Avaliação da severidade do cassava mosaic disease e cassava brown streak disease em germoplasma de Manihot esculenta Crantz e outras espécies do gênero Manihot de origem americana.

O principal objetivo deste estudo foi identificar acessos de mandioca da América Latina com resistência ao AfricanCassava Mosaic Disease (CMD) e Cassava Brown Streack Disease(CBSD). Em Uganda, onde estas viroses são endêmicas, quatro famílias F1 de Manihot esculenta Crantz selecionadas via seleção assistida por marcadores (SAM) ligado ao alelo de resistência CMD2, bem como espécies silvestres de Manihot foram avaliadas em condições de campo. A severidade do CMD nas folhas variou de 1,0 a 4,1 com maior severidade média (3,8) nas famílias F1 de Manihot esculenta Crantz. A severidade do CBSD nas folhas foi relativamente baixa (variação de 1,0 a 1,5). Por outro lado, a incidência e severidade dos sintomas de CBSD nas raízes das famílias F1 foram bastante elevadas (85% e 3,5, respectivamente). Entretanto, algumas espécies de Manihot não apresentaram nenhuma severidade do CBSD nas raizes, especialmente M. carthaginensis ssp. glaziovii e M. caerulescens. Portanto, os principais resultados deste projeto são: 1) a SAM para o CMD2 não foi eficaz para a seleção de fontes de resistência ao CMD contra as estirpes do vírus presentes em Uganda, 2) resistência genética ao CBSD foi observada em espécies de Manihot, que pode ser útil para geração de híbridos interespecífico

Validation of KASP markers associated with cassava mosaic disease resistance, storage root dry matter and provitamin A carotenoid contents in Ugandan cassava germplasm

Open Access Journal; Published online: 23 Nov 2022Introduction The intrinsic high heterozygosity of cassava makes conventional breeding ineffective for rapid genetic improvement. However, recent advances in next-generation sequencing technologies have enabled the use of high-density markers for genome-wide association studies, aimed at identifying single nucleotide polymorphisms (SNPs) linked to major traits such as cassava mosaic disease (CMD) resistance, dry matter content (DMC) and total carotenoids content (TCC). A number of these trait-linked SNPs have been converted to Kompetitive allele-specific polymerase chain reaction (KASP) markers for downstream application of marker assisted selection. Methods We assayed 13 KASP markers to evaluate their effectiveness in selecting for CMD, DMC and TCC in 1,677 diverse cassava genotypes representing two independent breeding populations in Uganda. Results Five KASP markers had significant co-segregation with phenotypes; CMD resistance (2), DMC (1) and TCC (2), with each marker accounting for at least 30% of the phenotypic variation. Markers located within the chromosomal regions for which strong marker-trait association loci have been characterised (chromosome 12 markers for CMD, chromosome 1 markers for DMC and TCC) had consistently superior ability to discriminate the respective phenotypes. Discussion The results indicate varying discriminatory abilities of the KASP markers assayed and the need for their context-based use for MAS, with PSY2_572 particularly effective in selecting for high TCC. Availing the effective KASP markers on cost-effective genotyping platforms could facilitate practical implementation of marker-assisted cassava breeding for accelerated genetic gains for CMD, DMC and provitamin A carotenoids

