Deployment of diversity for enhanced crop function

Mixtures of genotypes are the norm in natural and seminatural ecosystems and subsistence agriculture but have been replaced by pure genotypes in modern agriculture to maximise profitability in high-input systems. However, crop function with respect to the stability of yield and quality in particular tends to be lost in this process. Diversity can be reintroduced into cropping systems as a trait not only to confer stability but also to exploit synergies between component genotypes, compensating for potential performance losses against the best performing genotype in any given season or location. Quality need not be compromised, and research has demonstrated practical development and deployment approaches, which challenge the assumed benefits of current approaches to agronomy and achieve enhanced crop function.     

Landscape-scale crop diversity interacts with local management to determine ground beetle diversity

Crop diversification and maintenance of semi-natural habitats (grasslands and field boundaries) are suggested to enhance farmland biodiversity, but the relative importance of these factors remains poorly known. We evaluated how crop diversity and availability of semi-natural grasslands at a landscape-scale interacted with local farming management (three management types from low to high intensity: ley < winter wheat < sugar beet) in their effect on ground beetle assemblages in southern Sweden. Ground beetle diversity increased with crop diversity either independently of local management (Simpson species diversity), or only in the less intensively managed habitats (rarefied species richness). While ground beetle diversity in leys tended to increase with field boundary length, no such relationship was observed in winter wheat or sugar beet fields. In contrast, the landscape proportions of leys and semi-natural grasslands did not affect ground beetle species richness and diversity. We conclude that (a) semi-natural grasslands and leys may not function as source habitats at a landscape-scale if they comprise a low proportion of the total land-use, while (b) increasing crop diversity is correlated to ground beetle richness and diversity in agricultural landscapes dominated by arable land. The beneficial effect of landscape-scale crop diversification on farmland biodiversity may depend on the general level of agricultural intensity of a region.     

Functional group diversity of bee pollinators increases crop yield

Niche complementarity is a commonly invoked mechanism underlying the positive relationship between biodiversity and ecosystem functioning, but little empirical evidence exists for complementarity among pollinator species. This study related differences in three functional traits of pollinating bees (flower height preference, daily time of flower visitation and within-flower behaviour) to the seed set of the obligate cross-pollinated pumpkin Cucurbita moschata Duch. ex Poir. across a land-use intensity gradient from tropical rainforest and agroforests to grassland in Indonesia. Bee richness and abundance changed with habitat variables and we used this natural variation to test whether complementary resource use by the diverse pollinator community enhanced final yield. We found that pollinator diversity, but not abundance, was positively related to seed set of pumpkins. Bees showed species-specific spatial and temporal variation in flower visitation traits and within-flower behaviour, allowing for classification into functional guilds. Diversity of functional groups explained even more of the variance in seed set (r2=45%) than did species richness (r2=32%) highlighting the role of functional complementarity. Even though we do not provide experimental, but rather correlative evidence, we can link spatial and temporal complementarity in highly diverse pollinator communities to pollination success in the field, leading to enhanced crop yield without any managed honeybees.     

Impact of crop genetic diversity on a litter consumer

Belowground communities, and especially earthworms, provide numerous services in agroecosystems. In the current context of crop genetic erosion, this study aims at assessing the effect of wheat cultivar diversity on earthworm growth and survival. Our laboratory experiment was divided into two periods: (1) the “feeding period” (4 months), simulating conditions in autumn with cool temperatures and high food availability, and (2) the “non-feeding period” (1 month) simulating conditions in winter with cold temperatures and low food availability because litter had already been consumed without replacement. The aerial biomass of mixtures of wheat cultivars was used as food for earthworms and their growth was measured twice during the feeding period and once during the non-feeding period. We found two major effects on earthworm growth. Increasing the cultivar number (i) increased juvenile earthworm growth and earthworm survival and (ii) decreased adult earthworm growth. A consistent positive effect of nitrogen concentration of the biomass was found on growth and this effect was independent of biodiversity effects. This study suggests that the loss of litter intraspecific genetic diversity has an impact on earthworm populations, probably through mechanisms linked to the variance in edibility or the balanced diet hypotheses. While they remain to be tested in the field, these results suggest that the current losses of crop genetic diversity could potentially impact processes at the community or ecosystem level through a decrease in food quality.     

