QTL associated with resistance to cassava brown streak and cassava mosaic diseases in a bi-parental cross of two Tanzanian farmer varieties, Namikonga and Albert

Article purchasedCassava production in Africa is compromised by cassava brown streak disease (CBSD) and cassava mosaic disease (CMD). To reduce costs and increase the precision of resistance breeding, a QTL study was conducted to identify molecular markers linked to resistance against these diseases. A bi-parental F1 mapping population was developed from a cross between the Tanzanian farmer varieties, Namikonga and Albert. A one-step genetic linkage map comprising 943 SNP markers and 18 linkage groups spanning 1776.2 cM was generated. Phenotypic data from 240 F1 progeny were obtained from two disease hotspots in Tanzania, over two successive seasons, 2013 and 2014. Two consistent QTLs linked to resistance to CBSD-induced root necrosis were identified in Namikonga on chromosomes II (qCBSDRNFc2Nm) and XI (qCBSDRNc11Nm) and a putative QTL on chromosome XVIII (qCBSDRNc18Nm). qCBSDRNFc2Nm was identified at Naliendele in both seasons. The same QTL was also associated with CBSD foliar resistance. qCBSDRNc11Nm was identified at Chambezi in both seasons, and was characterized by three peaks, spanning a distance of 253 kb. Twenty-seven genes were identified within this region including two LRR proteins and a signal recognition particle. In addition, two highly significant CMD resistance QTL (qCMDc12.1A and qCMDc12.2A) were detected in Albert, on chromosome 12. Both qCMDc12.1A and qCMDc12.2A lay within the range of markers reported earlier, defining the CMD2 locus. This is the first time that two loci have been identified within the CMD2 QTL, and in germplasm of apparent East African origin. Additional QTLs with minor effects on CBSD and CMD resistance were also identified

The role of community based organisations' NGOs and farmers in technology transfer

Cassava (Manihot esculenta Crantz) and sweetpotato (Ipomoea batatas) are key food crops in Tanzania in general and in the lake zone in particular. Cassava and sweetpotato are household food security crops in addition to being a source of household income. Availability of the new improved varieties to farmers has been a problem in Tanzania. A project was initiated to transfer improved varieties of both cassava and sweetpotato in the lake zone to farmers. In collaboration with community-based organisations (CBOs), non-government organisations (NGOs) and farmer groups, the project distributed different varieties of cassava and sweet potato in Shinyangu and Mwanza regions through CIP/ASARECA. The CBOs, NGOs and farmer groups were very instrumental in facilitating the delivery of improved varieties to farmers. Through this informal seed delivery system using CBOs, NGOs, and farmers groups, farmers have been able to access, use and adopt improved varieties suitable for their areas. The project has also established a system that can be used for future technology transfer activities. through this system more than 988,000 and 1,752,711 cuttings of improved cassava and sweet potato varieties respectively were distributed in three regions of the lake zone. The distribution activity went hand in hand with training of farmers in rapid multiplication techniques of planting materials

An ESTderived SNP and SSR genetic linkage map of cassava (Manihot esculenta Crantz)

Published online: 15 march, 2012Cassava (Manihot esculenta Crantz) is one of the most important food security crops in the tropics and increasingly being adopted for agro-industrial processing. Genetic improvement of cassava can be enhanced through marker-assisted breeding. For this, appropriate genomic tools are required to dissect the genetic architecture of economically important traits. Here, a genome-wide SNPbased genetic map of cassava anchored in SSRs is presented. An outbreeder full-sib (F1) family was genotyped on two independent SNP assay platforms: an array of 1,536 SNPs on Illumina’s GoldenGate platform was used to genotype a first batch of 60 F1. Of the 1,358 successfully converted SNPs, 600 which were polymorphic in at least one of the parents and was subsequently converted to KBiosciences’ KASPar assay platform for genotyping 70 additional F1. High-precision genotyping of 163 informative SSRs using capillary electrophoresis was also carried out. Linkage analysis resulted in a final linkage map of 1,837 centi-Morgans (cM) containing 568 markers (434 SNPs and 134 SSRs) distributed across 19 linkage groups. The average distance between adjacent markers was 3.4 cM. About 94.2% of the mapped SNPs and SSRs have also been localized on scaffolds of version 4.1 assembly of the cassava draft genome sequence. This more saturated genetic linkage map of cassava that combines SSR and SNP markers should find several applications in the improvement of cassava including aligning scaffolds of the cassava genome sequence, genetic analyses of important agro-morphological traits, studying the linkage disequilibrium landscape and comparative genomics

Diallel analysis of field resistance to brown streak disease in cassava (Manihot esculenta Crantz) landraces from Tanzania

Published online: 13 June 2012Cassava brown streak disease (CBSD) is an economically important virus disease causing significant losses to cassava root yield and quality in east, central and southern Africa. Breeding for resistance in cassava requires an understanding of the underlying genetic control of CBSD resistance. Sources of CBSD resistance are available but little is known on the value of those varieties as parents for CBSD resistance breeding. Two resistant and two susceptible varieties were crossed in a half diallel design and 35 F1 progeny from each of the six families, plus parents, were screened at two locations in a randomised complete block design with four replications in warm sub-humid environments of coastal Tanzania in 2008. Screening for CBSD field resistance was done using disease severity scoring on a scale of 1–5. Significant variations in disease severity were observed for crosses, general combining ability (GCA) and specific combining ability (SCA) at both locations. The CBSD shoot symptom severity scores ranged from 1 to 4.4 while root necrosis ranged from 1.3 to 4.5. The contribution of GCA to the total sum of squares of crosses for disease scores ranged from 86.9 to 95.2 % compared to SCA that ranged from 4.8 to 14.2 %. Additive gene effects were more important than non-additive effects indicating that CBSD resistance is genetically determined and that selection should be successful to improve resistance. Selection of parents with good GCA effects will be important for success in CBSD resistance breeding

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 [Fst(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

Mapping of QTL associated with resistance to Cassava Brown Streak and Mosaic Diseases in outcrossing cassava cultivars locally cultivated in Tanzania

Cassava brown streak (CBSD) and mosaic (CMD) diseases are the major cassava production constraints in the East African region. Efforts to control these diseases were initiated in northeastern Tanganyika in the 1930’s using conventional breeding methods. Despite these efforts, CBSD is spreading, threatening central and western Africa. Identification of molecular markers associated with observed plant resistance and tolerance would facilitate marker assisted breeding leading to efficient breeding, knowledge-based deployment of resistance genes and pre-emptive breeding in West Africa. QTL analysis using a bi-parental F1 mapping population was undertaken to identify QTL for CMD resistance in the variety ‘Albert’ and CBSD resistance in the variety ‘Namikonga’. A one-step genetic linkage map composed of 986 SNP markers and 18 linkage groups spanning 1826.3 cM was generated and used to create a framework map of 242 loci. Phenotyping data was obtained from two disease hotspots in Tanzania from 226 F1 progeny. Significant QTL were identified on chromosomes VIII, XV and I for CMD, CBSD foliar and root symptoms respectively. The maximum LOD score of 19.32 explained variation 34.9% of variation for CMD resistance. CBSD foliar and root symptoms revealed different QTL with moderate LOD scores of up to 5.09 and 7.33, explaining 10.7 and 16.2% respectively of variation. Several other minor significant QTL for all traits were also identified. Genes within QTL regions are being characterized