1.3 Breeding for drought tolerance
1.3.4 Sweetpotato breeding
Mating designs are important in the breeding of any crop as they are used to evaluate parents, and develop F1 recombinant crosses that form the base population for selection (Hallauer et al., 2010). They also help to generate synthetic populations, and provide information for estimating gene action and genetic gains (Hallauer et al., 2010). In sweetpotato, mating designs such as polycross, diallel, or North Carolina design I may be used (Jones et al., 1987; Mwanga et al., 2002; Chiona, 2010).
The diallel mating design is used to study the polygenic action (Hayman, 1954; Hallauer et al., 2010). Its analysis is based on general combining ability (GCA) and specific combining ability (SCA) (Griffing, 1956b). GCA and SCA enable distinguishing between the average performance of parents and their crosses, respectively (Johnson and King, 1998; Acquaah, 2007). The GCA indicates the relative value of the population in terms of frequency of favourable genes, and identifies superior parents for use in intra-population breeding programmes (Acquaah, 2007). On the other hand, the SCA indicates the predominant direction of the deviations due to dominance in a population (Viana and Matta, 2003;
Acquaah, 2007).
An evaluation of at least five parents in a partial diallel has been reported to give reliable estimates of GCA (Stuber, 1980). Genetic component analysis done using partial diallel on SPVD resistance in Uganda indicated that GCA:SCA range of 0.51-0.87, meaning GCA effects were more important than SCA ratios (Mwanga et al., 2002). Resistant parents exhibited high GCA meaning additive gene effects were predominant in the inheritance of resistance to SPVD. The narrow sense heritability of 31-41% and broad sense heritability of 73-98% (moderate–high) was reported meaning genetic gain for SPVD resistance could be accomplished easily by mass selection (Mwanga et al., 2002).
After making crosses in sweetpotato breeding, visual selection that eliminates genotypes that do not meet the lowest acceptable values for each trait is done (Grüneberg et al., 2009).
This is followed by index selection for yield and nutritional quality based on the desired genetic gain. Further, the remaining clones are selected against pests and diseases done by visual selection. The last 100-200 clones enter into late breeding stages, but are also used as parental materials for the next cycle of recombination and selection (Grüneberg et al., 2009).
Each sweetpotato seedling is potentially a new variety (Chiona, 2010). The seedlings are clonally propagated, whereby; about 40-60 cuttings per plant are produced under rapid multiplication (Stathers et al., 2005). The breeder may reduce the selection cycles from five
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years to about a year or two and get the best genotype for tolerance to abiotic and biotic factors using accelerated breeding scheme (ABS) (Grüneberg et al., 2009). Use of higher selection intensity (5-20%) and many environments results in optimum genetic gain (Wricke and Weber, 1986). Crosses and multiplication are done during early selection and late selection stages, reducing the phases into 3 stages in 3 years and, the clone that fails test in each stage are discarded (independent culling) (Wricke and Weber, 1986). About 90 cuttings of sweetpotato per plant can be obtained in 3-4 months (Stathers et al., 2005).
Visual evaluation and selection is conducted in poor resource environment and selected clones are further evaluated in more environments (Pesek and Baker, 1969). Potential clones selected from previous selection cycle are subjected to high selection pressure, upon which 2-5 clones are finally selected for variety release (Grüneberg et al., 2009). The five phases of selections in sweet potato are summarized in Fig. 1.1.
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Parent A X Parent B Parental crosses: many crosses are done as much as possible
A1F A1F A1F A1F ↓ B1F B1F B1F B1F F1 seedling developed
↓
FA1 clones FA1 clones FA1 clones FA1 clones FB1 clones FB1 clones FB1 clones FB1 clones
A clones: Evaluation plots, visual screening, few traits measured, visual selection done, small single row plots of 3-5 plants used, not replicated. The evaluation get close to optimum response to selection, usually done in two sites (Grüneberg et al., 2004)
A1 F A1 F A1 F A1 F ↓ B1 clonesF B1 clonesF B1 clonesF B1 clonesF
B clones: Promising clones selected and planted in plots of 2-3 locations
A1 F A1 F ↓ B1 clonesF B1 clonesF
C clones: Good clones are selected for further trials
↓
FA1 FB1 clones
D-clones: Selection for advanced trials and potential release
Figure 1.1: Breeding process of sweetpotato Source: Grüneberg et al., 2009
For example, a mass selection programme of 350 sweetpotato genotypes from Taiwan, Japan, New Guinea, Hawaii, Nigeria, Thailand, Peru, Cuba, Philippines, Puerto Rico, New Zealand, Guatemala, Uruguay, Cook Islands, Marquesas Islands, Spain, and Canada from the previous wide gene populations was used by Jones in mass selection (Jones et al., 1987). The plants were allowed to undergo open-pollination. About 3000 seeds were collected, germinated and grown in the greenhouse. Among them, 700 genotypes representing all possible crosses were trellised. After open pollination, seed from 200 of the 700 plants were selected to start the next cycle. In the third cycle the number selected as seed parents was reduced to about 100 among 700 trellised plants. After a two-year evaluation, the best 25-30 selections are identified and their seed used to start the next
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selection cycle. Selections from the mass selection populations can be tested directly for genotype potential or may provide a source of parental material for a polycross nursery.