Exploring the <i>Bush yam</i> (<i>Dioscorea praehensilis</i> Benth) as a Source of Agronomic and Quality Trait Genes in White <i>Guinea yam</i> (<i>Dioscorea rotundata</i> Poir) Breeding

Yam (Dioscorea spp.) is an important food security crop in the tropics and subtropics. However, it is characterized by a narrow genetic base within cultivated and breeding lines for tuber yield, disease resistance, and postharvest traits, which hinders the yam breeding progress. Identification of new sources of desirable genes for these traits from primary and secondary gene pools is essential for this crop improvement. This study aimed at identifying potential sources of genes for yield and quality traits in a panel of 162 accessions of D. praehensilis, a semi-domesticated yam species, for improving the major yam species, D. rotundata. Significant differences were observed for assessed traits (p D. praehensilis genotypes out-performing the best D. rotundata landraces for tuber yield (23.47 t ha−1), yam mosaic virus (YMV) resistance (AUDPC = 147.45), plant vigour (2.43) and tuber size (2.73). The study revealed significant genotypic (GCV) and phenotypic (PCV) coefficients of variance for tuber yield, YMV severity score, and tuber flesh oxidation. We had also a medium-to-high broad-sense heritability (H2b) for most of the traits except for the dry matter content and tuber flesh oxidation. This study identified some promising D. praehensilis genotypes for traits such as high yield potential (WNDpr76, CDpr28, CDPr7, EDpr14, and WNDpr63), resistance to YMV (WNDpr76, CDpr7, EDpr14, CDpr28, and EDpr13), high dry matter content (WNDpr76, CDpr28, and WNDpr24), low tuber flesh oxidation (WNDpr76, CDpr5, WNDpr31, CDpr40, and WNDpr94) and high number of tubers per plant (WNDpr76, CDpr7, CDpr68, CDpr29, and CDpr58). These genotypes could, therefore, be employed in breeding programmes to improve the white Guinea yam by broadening its genetic base

Variation in the susceptibility of okra (Abelmoschus esculentus L. Moench) genotypes to okra mosaic virus and Podagrica species under field conditions

A total of 21 okra (Abelmoschus escuentus L. Moench) genotypes were screened for their reactions against okra mosaic disease (OMD) and flea beetles (Podagrica species) infestations in field trials which were conducted from May to October, 2015 (wet season) and November 2015 to March 2016 (dryseason), in order to identify sources of resistance and or tolerance. The trials were laid out in a randomised complete block design (RCBD) with four replications. Field resistance in the genotypes was assessed at 2, 6 and 10 weeks after planting using a 0 to 5 visual scale based on disease symptoms(where 1 denotes no symptom and 5, very severe symptom). Enzyme linked immunosorbent assay (ELISA) was performed to detect the presence of Okra mosaic virus (OkMV) in the okra genotypes.Populations of the flea beetle (Podagrica spp.), the vector of OkMV, and the associated leaf and fruit damage were also assessed. All the okra genotypes exhibited a varying range of disease symptoms and the flea beetle infestations, and lacked immunity. Genotypes GH2052, GH2063, GH2026, GH3760,GH5302, GH5332, GH5793, GH6105 and UCCC6 exhibited mild symptoms of OMD, and were less susceptible to flea beetle infestation and associated leaf damage during both seasons. Using ELISA, OkMV was detected in all the 21 genotypes. The mean number of fruits per plant and the mean fruit yield (t ha-1) differed significantly (P&lt;0.05) among the okra genotypes. Genotype GH5332 had the highest fruit yield of 11.88 t ha-1 followed by genotype GH6105 (9.34 t ha-1). Percentage fruit damage due to the flea beetle infestation differed significantly among the okra genotypes, ranging between 43.7 and 91.2% and from 47 to 84% in both trials respectively

The search for yield predictors for mature field-grown plants from juvenile pot-grown cassava (Manihot esculenta Crantz).

Cassava is the 6th most important source of dietary energy in the world but its root system architecture (RSA) had seldom been quantified. Ability to select superior genotypes at juvenile stages can significantly reduce the cost and time for breeding to bridge the large yield gap. This study adopted a simple approach to phenotyping RSA traits of juvenile and mature cassava plants to identify genotypic differences and the relationships between juvenile traits and harvest index of mature plants. Root classes were categorised and root and shoot traits of eight (8) juvenile pot-grown cassava genotypes, were measured at 30 and 45 days after planting (DAP). The same or related traits were measured at 7 months after planting of the same genotypes grown in the field while yield and yield components were measured in 12-months old field-grown plants. The field experiment was done in 2017 and repeated in 2018. Differences between genotypes for the measured traits were explored using analysis of variance (ANOVA) while traits in juvenile plants were correlated or regressed onto traits measured in 7- and 12-months old plants. The results show significant genotypic variations for most of the traits measured in both juvenile and 7-months old plants. In the 12-months old plants, differences between genotypes were consistent for both 2017 and 2018. Broad-sense heritability was highest for the number of commercial roots (0.87) and shoot fresh weight (0.78) and intermediate for the total number of roots (0.60), harvest index (0.58), fresh weight of roots (0.45). For all the sampling time points or growth stages, there were greater correlations between traits measured at a particular growth stage than between the same traits at different growth stages. However, some juvenile-mature plant trait relationships were significant, positive and consistent for both 2017 and 2018. For example, total root length and the total number of roots in 30 DAP, and branching density of upper nodal roots in 45 DAP, positively correlated with harvest index of 12-months old plants in both 2017 and 2018. Similarly, the diameter of nodal roots, for example, had a negative, significant correlation with fresh shoot biomass of mature plants in both 2017 and 2018. Regression of traits measured in 30 DAP explained up to 22% and 36% of the variation in HI of mature plants in 2017 and 2018, respectively. It is concluded that the simple, rapid, inexpensive phenotyping approach adopted in this study is robust for identifying genotypic variations in juvenile cassava using root system traits. Also, the results provide seminal evidence for the existence of useful relationships between traits of juvenile and mature cassava plants that can be explored to predict yield and yield components