Prospects for Genomic Selection in Cassava Breeding

Article purchased; Published online: 28 Sept 2017Cassava (Manihot esculenta Crantz) is a clonally propagated staple food crop in the tropics. Genomic selection (GS) has been implemented at three breeding institutions in Africa to reduce cycle times. Initial studies provided promising estimates of predictive abilities. Here, we expand on previous analyses by assessing the accuracy of seven prediction models for seven traits in three prediction scenarios: cross-validation within populations, cross-population prediction and cross-generation prediction. We also evaluated the impact of increasing the training population (TP) size by phenotyping progenies selected either at random or with a genetic algorithm. Cross-validation results were mostly consistent across programs, with nonadditive models predicting of 10% better on average. Cross-population accuracy was generally low (mean = 0.18) but prediction of cassava mosaic disease increased up to 57% in one Nigerian population when data from another related population were combined. Accuracy across generations was poorer than within-generation accuracy, as expected, but accuracy for dry matter content and mosaic disease severity should be sufficient for rapid-cycling GS. Selection of a prediction model made some difference across generations, but increasing TP size was more important. With a genetic algorithm, selection of one-third of progeny could achieve an accuracy equivalent to phenotyping all progeny. We are in the early stages of GS for this crop but the results are promising for some traits. General guidelines that are emerging are that TPs need to continue to grow but phenotyping can be done on a cleverly selected subset of individuals, reducing the overall phenotyping burden.Bill & Melinda Gates FoundationUKaidCGIAR Research Program on Roots, Tubers and BananasPeer Revie

Genome-wide association mapping and genomic prediction for CBSD resistance in Manihot esculenta

Open Access Journal; Published online: 24 Jan 2018Cassava (Manihot esculenta Crantz) is an important security crop that faces severe yield loses due to cassava brown streak disease (CBSD). Motivated by the slow progress of conventional breeding, genetic improvement of cassava is undergoing rapid change due to the implementation of quantitative trait loci mapping, Genome-wide association mapping (GWAS), and genomic selection (GS). In this study, two breeding panels were genotyped for SNP markers using genotyping by sequencing and phenotyped for foliar and CBSD root symptoms at five locations in Uganda. Our GWAS study found two regions associated to CBSD, one on chromosome 4 which co-localizes with a Manihot glaziovii introgression segment and one on chromosome 11, which contains a cluster of nucleotide-binding site-leucine-rich repeat (NBS-LRR) genes. We evaluated the potential of GS to improve CBSD resistance by assessing the accuracy of seven prediction models. Predictive accuracy values varied between CBSD foliar severity traits at 3 months after planting (MAP) (0.27–0.32), 6 MAP (0.40–0.42) and root severity (0.31–0.42). For all traits, Random Forest and reproducing kernel Hilbert spaces regression showed the highest predictive accuracies. Our results provide an insight into the genetics of CBSD resistance to guide CBSD marker-assisted breeding and highlight the potential of GS to improve cassava breeding

Prospects for Genomic Selection in Cassava Breeding

Article purchased; Published online: 28 Sept 2017Cassava (Manihot esculenta Crantz) is a clonally propagated staple food crop in the tropics. Genomic selection (GS) has been implemented at three breeding institutions in Africa to reduce cycle times. Initial studies provided promising estimates of predictive abilities. Here, we expand on previous analyses by assessing the accuracy of seven prediction models for seven traits in three prediction scenarios: cross-validation within populations, cross-population prediction and cross-generation prediction. We also evaluated the impact of increasing the training population (TP) size by phenotyping progenies selected either at random or with a genetic algorithm. Cross-validation results were mostly consistent across programs, with nonadditive models predicting of 10% better on average. Cross-population accuracy was generally low (mean = 0.18) but prediction of cassava mosaic disease increased up to 57% in one Nigerian population when data from another related population were combined. Accuracy across generations was poorer than within-generation accuracy, as expected, but accuracy for dry matter content and mosaic disease severity should be sufficient for rapid-cycling GS. Selection of a prediction model made some difference across generations, but increasing TP size was more important. With a genetic algorithm, selection of one-third of progeny could achieve an accuracy equivalent to phenotyping all progeny. We are in the early stages of GS for this crop but the results are promising for some traits. General guidelines that are emerging are that TPs need to continue to grow but phenotyping can be done on a cleverly selected subset of individuals, reducing the overall phenotyping burden

Varietal response of cassava root yield components and root necrosis from cassava Brown streak disease to time of harvesting in Uganda

