Search

Over the last decade, Bioversity International has worked with national, regional and local partners in eight countries (Burkina Faso, Ethiopia, Hungary, Mexico, Morocco, Nepal, Peru, and Vietnam) on the maintenance and use of crop genetic diversity on farm, particularly that found in traditional varieties (or landraces). The work has involved investigating the extent and distribution of diversity in over 27 crops and exploring with farmers and rural communities the management practices used to maintain traditional varieties. The results of this collaboration have (i) provided tools to assess the amount and distribution of crop genetic diversity in production systems (ii) increased our understanding of when, where and how this diversity will be maintained, (iii) identified practices, communities and institutions that support maintenance and evolution of crop genetic diversity in production systems, and (iv) provided possible mechanisms for ensuring that the custodians of these systems and genetic...

Plant genetic resources are conserved so that they can be used to improve crop plant pro- duction and in other ways. However, it is often asserted that use ofex situconserved germplasm is inadequate and that genetic diversity maintained in genebanks is underutilized. In part, this reflects an incomplete recognition of what constitutes use of plant genetic resources, and of the many different ways in which material from genebanks contributes to improved agricultural production. Based on recent information from surveys of distribution of germplasm from genebanks, and from surveys of users, we suggest that the evidence indicates that there is substantial use ofex situconserved materials for a wide range of different uses. We suggest that barriers to use ofex situconserved germplasm may often result from a lack in numbers of users, and from limitations in capacity to effectively utilize the genetic diversity present in genebanks to reduce genetic vulnerability and increase sustainability in modern production systems.

This chapter surveys the conceptual basis of indicators of genetic diversity, genetic erosion, and genetic vulnerability. These are summary measures of genetic diversity in cultivated plants and their wild relatives that guide decisions, monitor progress, and warn of emerging issues of genetic resources for resilient agricultural production. Such indicators measure the genetic diversity currently present in agricultural populations on farm and held in germplasm collections, and aim to detect genetic erosion, or serious loss of diversity in time, and to warn of vulnerability due to adverse deployment of genetic diversity in space. While diversity itself encompasses many concepts, richness diversity—the number of different kinds of individuals regardless of their frequencies—is the most important theme, followed by evenness diversity—how similar the frequencies of the different variants are. Many variables are plausible as indicators of diversity. The more practical are based on number of individuals or area occupied in situ and on the number of accessions and the number of genebanks ex situ. Genetic erosion is measurable as the proportion of richness of genetic diversity no longer existing in current populations, when compared with the crop a decade previously or predicted to be lost in the next decade without remedial action. Genetic vulnerability is inversely related to richness diversity that is present locally, particularly if it is known to possess adaptation to exotic or new mutant pathotypes or insect strains or environments. Census information forms the primary data. For cultivated species, these data are based on the farmer’s unit of diversity management, most often their named varieties, their number, relative frequencies and divergence over various units of spatial and temporal scale. For wild species, the analogous units of diversity are the lowest recognized (e.g. subspecies, morphological types, ecotypes). Census data should be supplemented and validated using more direct assays at the DNA level with molecular techniques in carefully constructed samples.

Biodiversity refers to variation within the living world, while genetic diversity represents the heritable variation within and between populations of organisms, and in the context of this paper, among plant species. This pool of genetic variation within an inter-mating population is the basis for selection as well as for plant improvement. Thus, conservation of this plant genetic diversity is essential for present and future human well-being. During recent years, there has been increasing awareness of the importance of adopting a holistic view of biodiversity, including agricultural biodiversity, conservation for sustainable utilization and development. These principles have been enshrined in the Convention on Biological Diversity and the Global Plan of Action of the Food and Agriculture Organization of the United Nations. The emphasis is now to understand the distribution and extent of genetic diversity available to humans in plant species, so that the genetic diversity can be safely conserved and efficiently used. It is generally recognized that plant genetic diversity changes in time and space. The extent and distribution of genetic diversity in a plant species depends on its evolution and breeding system, ecological and geographical factors, past bottlenecks, and often by many human factors. Much of the large amount of diversity of a species may be found within individual populations, or partitioned among a number of different populations. A better understanding of genetic diversity and its distribution is essential for its conservation and use. It will help us in determining what to conserve as well as where to conserve, and will improve our understanding of the taxonomy and origin and evolution of plant species of interest. Knowledge of both these topics is essential for collecting and use of any plant species and its wild relatives. In order to mange conserved germplasm better, there is also a need to understand the genetic diversity that is present in collections. This will help us to rationalize collections and develop and adopt better protocols for regeneration of germplasm seed. Through improved characterization and development of core collections based on genetic diversity information, it will be possible to exploit the available resources in more valuable ways.

A sesame core collection containing 453 accessions was established by assessing genetic diversity of 4251 accessions collected in China. The accessions in the whole collection were grouped based on their origin, varietal and agro-ecological type. A computer program was used to cluster the data of 14 traits from the random small sample. Ward's method was chosen as the desirable method for clustering sesame germplasm resources. A set of technical methods for establishing core collection has been developed through determining the number of selected accessions based on the diversity distribution. The agronomic traits of the pre-selected core collection were investigated and tested in three ecological locations in China for two years. Among six qualitative traits, including plant type and flower color in a total of 5304 accessions, 97.85% was completely the same as the original data, and three quantitative traits, including growth period and plant height, were significantly and positively related with the original data. The representativeness assessment of the final core collection indicated that 35 phenotypes of 10 qualitative traits in the core collection were basically similar to those of the pre-selected core collection. Six feature values such as mean, standard deviation etc. of four quantitative traits including oil content and 1000-seed weight were close to those of the pre-selected core collection. The final core collection possessed a fair representativeness of the pre-selected core collection.