Table_1_Genetic architecture of post-harvest tuber quality traits in bush yam (Dioscorea praehensilis Benth.) germplasm through association mapping.docx

IntroductionBush yam (Dioscorea praehensilis Benth.) is an important semi-domesticated food crop in West Africa. Limited information on the genetic architecture and its poor post-harvest tuber quality traits significantly hinder its use as food and source of income. Hence, dissecting the genetics underlying the expression of its post-harvest tuber quality traits is essential for establishing proper breeding schemes.MethodsIn this study, 138 D. praehensilis accessions collected in Ghana were sequenced using Diversity Array Technology (DArTSeq). The materials were profiled for dry matter content (DMC), tuber flesh oxidation (TBOXI) and for tuber flesh hardness (TBhard) during two cropping seasons.Results and discussionDiversity assessment using population structure, principal component analysis and hierarchical clustering methods revealed the presence of three major groups. Six genetic models were used for the trait association analysis using multiple random locus mixed linear model (MrMLM). Sixteen SNP markers distributed across the yam genome were identified to be associated with the evaluated traits. The associated SNP markers displayed a phenotypic variance ranged from 4.22% in TBHard to 16.92% in TBOXI. A total 25 putative candidate genes were identified around the SNP markers. The putative genes were identified to play key roles in tuber bulking, oxidative browning and starch hydroxylase. This study provides a valuable insight on the genetics underlying tuber quality traits in bush yam and opens avenues for developing genomic resources to improve D. praehensilis.</p

Image_1_Genetic architecture of post-harvest tuber quality traits in bush yam (Dioscorea praehensilis Benth.) germplasm through association mapping.pdf

IntroductionBush yam (Dioscorea praehensilis Benth.) is an important semi-domesticated food crop in West Africa. Limited information on the genetic architecture and its poor post-harvest tuber quality traits significantly hinder its use as food and source of income. Hence, dissecting the genetics underlying the expression of its post-harvest tuber quality traits is essential for establishing proper breeding schemes.MethodsIn this study, 138 D. praehensilis accessions collected in Ghana were sequenced using Diversity Array Technology (DArTSeq). The materials were profiled for dry matter content (DMC), tuber flesh oxidation (TBOXI) and for tuber flesh hardness (TBhard) during two cropping seasons.Results and discussionDiversity assessment using population structure, principal component analysis and hierarchical clustering methods revealed the presence of three major groups. Six genetic models were used for the trait association analysis using multiple random locus mixed linear model (MrMLM). Sixteen SNP markers distributed across the yam genome were identified to be associated with the evaluated traits. The associated SNP markers displayed a phenotypic variance ranged from 4.22% in TBHard to 16.92% in TBOXI. A total 25 putative candidate genes were identified around the SNP markers. The putative genes were identified to play key roles in tuber bulking, oxidative browning and starch hydroxylase. This study provides a valuable insight on the genetics underlying tuber quality traits in bush yam and opens avenues for developing genomic resources to improve D. praehensilis.</p

Table_2_Genetic architecture of post-harvest tuber quality traits in bush yam (Dioscorea praehensilis Benth.) germplasm through association mapping.docx

IntroductionBush yam (Dioscorea praehensilis Benth.) is an important semi-domesticated food crop in West Africa. Limited information on the genetic architecture and its poor post-harvest tuber quality traits significantly hinder its use as food and source of income. Hence, dissecting the genetics underlying the expression of its post-harvest tuber quality traits is essential for establishing proper breeding schemes.MethodsIn this study, 138 D. praehensilis accessions collected in Ghana were sequenced using Diversity Array Technology (DArTSeq). The materials were profiled for dry matter content (DMC), tuber flesh oxidation (TBOXI) and for tuber flesh hardness (TBhard) during two cropping seasons.Results and discussionDiversity assessment using population structure, principal component analysis and hierarchical clustering methods revealed the presence of three major groups. Six genetic models were used for the trait association analysis using multiple random locus mixed linear model (MrMLM). Sixteen SNP markers distributed across the yam genome were identified to be associated with the evaluated traits. The associated SNP markers displayed a phenotypic variance ranged from 4.22% in TBHard to 16.92% in TBOXI. A total 25 putative candidate genes were identified around the SNP markers. The putative genes were identified to play key roles in tuber bulking, oxidative browning and starch hydroxylase. This study provides a valuable insight on the genetics underlying tuber quality traits in bush yam and opens avenues for developing genomic resources to improve D. praehensilis.</p