Clair Hershey

Yield Stability of Cassava During Prolonged Mid-Season Water Stress

Yield stability in cassava requires genotypes that produce well under the variable moisture conditions encountered during the growth cycle. Plant characteristics related to yield stability were studied in two cassava clones subjected to 105 days of water stress in a field drainage lysimeter. Stress conditions commenced 117 days after planting, and the plants were allowed to recover at the end of the stress period for the rest of the growth cycle. Water stress restricted the growth of leaves and stems, but root yields were increased or remained unaffected. Leaf water potential varied little with stress, but gas exchange rates were about 75% those of the control throughout the stress period in both cultivars. Under stress, the plants partially closed their stomata and extracted deep soil moisture slowly. A high yield in both wet and stressed environments was associated with high mean LAI, better leaf retention, and greater partitioning of shoot biomass into leaf formation.

Conventional breeding, marker-assisted selection, genomic selection and inbreeding in clonally propagated crops: a case study for cassava

Key messageConsolidates relevant molecular and phenotypic information on cassava to demonstrate relevance of heterosis, and alternatives to exploit it by integrating different tools. Ideas are useful to other asexually reproduced crops.AbstractAsexually propagated crops offer the advantage that all genetic effects can be exploited in farmers’ production fields. However, non-additive effects complicate selection because, while influencing the performance of the materials under evaluation, they cannot be transmitted efficiently to the following cycle of selection. Cassava can be used as a model crop for asexually propagated crops because of its diploid nature and the absence of (known) incompatibility effects. New technologies such as genomic selection (GS), use of inbred progenitors based on doubled haploids and induction of flowering can be employed for accelerating genetic gains in cassava. Available information suggests that heterosis, non-additive genetic effects and within-family variation are relatively large for complex traits such as fresh root yield, moderate for dry matter or starch content in the roots, and low for defensive traits (pest and disease resistance) and plant architecture. The present article considers the potential impact of different technologies for maximizing gains for key traits in cassava, and highlights the advantages of integrating them. Exploiting heterosis would be optimized through the implementation of reciprocal recurrent selection. The advantages of using inbred progenitors would allow shifting the current cassava phenotypic recurrent selection method into line improvement, which in turn would allow designing outstanding hybrids rather than finding them by trial and error.

A global alliance declaring war on cassava viruses in Africa

James Legg & Eklou Attiogbevi Somado & Ian Barker & Larry Beach & Hernan Ceballos & Willmer Cuellar & Warid Elkhoury & Dan Gerling & Jan Helsen & Clair Hershey & Andy Jarvis & Peter Kulakow & Lava Kumar & Jim Lorenzen & John Lynam & Matthew McMahon & Gowda Maruthi & Doug Miano & Kiddo Mtunda & Pheneas Natwuruhunga & Emmanuel Okogbenin & Phemba Pezo & Eugene Terry & Graham Thiele & Mike Thresh & Jonathan Wadsworth & Steve Walsh & Stephan Winter & Joe Tohme & Claude Fauquet

Cassava Breeding: Current Status, Bottlenecks and the Potential of Biotechnology Tools

Cassava is an important energy source in the diets of millions of people in tropical and subtropical regions of the world. It is a key subsistence crop, and its industrial uses are steadily growing. In spite of its economic and social relevance, relatively little investment has been made for research on cassava. However, conventional breeding resulted in more stable production through enhanced tolerance to biotic and abiotic stresses; increased productivity, both in fresh root production and increased dry matter content; and, more recently, improvements in qualitative traits such as starch quality and increased carotenoids content. The inbreeding of cassava has been identified as a key step for more efficient genetic improvement of the crop, therefore, research is underway to develop protocol(s) for the production of doubled haploids. Marker-assisted selection has been successfully applied to cassava, but in a more modest scale compared with other crops. More support and emphasis is needed on practical applications of molecular marker technology in cassava improvement. The availability of more efficient genotyping approaches and the cassava genome sequence promise to increase the impact of biotechnology tools on cassava improvement. Efficient and reliable phenotyping of cassava remains a challenging goal to achieve in the near future.

Cassava ( Manihot esculenta Crantz)

Cassava is a key food security staple and a competitive feedstock for multiple industrial processes and end uses. Farmers grow hybrids which are reproduced vegetatively. Several programs have used the same breeding scheme for the last 40 years. Significant progress has been made, particularly with the first improved varieties released in the 1980s and 1990s. However, gains slowed down since then. Biotechnology tools, after more than two decades, have not yet had impact on increasing yields. Results from ongoing genomic selection show promising results for high-heritability traits, but not for fresh root yield (FRY). Key challenges to increase FRY are the strong influence of nonadditive genetic effects and the heterozygous nature of breeding parents. There is large within-family variation masking the true breeding value of each progenitor. To improve yields, breeding must shift from making crosses where breeders hope to find hybrids that are superior to those already available (a strategy that has made only slow progress in the last decade or two) to develop and improve inbred progenitors that can produce more reliably better hybrids. The use of inbred progenitors and implementation of reciprocal recurrent selection should be an efficient way to exploit heterosis and epistasis, which are large components in the determination of FRY. Induction of flowering would also accelerate genetic gains. In the near-term future, available molecular markers can be used to improve the breeding value of progenitors rather than in selection of segregating progenies. They can also be used to screen germplasm collections in search of useful traits.

