Breeding dual-purpose Sweetpotato [Ipomoea batatas (L.) Lam.] varieties in Rwanda.
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2017
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Abstract
Sweetpotato [Ipomoea batatas (L.) Lam] is a multi-purpose crop where the fresh roots are used for human food and aboveground biomass for animal feed. In Rwanda, sweetpotato plays a key role in the mixed crop-livestock farming systems providing economic opportunities from livestock and crop production enterprises. However, dual-purpose sweetpotato varieties (DPSVs) with farmer-preferred traits and enhanced yields are yet to be developed and deployed in sub-Saharan Africa including in Rwanda. Therefore, the objectives of this study were: (i) to assess the role of sweetpotato in the crop-livestock farming system practised in Rwanda, to identify farmer-preferred traits and to establish farmer-led priorities in breeding dual-purpose sweetpotato varieties, (ii) to assess the level of phenotypic diversity present among sweetpotato varieties grown in Rwanda, and to select suitable parents for breeding DPSVs, (iii) to characterize diverse sweetpotato germplasm using simple sequence (SSR) markers to identify potential parents for breeding DPSVs, and (iv) to determine gene action and heritability of storage root and aboveground biomass yields, and yield components, in sweetpotato varieties, and to undertake early clonal selections for future release of DPSVs.
In the first study, a participatory rural appraisal (PRA) was undertaken in the following three selected districts of Rwanda: Bugesera, Huye and Nyagatare. Data were collected through semi-structured interviews, focus group discussions and a transect walk. All respondents wanted to grow new sweetpotato varieties with improved storage root production combined with high aboveground biomass. About 87.7, 66.6 and 51.1% of the respondents indicated that root-related traits of the crop such as high dry matter content, red skin colour and yellow flesh colour were additional preferred traits, respectively.
Secondly, fifty one diverse sweetpotato genotypes were evaluated in field trials conducted at the Rubona and Karama experimental stations of the Rwanda Agriculture Board (RAB) using a 6 x 9 unbalanced alpha lattice design with three replications. The top two genotypes selected for their high yields of storage roots were RW11-4923 and RW11-2419, with yields of 20.91 t.ha-1 and 20.18 t.ha-1, respectively. The genotypes RW11-4923 and Wagambolige were the best performers for aboveground yields, producing 23.67 t.ha-1 and 23.45 t.ha-1 of vines, respectively. The genotype Ukerewe performed well for its dry root yield (7.09 t.ha-1), while RW11-4923 had the highest mean dry vine yield (5.17 t.ha-1). The genotypes RW11-2910 and 8-1038 had root-to-vine ratios of 2.0 and 1.5, respectively. Two main phenotypic groups with 10 sub-groups were detected through cluster analysis and 24 sweetpotato clones were selected for their combination of high storage root yields, heavy vine production and prolific flowering ability.
Thirdly, the above 24 selected sweetpotato genotypes were genotyped with nine highly polymorphic SSRs. Cluster analysis allocated the test genotypes into three distinct genetic groups: I, II and III, with 6, 5 and 13 genotypes, respectively. Eight genetically diverse clones were selected, namely SPK004 and K5132/61 (from Group I), 4-160, Ukerewe, RW11-2910 (Group II), RW11-1860, Wagabolige, 2005-179 (Group III), with key agronomic traits for breeding DPSVs.
Finally, a half-diallel mating design was used and crosses were performed involving eight parents selected for their complementary traits including storage root and aboveground biomass production, dry matter content and farmer-preferred traits. A total of 28 families and 8 parents were field evaluated at Rubona, Karama and Ngoma research stations of RAB. Families had highly significant (P < 0.001) differences for fresh root yield (FRY), root dry matter content (RDMC), dry root yield (DRY), fresh vine yield (FVY), vine dry matter content (VDMC), dry vine yield (DVY), total biomass on dry weight basis (TBDW), root-to-vine ratio (R:V) and harvest index (HI). The general combining ability (GCA) and specific combining ability (SCA) effects were significant for FRY, RDMC, DRY, R:V, HI and VDMC. The GCA/SCA ratios were 0.75, 0.81 and 0.88 for DRY, RDMC and FRY, respectively, suggesting that additive gene action was more important than non-additive gene action in the expression of these parameters. Conversely, the GCA/SCA ratio was relatively lower, ranging between 0.09 and 0.28 for vine and root-vine combined parameters, suggesting that the non-additive component of the genetic variance, either dominance or epistasis, was more influential in controlling the traits. This implies that parental performance cannot necessarily be the basis of progeny performance prediction for these traits. The broad-sense heritability (H2) values were above 0.5 for all assessed traits, with FRY, HI and RDMC having higher estimates of 0.80, 0.81 and 0.92, in that order. RDMC had a high narrow-sense coefficient of genetic determination (NSCGD) of 0.80, while this parameter varied between 0.09 and 0.49 for the rest of the tested traits. The parent K5132/61 was the best combiner for FRY and HI, while the parents RW11-1860, RW11-2910, SPK004 and Ukerewe were best general combiners for RDMC. The parent Wagabolige was the best general combiner for FRY, DRY and R:V. Based on desirable SCA effects for FVY, DVY, TBDW, RDMC, R:V and FRY, the most promising families selected in this study were K5132/61 x Wagabolige, 4-160 x 2005-179, K5132/61 x RW11-1860 and RW11-2910 x 2005-179.
Overall, the study developed promising families with high storage root and aboveground biomass yields. From these families, novel progenies were selected and are recommended for advanced clonal selection across multiple sites to release DPSVs in Rwanda or similar agro-ecologies in SSA.
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Doctor of Philosophy in Plant Breeding. University of KwaZulu-Natal, Pietermaritzburg 2017.