore propagated vegetatively e.g. potato, sugar cane.
Polyploidy is of two types; Autopolyploidy and Allopoly ploidy.
Auto polyploidy: This is a type of polyploidy in which there is a numerical increase of the same genome e.g. autriploid (AAA), auto tetraploid (AAAA). Some of the crop and garden plants are auto polyploids e.g. maize, rice, etc. auto ploidy induces gigas effect.
Allopoly ploidy:  It was developed through hybridization between two species followed by doubling of chromosomes e.g. (AABB) Allotraploid is the common type. Allopolyploids functions as new species e.g. wheat, American cotton. Auto allopolyploidy is a type of allopolyploidy in which one genome is more than diploid state. Common auto allopolyploids are hexaploid (AAAABB) e.g. Helianthus tuberous.
INDUCTIONN OF POLYPLOID
Polyploidy in plant can be induced by colchicines treatment, colchicines is an alkaloid obtained from the corms of colchicium autonale (Lillaceae), colchicines inhibits the formation of the spindle in the dividing cells and hence chromosomes do not separate at anaphase.
Thus  a restitution nucleus (A nucleus in which the chromosomes have divided but could not divide into 2 daughter cells) is formed. The effect of colchicines is temporary, so as a cell recovers, a new spindle is formed and restitution nucleus undergoes normal mitosis as a polyploidy cell.
            ROLE OF AUTOPOLYPLOIDS IN PLANT BREEDING
·         It is used to increase fruit size.
·         It is used to increase and develop seedless varieties.
ROLE OF ALLOPOLYPLOIDS IN PLANT IMPROVEMENT
·         It is useful in interspecific and interheneric hybridization
·         Allopolyploid are used un cultivating vegetative propagated crops.
·         Genera raised through inter- generic allopolyploidy include Triticale (Tritium x secale) and Ralphano brassia (Raphanus and Brassica).
POLYPLOIDS IN VEGETATIVE PROPAGATED CROPS
                        ORIGIN OF TOBACCO
There was evidence that Nicotiana tabacum (2A= 48) is an allotetraploid between N. sylvestris (2n = 24) and Nicotiana tomentosa or some allied species (2n = 24) Nicotiana tobacum is known only under cultivation and does not occur as a natural species.
                        ORIGIN OF TRITICALE
It is a man – made cereal, an allopolyploid between Triricum (wheat) and Secour (rye). The release varieties of Triticale are hexaploid (2n = 42) and have been synthesized by doubling the chromosome complements of sterile hybrids between Tritocum turgidum (Durum wheat 2n = 28) and S. cereal (rye, 2n = 14).
ROLE OF PLANT BREEDING IN VEGETATIVE PROPAGATION
·         All the sugar cane varieties that are cultivated today are interspecific hybrids.
·         Plant breeding has also given us improved varieties of crops like Sonora- 64 of wheat.
·         Through mutation breeding more than 200 varieties of crops have been developed.
·         Disease resistance in plants have been introduced through breeding.
                                    CONCLUSION
Vegetatively propagated crops which possess high uniform and hybrids vigour are very important set of food and cash crop, promoting diversification and producing nutritional and food security.
The possibility of combining sexual and asexual reproduction systems in almost all vegetatively propagated horticultural crops is very important for both broaden of the genetic base, and developing new countries. The inter and intra specific hybridization are the best approach to increase the genetic base of cultivated germoplasm and the cross ability with related species and to combine traits in one individual. Hybridization also increases the genetic diversity for traits less or still not exploited in breeding programs and to reduce the genetic vulnerability of the crops.
The different breeding methods used in the improvement of the vegetatively propagated crops have been developed to increase the effectiveness of the new species. These species of vegetative propagated crops also depends upon high quality propagules of their parent plants.





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