Role and economic importance of crop genetic diversity in food security

Determination of genetic diversity and their relationships among breeding materials is very crucial in crop improvement strategies. Characterization and evaluation of germplasm is pre-requisite to screen out the desired genetic materials for the genetic improvement programs. The collection of germplasm relies on the several number of accessions it possesses and the genetic materials available in those accessions for yield and yield components. Climate change and geographical isolation are identifi ed as two majors in the formation of new species. The other sources of germplasm diversifi cation and evolution are biotic factors like competition and predation among themselves. Phenotypic characters are the most important conventional tools to analyse variation among the genetic materials and the visible morphological traits are very crucial tools in genetic diversity investigation. Plant breeding is primarily relied on the variation exist in the genetic diversity of cultivated and their wild relatives together for further improvements. Plant phenotyping is defi ned as the investigation of plant characters by researchers for yield, quality and resistance to biotic and abiotic stresses. Genetic variation and selection are the two basic principles of plant breeding. Additive (heritable) and non-additive variance (dominance and epitasis) are the important components of genetic variance of any quantitative traits. Hence, it is important to decompose the visible phenotypic variation into heritable and non-heritable components with suitable genetic components like genotypic coeffi cient of variation, heritability and genetic advance. Genetic diversity is the totality of genetic difference of genetic variation in the genetic make-up of a species. Genetic diversity ha paramount role in the perpetuation of a species through offering adaptation mechanisms to biotic and abiotic environmental stresses and enables change in the genetic composition to cope with changes in the environment. Eventually, plant genetic diversity is playing a key role in the continuation of agricultural development with signifi cant improvement in different morphological and agronomical characteristics. Selection for improvement highly depends on inherent levels of genetic diversity present at the time in the species, rate of evolutionary response and adaptation to the environmental conditions. As the genetic diversity increases the ability to adapt to changing environments also increases within a given species. Especially, when the climate fl uctuation, new pests and diseases are occurred, the species which have huge genetic diversity capable of overcome the challenges. Since crop plant improvement program is integrated with different research disciplines, the availability and accessibility of diverse genetic materials ensure the sustainability of global food production network. Review Article Role and economic importance of crop genetic diversity in food security


Introduction
Biological diversity is the existence of variation among and within the living world for the betterment of improvement especially in crop plants [1]. Biological diversity is necessarily decomposed into three different major components such as genetic diversity, species diversity and ecosystem diversity [2]. Genetic diversity is the availability of variability of heritable traits in a population of the given species [3]. Genetic variation is defi ned as the differences in DNA sequence, biochemical characteristics, physiological and morphological characters such as plant height, fl ower position, fl ower color and other different function. Ramanatha & Hodgkin, [2] described genetic diversity as the presence of difference in alleles, genotypes, the result of their performance (phenotypes) and the overall sum of genome. The utilization of genetic diversity is relevant for making advanced improvement for crop plants [4]. In the presence of narrow genetic diversity within crop species are susceptible to emerging pathogens and other several constraints leading to loss of productivity and this problem leads to serious decline in the areas of adaptation [5]. Genetic variation is a major driver of evolutionary diversifi cation and source of phenotypic variation.
The achievement in the crop improvement primarily relies on the broad base of degree genetic divergence [6]. Genetic Genetic diversity is the natural gift that arised due to mutation, gene fl ow hybridization and polyploidy of genetic materials. Genetic variation is the allelic differences of genes in DNA or RNA arrangements in the genetic pool of a population. Genetic diversity is the broadest term consisting all the variation existing between the different genetic materials in relation to genetic make-up of crop species. There is three level of categories of biological diversity. These are: ecosystem diversity which is the representation of variability between distinguished communities of species at the highest hierarchy. The second level of biological diversity is diversity in species which is indicating the different species within a community and the third biological diversity is genetic diversity which is the diversity present within distinguished cultivars of species.
Genetic diversity has signifi cant role in ensuring food security through increasing farmer's income and plays in current and future food production [11]. The importance of plant genetic diversity is very large especially from the very beginning of agriculture, natural genetic variability has been exploited within crop species to meet subsistence food requirement, and now it is being focused to surplus food for growing populations. Today's crop gene banks have emerged in response to two different intended purposes: fi rst, the mobilization, management, and long-term storage of materials that can be readily used in crop variety improvement programs and second, the long-term conservation of crop genetic diversity, for the potential future use of humanity. Crop genetic resources are the basis of agricultural production and signifi cant economic benefi ts have resulted from their conservation and use. Genetic resources provide the fundamental mechanics that enable plants to convert soil, water and sunlight into something of critical value to humans-food. Diverse genetic resources allow humans to select and breed plants with desired characteristics, thus increasing agricultural productivity. Genetic diversity is the backbone of a nation's food security and the basis of economic development as a whole.