Native crop diversity in Aridoamerica: Conservation of regional gene pools

Scholars have seldom considered the native crop diversity in northwest Mexico and the U.S. Southwest as resources of the same cohesive ecological and cultural region. The term Aridoamerica is introduced to describe this overlooked center of plant domestication and diversification, which is distinct from centers of Mesoamerica and the Mississippi Valley. To understand why certain of its landraces are unique, the systematic relationships and gene-pool relations of crops found prehistorically and protohistorically in Aridoamerica are reviewed. Signifcant crop/ weed introgression continues where indigenous agriculture persists, but native fields are being rapidly abandoned or converted. In planning in situ and ex situ conservation efforts to maintain this diversity, both cultural factors and plant population genetics must be considered.     

Generating and maintaining diversity at the elite level in crop breeding

Most breeding programs develop elite genotypes that are well adapted to the normal range of environmental conditions in the target production region. These elite lines have similar essential alleles for desirable end use characteristics, agronomics, disease resistance, and adaptation in the target region. The genetic makeup of these elite lines is complex. Intermating among the elite lines will often produce new variability through recombination with minimal risk of introducing new undesirable features, and is the source of most new cultivars. Eventually, this variation will be exhausted and new alleles must be introduced into the elite breeding population. Introducing desirable alleles from exotic germplasm may "pollute" the elite gene pool with undesirable alleles. Exotic germplasm may also disrupt essential allele combinations for adaptation, quality, and agronomic performance. New desirable alleles from exotic germplasm can be introgressed into an elite population in a systematic way through limited backcrossing with a minimal disturbance to the finely tuned elite background. Combining recurrent selection within elite germplasm with a systematic introgression from exotic germplasm in the recurrent introgressive population enrichment (RIPE) system has created an open-ended, continually improving, and sustainable elite population breeding system, which is simple, effective, and a regular source of new cultivars.     

Simple sequence repeat marker diversity in cassava landraces: genetic diversity and differentiation in an asexually propagated crop

Cassava (Manihot esculenta) is an allogamous, vegetatively propagated, Neotropical crop that is also widely grown in tropical Africa and Southeast Asia. To elucidate genetic diversity and differentiation in the crop's primary and secondary centers of diversity, and the forces shaping them, SSR marker variation was assessed at 67 loci in 283 accessions of cassava landraces from Africa (Tanzania and Nigeria) and the Neotropics (Brazil, Colombia, Peru, Venezuela, Guatemala, Mexico and Argentina). Average gene diversity (i.e., genetic diversity) was high in all countries, with an average heterozygosity of 0.5358 ± 0.1184. Although the highest was found in Brazilian and Colombian accessions, genetic diversity in Neotropical and African materials is comparable. Despite the low level of differentiation [F_st(theta) = 0.091 ± 0.005] found among country samples, sufficient genetic distance (1-proportion of shared alleles) existed between individual genotypes to separate African from Neotropical accessions and to reveal a more pronounced substructure in the African landraces. Forces shaping differences in allele frequency at SSR loci and possibly counterbalancing successive founder effects involve probably spontaneous recombination, as assessed by parent-offspring relationships, and farmer-selection for adaptation.Communicated by H.C. Becker     