Propagating cassava (Manihot esculenta) by sexual seed

Commercial cassava production from true cassava seeds (TCS) appears to be a promising option for reducing or eliminating several of the production constraints associated with vegetative propagation. The most important contribution of TCS would be to reduce virus build-up in vegetative material and to resolve the problems of stake storage, low multiplication rate and the long growth cycle. Preliminary results suggest that the root yield potential of TCS is comparable with that of traditional vegetative propagation. Improvements in the capacity of true seed progenies to germinate and establish in the field can be achieved either genetically or through pre-planting seed treatments such as coating the seed with rock phosphate. Development of a successful TCS technology will require a multidisciplinary approach, involving basic studies in cassava breeding, physiology, agronomy, socio-economics and other fields.

Sensitivity of cassava (Manihot esculenta Crantz) clones to environmental changes

Abstract The performance of 15 cassava (Manihot esculenta Crantz) clones in 14 environments in Colombia was analyzed to determine the possibility of improving stability of root-yield in cassava in association with minimum acceptable yields. The specific objectives were to study the relationship among agronomic traits, to evaluate genotypic sensitivity to changes in the environment, and to characterize and determine the representativeness of the evaluation environments. Correlations found between root yield and related physiological or quality traits were in a favorable direction for breeding purposes. This indicates that when selecting for a complex set of traits, indices might be established with major emphasis on traits with high heritability and/or stability. For some traits (number of commercial roots and length of stem with attached leaves) the range of genetic variability was broader in favorable environments. Variation among evaluation sites was greater than variation across years. The results indicate that intermediate to low genotypic sensitivity in terms of cassava root yield andry matter content can be combined with improved potential for dry matter production per unit area. In order to improve the performance and stability of cassava gene-pools, representative sites should be selected within the priority agro-ecosystem, to evaluate the genetic base for at least two years before selection is made. Association between the mean and the sensitivity coefficient for different traits was either nonsignificant or positive for breeding purposes. Improvement in the mean of traits can be made independently from, or in relation to, genotypic ability to react to environmental changes.

Genetic Biotechnologies and Cassava-Based Development

Cassava (Manihot esculenta Crantz) possesses a number of characteristics which make it an irreplaceable food security for smallholder farmers in certain areas of the tropics, where climate, soils, or societal stresses create particularly difficult conditions. Yet because of other characteristics, and especially historical factors, cassava’s value for improving the quality of life of farm families who depend on it is far less than it could be. One resuit is that cassava remains relatively more important in poor, unfavored areas where there are few crop alternatives — areas where smallholder farmers predominate. Appropriate research and development (R&D) to improve cassava’s productivity and value therefore has unusual and direct linkages to global development objectives: food security, poverty alleviation, equity, and environmental protection.

Toward better understanding of postharvest deterioration: biochemical changes in stored cassava (Manihot esculenta Crantz) roots:

Abstract Food losses can occur during production, postharvest, and processing stages in the supply chain. With the onset of worldwide food shortages, interest in reducing postharvest losses in cassava has been increasing. In this research, the main goal was to evaluate biochemical changes and identify the metabolites involved in the deterioration of cassava roots. We found that high levels of ascorbic acid (AsA), polyphenol oxidase (PPO), dry matter, and proteins are correlated with overall lower rates of deterioration. On the other hand, soluble sugars such as glucose and fructose, as well as organic acids, mainly, succinic acid, seem to be upregulated during storage and may play a role in the deterioration of cassava roots. Cultivar Branco (BRA) was most resilient to postharvest physiological deterioration (PPD), while Oriental (ORI) was the most susceptible. Our findings suggest that PPO, AsA, and proteins may play a distinct role in PPD delay.

Cassava Breeding I: The Value of Breeding Value

Breeding cassava relies on several selection stages (single row trial-SRT; preliminary; advanced; and uniform yield trials—UYT). This study uses data from 14 years of evaluations. From more than 20,000 genotypes initially evaluated only 114 reached the last stage. The objective was to assess how the data at SRT could be used to predict the probabilities of genotypes reaching the UYT. Phenotypic data from each genotype at SRT was integrated into the selection index (SIN) used by the cassava breeding program. Average SIN from all the progenies derived from each progenitor was then obtained. Average SIN is an approximation of the breeding value of each progenitor. Data clearly suggested that some genotypes were better progenitors than others (e.g., high number of their progenies reaching the UYT), suggesting important variation in breeding values of progenitors. However, regression of average SIN of each parental genotype on the number of their respective progenies reaching UYT resulted in a negligible coefficient of determination (r2 = 0.05). Breeding value (e.g., average SIN) at SRT was not efficient predicting which genotypes were more likely to reach the UYT stage. Number of families and progenies derived from a given progenitor were more efficient predicting the probabilities of the progeny from a given parent reaching the UYT stage. Large within-family genetic variation tends to mask the true breeding value of each progenitor. The use of partially inbred progenitors (e.g., S1 or S2 genotypes) would reduce the within-family genetic variation thus making the assessment of breeding value more accurate. Moreover, partial inbreeding of progenitors can improve the breeding value of the original (S0) parental material and sharply accelerate genetic gains. For instance, homozygous S1 genotypes for the dominant resistance to cassava mosaic disease (CMD) could be generated and selected. All gametes from these selected S1 genotypes would carry the desirable allele and 100% of their progenies would be resistant. Only half the gametes produced by the heterozygous S0 progenitor would carry the allele of interest. For other characteristics, progenies from the S1 genotypes should be, at worst, similar to those generated by the S0 progenitors.