Advantages of genetic diversity
Genetic diversity is the base for crop improvement and existence of crop pant in nature. It clear that the genetic diversity offers opportunity for improvement of cultivars with desired traits which consist both farmer-preferred traits and breeder-preferred traits. To meet subsistence food requirement, genetic variability has been used in beginning of agriculture. Nowadays, climate adapted cultivar development is the issues of plant breeder since the climate components are fl uctuated and causing adverse problems on the normal growth and development of crop plants. The availability of genetic diversity directly related with presence of desired alleles and help to develop in breeding climate resilient varieties. Sustainability of crop production and food security is being threatened by the increasing unpredictability and severity of drought stress due to global climate changes. The incorporation of the adapted natural genetic variations into breeding programs can enrich the current genetic diversity of stress tolerance and improve yield under stress. Genetic diversity enable for the development of high yields of farmers and breeders preferred improved quality cultivars. Genetic analysis which is a good indicator of genetic diversity. This system is required to be tested on wide ranges of crops to be verifi ed [7]. The potential improvement of crop plants determined by the magnitude of genetic diversity available in provided crop species. Characterization followed by cataloguing of genetic materials are an essential prerequisite for a successful improvement program. Genetic diversity analysis tools are used to measure the degree of genetic divergence amongst different populations [8]. The presence of genetic variation in plant populations is useful for conservation and breeding programs. This day the crop genetic diversity is decreased due to several reasons as compared to the previous one.
It is crucial to enhance the crop productivity through the providing the appropriate protection and conservation to genetic diversity of crops and the management practices of growing environments should be modifi ed. Eventually, human population is increasing alarming and becoming beyond the expectation of life standard which caused the scarcity of natural resources [10]. Therefore, knowledge of genetic variability is the key component in selecting genotype that withstand to changing environments, including new pests, diseases and new climatic conditions for the future breeding program. The objective of the paper was to understand the role and economic importance of crop genetic diversity in food security.

Concept of diversity and its impact in crop improvement
Genetic diversity is defi ned as the availability of genetic variation which is heritable traits in a population of given species [3]. For the development of climate resilient cultivars, the existence of genetic diversity in the form of wild species, related species, breeding stocks and mutant lines are the source of desirable alleles which assist plant breeders [10]. Agriculture Organization reported the depletion of genetic diversity as the most serious environmental concerns [12]. In general, genetic diversity strictly the amount genetic variation available between crop species [13].

Variability and adaptability
The largest genetic variation of crop species offers the greater opportunities for improvement to adapt to environmental conditions. Adaptability is the better survival

Effects of genetic erosion
Genetic erosion is the depletion of genetic variability due to several factors over a particular period of time in a particular location. The loss can include individual genes or combination of genes. Genetic loss is the reduction of genetic diversity over time [29]. Genetic loss primarily caused due to the as an alleviation in evenness [32]. Genetic loss as depletion in evenness originates from the variability indicators used in population genetics, such as Shannon's index [33] and Nei's gene diversity index [34]. Genetic diversity is measured using frequencies of genes within a group of genotypes in a specifi ed region. Diversity level is reduced because of dominant single genotypes or alleles. Genetic loss at ex situ conservation can be occurred because of depletion of genes as a result of regeneration and storage practices [35]. The main cause of narrow genetic base is the replacement of diversity of land races with few modern varieties. Genetic erosion has negative developmental effect when loss of genetic diversity has profoundly narrowed the genetic base of modern crop varieties [36]. Narrow genetic base is defi ned as the loss of genetic diversity and commonly refers to the reduction in the quantities of specimens of a species [37,38]. Green revolution was the transition of cultivation of landraces to modern varieties to increase the agricultural productivity using improved varieties, excessive agricultural inputs and mechanized agriculture. Replacement of landraces that evolved with and has been genetically improved by traditional agriculturists, but has not been infl uenced by modern breeding practices or traditional varieties by modern varieties or high yielding varieties is one of the most important reasons. The landraces of a primary centre of origin are assumed to contain many valuable genes particularly for resistance or tolerance to various biotic and abiotic stresses and hence hold promise for their utilization in future plant-breeding programs. The term genetic erosion is sometimes used in a narrow sense, i.e. the loss of genes or alleles, as well as more broadly, referring to the loss of varieties. There are a number of different ways to represent the problems of genetic erosion. One of the most useful indicators is the narrowness of the food base. Narrow genetic base is the depletion in population variation because of inbreeding and genetic drift which is largely causes the endangerment of small isolated populations. Narrowing of genetic diversity might result the complete loss of crop plants.

Conclusion
Genetic diversity is the extent of genetic variation available among crop species to use in improvement program. The presence of suffi cient genetic variation is a key for the success of breeding program. Genetic diversity has paramount importance for the development of superior varieties in terms of yield and other desirable traits. It is also very crucial in the production of superior hybrids and desirable recombinants. Genetic diversity determines the effi ciency and effectiveness of improvement which may result in enhanced food production. From plant breeding aspects, classifi cation of genetic variability to respective heterotic group is critical for the development of vigorous and outstanding hybrids in terms of economically important traits. Genetic diversity is providing vital protection to other nature against climate change, pests and diseases stresses.
Creating suffi cient genetic variation for golden crop improvement is becoming challenges to keep improving genetic yield potential. Nowadays, plant breeders are utilizing genetic materials without knowing its genetic background such as exotic non-adapted, exotic adapted and existing genetic material as a source of new alleles that protect and improve genetic gain through selection. In ensuring food and nutritional security, genetic diversity is contributing very amble quantity. Knowledge of genetic diversity of the genetic material is very critical in crop improvement. Effective selection is highly important in any crop improvement where the suffi cient genetic variation is available for different characters. The genetic variability analysis of crop cultivars for different agronomical and morphological characters are very critical in providing opportunity to select a number of promising cultivars. Genetic variation is the basic foundation