Whether new combinations of genes that result from hybridization and introgression between wild and cultivated taxa are maintained, with the resultant development of populations with new characteristics, depends on natural selection, and in the case of crops, on human selection. While many cases of deliberate introgression of desirable traits into crop cultivars as part of breeding programmes are known, the extent and significance of natural or farmer-assisted introgression is uncertain. A range of techniques have been used to document natural hybridization and introgression of agricultural crops and their wild relatives in many crops including maize, wheat, barley, oats, pearl millet, foxtail millet, quinoa, hops, hemp, potato, cocona, casava, common bean, cowpea, pigeon pea, carrots, squash, tomato, radish, letuce, chilli, beets, sunflower, cabbage, and rasberries. However, the majority of these studies are based on morphological characters, and few have investigated the frequency with which such new types are produced and retained in natural and agroecosystems for farmer selection. Even more limited is information on the role of farmers in recognizing and selecting new genetic variation from the natural introgression of crops with their wild relatives, and the impact, once selected, of these new genetic combinations on the crop diversity. Molecular evaluation of natural introgression linked to investigations of farmer recognition and use of introgressed types provide ways of evaluating whether farmer selection for introgressed types is a significant process in increasing the genetic diversity of crop plants.

Cacao (Theobroma cacao L.) is indigenous to the Amazon basin, but is generally believed to have been domesticated in Mesoamerica for the production of chocolate beverage. However, cacao’s distribution of genetic diversity in South America is also likely to reflect pre-Columbian human influences that were superimposed on natural processes of genetic differentiation. Here we present the results of a spatial analysis of the intra-specific diversity of cacao in Latin America, drawing on a dataset of 939 cacao trees genotypically characterized by means of 96 SSR markers. To assess continental diversity patterns we performed grid-based calculations of allelic richness, Shannon diversity and Nei gene diversity, and distinguished different spatially coherent genetic groups by means of cluster analysis. The highest levels of genetic diversity were observed in the Upper Amazon areas from southern Peru to the Ecuadorian Amazon and the border areas between Colombia, Peru and Brazil. On the assumption that the last glaciation (22,000–13,000 BP) had the greatest pre-human impact on the current distribution and diversity of cacao, we modeled the species’ Pleistocene niche suitability and overlaid this with present-day diversity maps. The results suggest that cacao was already widely distributed in the Western Amazon before the onset of glaciation. During glaciations, cacao populations were likely to have been restricted to several refugia where they probably underwent genetic differentiation, resulting in a number of genetic clusters which are representative for, or closest related to, the original wild cacao populations. The analyses also suggested that genetic differentiation and geographical distribution of a number of other clusters seem to have been significantly affected by processes of human management and accompanying genetic bottlenecks. We discuss the implications of these results for future germplasm collection and in situ, on farm and ex situ conservation of cacao.

Varietal data from 27 crop species from five continents were drawn together to determine overall trends in crop varietal diversity on farm. Measurements of richness, evenness, and divergence showed that considerable crop genetic diversity continues to be maintained on farm, in the form of traditional crop varieties. Major staples had higher richness and evenness than nonstaples. Variety richness for clonal species was much higher than that of other breeding systems. A close linear relationship between traditional variety richness and evenness (both transformed), empirically derived from data spanning a wide range of crops and countries, was found both at household and community levels. Fitting a neutral “function” to traditional variety diversity relationships, comparable to a species abundance distribution of “neutral ecology,” provided a benchmark to assess the standing diversity on farm. In some cases, high dominance occurred, with much of the variety richness held at low frequencies. This suggested that diversity may be maintained as an insurance to meet future environmental changes or social and economic needs. In other cases, a more even frequency distribution of varieties was found, possibly implying that farmers are selecting varieties to service a diversity of current needs and purposes. Divergence estimates, measured as the proportion of community evenness displayed among farmers, underscore the importance of a large number of small farms adopting distinctly diverse varietal strategies as a major force that maintains crop genetic diversity on farm.

Summary - In recent years the possible use of wild relatives of crop species in plant breeding programs has been the subject of increased attention and this has stimulated the collection and ex situ maintenance of many wild plant species. These activities raise a number of questions concerning the number of wild species with which those interested in the conservation and utilization of plant genetic resources should be concerned, their relationships to crop species, the scale on which wild species' collection should be undertaken, the most suitable conservation methods for such material and the description of the genetic diversity available in these wider gene pools. Molecular genetic techniques have already provided useful information on such topics as the relationships between crop species and their wild relatives, the extent of genetic diversity in such species and genetic stability following ex situ storage. There remain significant opportunities for applying these techniques in studies of the distribution of genetic diversity in wild relatives of crop plants, and in the location, collection and maintenance of that diversity.