Genetic diversity trends in twentieth century crop cultivars: a meta analysis

In recent years, an increasing number of papers has been published on the genetic diversity trends in crop cultivars released in the last century using a variety of molecular techniques. No clear general trends in diversity have emerged from these studies. Meta analytical techniques, using a study weight adapted for use with diversity indices, were applied to analyze these studies. In the meta analysis, 44 published papers were used, addressing diversity trends in released crop varieties in the twentieth century for eight different field crops, wheat being the most represented. The meta analysis demonstrated that overall in the long run no substantial reduction in the regional diversity of crop varieties released by plant breeders has taken place. A significant reduction of 6% in diversity in the 1960s as compared with the diversity in the 1950s was observed. Indications are that after the 1960s and 1970s breeders have been able to again increase the diversity in released varieties. Thus, a gradual narrowing of the genetic base of the varieties released by breeders could not be observed. Separate analyses for wheat and the group of other field crops and separate analyses on the basis of regions all showed similar trends in diversity.     

农作物遗传多样性农家保护的现状及前景

农作物地方品种的有效保护是农业生物多样性的可持续利用的基础。由于现代农业的集约化生产方式使大量农作物地方品种被少数高产改良品种所取代,造成农作物基因库的严重“基因流失”（genetic erosion)。农家保护是在农业生态系统中进行的动态保护,被保护的生物多亲性可在其生境中继续进化而产生新的遗传变异,在而是农业生物多样性就地保护的重要途径。然而,尽管人们对作物品种资源农家保护的兴趣不断增长,也有大量有关农家的保护的研究和案例分析,但目前为止还没有比较成功的农家保护实例报道。因此,对农家保护的机制及科学问题进行深入的研究,并寻求一条新的途径来充分发挥农家保护应有的作用,显得格外重要。利用生物多样性布局的水稻混合间栽的生产模式,不仅解决了病害控制的问题,而且也保护了水稻地方品种的多样性。这种混合间栽的生物多样性布局和生产方式可能成为农保护的一条新途径。     

Dynamic management of crop diversity: From an experimental approach to on-farm conservation

In agricultural systems, biodiversity includes diversity within species and among species and provides many benefits for production, resilience and conservation. This article addresses the effects of a strategy of in situ conservation called dynamic management (DM) on population evolution, adaptation and diversity. Two French DM initiatives are considered, the first one corresponding to an experimental context, the second to an on-farm management. Results from a study over 26 years of experimental DM of bread wheat (Triticum aestivum L.) are first presented, including the evolution of agronomic traits and genetic diversity at neutral and fitness related loci. While this experiment greatly increased scientific knowledge of the effects of natural selection on cultivated populations, it also showed that population conservation cannot rely only on a network of experimental stations. In collaboration with a farmers' network in France, researchers have begun studying the effects of on-farm DM (conservation and selection) on diversity and adaptation. Results from these studies show that on-farm DM is a key element for the long-term conservation and use of agricultural biodiversity. This method of in situ conservation deserves more attention in industrialised countries.     

Impact of crop management on intraspecific diversity of Pseudomonas corrugata in bulk soil

Colonization at sugar beet root surfaces by seedling-inoculated biocontrol strain Pseudomonas fluorescens DR54 and native soil bacteria was followed over a period of 3 weeks using a combination of immunofluorescence (DR54-targeting specific antibody) and fluorescence in situ hybridization (rRNA-targeting Eubacteria EUB338 probe) techniques with confocal laser scanning microscopy. The dual staining protocol allowed cellular activity (ribosomal number) to be recorded in both single cells and microcolonies of strain DR54 during establishment on the root. After 2 days, the population density of strain DR54 reached a constant level at the root basis. From this time, however, high cellular activity was only found in few bacteria located as single cells, whereas all microcolony-forming cells occurring in aggregates were still active. In contrast, a low density of strain DR54 was observed at the root tip, but here many of the bacteria located as single cells were active. The native population of soil bacteria, comprising a diverse assembly of morphologically different forms and size classes, initiated colonization at the root basis only after 2 days of incubation. Hence the dual staining protocol allowed direct microscopic studies of early root colonization by both inoculant and native soil bacteria, including their differentiation into active and non-active cells and into single or microcolony-forming cells.