Published online: 21 Feb 2019Cassava brown streak disease (CBSD) is the most important biotic constraint threatening cassava (Manihot esculenta Crantz) production in eastern and southern Africa, and the food and income security of millions of rural farmers. With no tangible solution in view, farmers cope with the disease by harvesting the crop early, a practice that also leads to yield losses owing to small root size. This study evaluated CBSD root necrosis (RN), the total fresh root yield (TFRY), percentage marketable fresh root yield (PMFRY) and dry matter content (DMC) of six cassava varieties (MM, 2006/90, MM, 2006/123, MM, 2006/128, MM, 2006/130, MM 96/4271, and TME14) at different times of harvesting after the recommended 12 months after planting (MAP). Trials were planted during April 2012 at Sendusu and Serere located in the Wakiso and Serere districts of Uganda, respectively. The two sites are distinct in climatic, soil and CBSD disease pressure conditions. A split-plot design was used with cassava varieties as the main plots and times of harvesting (12, 16, 20 and 24 MAP) as sub-plots. Highly significant differences (P ≤ 0.001) were detected between sites (S), varieties (V) and time of harvesting (T) for TFRY, PMFRY and RN, whereas only V effects were highly significant (P ≤ 0.001) for DMC. The V × S interactions were highly significant for TFRY, PMFRY and RN, while the V × T interactions were highly significant for PMFRY and significant (P ≤ 0.01) for TFRY and DMC. The PMFRY decreased by about 14–27% as the plants remained in the field after 12 MAP. There was a great difference among clones, depending on location. Variety MM 2006/128 performed the best, with virtually no root damage even at 24 MAP at both sites. The mean CBSD root necrosis severity score among the varieties at each site increased from 2.6 (at 12 MAP) to 3.1 (at 24 MAP) at Sendusu, and from 1.9 (at 12 MAP) to 2.4 (at 24 MAP) at Serere. However, for MM2006/128 which was the best performing variety, the root necrosis score remained about 1.0 at Sendusu and about 1.3 at Serere. Time of harvesting had no significant effect on DMC. This study showed it is possible to breed new cassava varieties that combine tolerance to CBSD with long periods of in-ground storability. Genotype and environment had a profound effect on all the traits analysed, suggesting that genotypes should be selected for specific environments and harvest times. The results indicate that some of the clones (MM, 2006/128, MM, 2006/123 and MM, 2006/130) investigated in this study will be useful in the fight against CBSD in the region especially for the mid-altitude areas, which have recently been seriously affected by the virus disease

A breeding scheme for local adoption of cassava (Manihot esculenta Crantz)

In many rural communities, cassava mosaic disease (CMD) resistant varieties are being rejected by farmers owing to their inferior root qualities when compared to locally adapted varieties. In response to this challenge, we implemented a breeding scheme whose objective was to combine CMD resistance with farmer preferred root qualities, whose genes were respectively sourced for elite and local varieties. We targeted to achieve this goal within five years that comprised of: i) hybridization of complementary parental lines, ii) seedling evaluation trial (SET); iii) clonal evaluation trial (CET); iv) modified preliminary yield trial (MPYT) and v) modified uniform yield trial (MUYT). At SET and CET, emphasis was placed on traits of moderate to high heritability while for MPYT and MUYT emphasis was on traits of low heritability. Generated F 1 progeny (4080 half sibs) were established in SET of which 1014 seedlings were selected and advanced to the CET. At CET, only 143 clones were selected and advanced. Under MPYT, slightly less than 50% of the clones were selected, while under MUYT, (8 to 40 clones per site) were selected. Clones selected per site were characterized by: DMC (28 to 38%); ii) HI (0.26 to 0.62); iii) yield (14 to 59 t/ha), resistance to CMD and desirable farmer root qualities. Given this outcome, we have demonstrated the utility of this scheme in accelerating development of locally adapted cassava varieties and thus propose the scheme be referred to as "speed cassava breeding"