23 Buerno (1986) reported important genotype x location and genotype x location x year interaction for FSRY when testing a number of genotypes in the humid tropics of Brazil. Huhn (1996) reported that the dry matter content of cassava storage roots had high cultivar-by-year interaction and cultivar-temperature interaction.
24 can be included (Jennings and Iglesias, 2002; Ceballos et al., 2004). Table 1.1 adopted from (Ceballos et al., 2012) illustrates the selection scheme currently used at CIAT. Ceballos et al.
(2012) indicated that the scheme represents the way most cassava-breeding projects operate, beginning with the crossing of elite genotypes, including multiple rounds of selection, and ending with a few genotypes reaching the stage of regional trials across several locations. They, however, report that variations among cassava-breeding programmes regarding the numbers of genotypes and plants representing them through the different stages exist. Selection starts with nurseries planted with seedlings derived from botanical seeds. Considering the low correlation between the performance at seedling and clonal propagation stages, Jennings and Iglesias (2002) and Ceballos et al. (2004; 2012) suggested that early selections should be based on highly heritable traits only, such as plant type, branching habit and, particularly, reaction to diseases as indicated by Hahn et al. (1980b) and Morante et al. (2005). The second stage of selection, called clonal evaluation trial (CET) uses the few surviving genotypes from the seedling stage that can produce 6 to 10 vegetative cuttings required for CET. The capacity to produce this number of cuttings is another selection criterion utilised at the F1 stage Ceballos et al. (2004; 2012). The selection at CET depends largely on HI (Kawano 2003; Morante et al. 2005), plant type (Kawano et al., 1978), dry mass content and cyanogenic glycosides (Iglesias and Hershey, 1994). The first three evaluation stages are carried out at one location (Table 1.1). Subsequent selections, advanced yield trial, regional trial - I and regional trial - II are conducted at more locations (Jennings and Iglesias, 2002; Ceballos et al., 2004; 2012). The common characteristic with all cassava breeding programmes is that from the early stages to the later stages of breeding, the number of genotypes evaluated is reduced, whereas the number of test locations increases.
Table 1.1: Description of evaluation and selection stages utilised in the International Center for Tropical Agriculture cassava breeding programme (Ceballos et al., 2012)
Time (months)
Stage Plants per
plot (number)
Repetitions Locations Genotypes evaluated
18 Crossing blocks -.- -.- -.-
19 - 30 F1 1 1 1 2500
31 - 42 Clonal evaluation trial (CET) 6 - 8 1 1 1500-3000
43- 54 Preliminary yield trial (PYT) 10 3 1 100-300
55 - 56 Advanced yield trial (AYT) 20 3 1 - 2 75-150
67 - 78 Regional trial (RT) - I 25 3 2 - 6 20-40
79 - 90 Regional trial (RT) - II 25 3 5 - 10 20-40
Source: Ceballos et al. (2012)
25 1.11.2 Breeding methods
Breeding methods in cassava are essentially defined by the mode of its reproduction, genetic variability available and breeding objectives (Fukuda et al., 2002). Cassava presents sufficient segregation in the first generation after hybridisation because it is a highly heterozygous species (El-Sharkawy, 2012; Ceballos et al., 2012). Once a superior cassava hybrid has been identified in the first generation, its genotype is fixed by vegetative propagation, which is an advantage in breeding cassava (Fukuda et al., 2002; Grüneberg et al., 2009). The main disadvantages are the need to work with large populations, the difficulty in getting a precise estimation of the performance of the genotypes generated, and the low rate of vegetative propagation (Ceballos et al., 2004; 2012). Fukuda et al. (2002) indicated that there are no classic breeding methods developed for the vegetatively propagated crops and that normally the methods developed for self-pollinating crops are the ones applied to cassava, with some modifications because of cassava’s specific characteristics. The main breeding methods used in cassava cultivation are cultivar introduction and selection, intra- and inter- specific hybridisations and breeding of polyploids.
A. Cultivar introduction and selection
Cultivar introduction and selection are one of the key breeding methods used by most national cassava breeding programmes in Africa. The process involves recruiting genotypes from established cassava breeding programmes, like the International Center for Tropical Agriculture (CIAT) and International Institute for Tropical Agriculture (IITA), followed by field evaluation (Fukuda et al., 2002). Fukuda et al. (2002) indicated that this method is not only simplest and least expensive method, but also has greatest chance of success because of the wide genetic diversity exploited. Assessment and selection of the cultivars introduced involve formation of a study collection, followed by yield, pest and diseases evaluations and finally trials with producer participation in various localities and years (Fukuda et al., 2002).
B. Intraspecific hybridisation
Crossing among cassava parental genotypes of the same species, followed by selection among the progeny is the most common method used in cassava breeding (Fukuda et al., 2002; Jennings and Iglesias, 2002; Ceballos et al., 2012). The success of this method depends basically on correct parent choice and an efficient selection of genotypes within the progeny resulting from each cross. Parent selection is based on the phenotypic assessment of the genotypes and/or their general and specific combining abilities, estimated by the performance of the respective progeny. A large population should be used to obtain the
26 desirable recombinants. Since cassava genotypes are highly heterozygous for most of the gene loci, segregation occurs in the first generation. The F1 hybrids are first selected from within the segregating families (progeny). Then each selected individual is propagated vegetatively and the new genotypes assessed by yield trials (Fukuda et al., 2002; Jennings and Iglesias, 2002; Ceballos et al., 2004; Lebot, 2009).
C. Interspecific hybridisation
Successful crosses between cultivated cassava and its related wild species have been reported (Nichols, 1947; Jennings, 1957). Ceballos et al. (2012) reported that several traits of commercial importance have been found in wild relatives of cassava and that they could be introgressed into the cassava gene pool. They further found that among the most relevant traits are the tolerance to postharvest physiological deterioration (PPD) in M. walkerae, increased protein content in M. tristis and M. peruviana, resistance to the cassava green mite in M. esculenta sub spp. flavellifolia, and amylose-free starch in M. crassisepala and M.
chlorosticta. Blair et al. (2007) suspected that the resistance to cassava mosaic disease and the hornworm originated from in segregating progenies from crosses involving M. glaziovii as one of the progenitors. Nassar and Ortiz (2008) reported improved nutritional quality in wild relatives of cassava. However, Fukuda et al. (2002) recommended that although interspecific hybridisation in cassava has potential, it should only be done after completely understanding its merits and demerits and whenever the modifications of some characteristics of M.
esculenta are very necessary/desired.
D. Breeding of polyploids
This breeding method is based on the premise that polyploidy is associated with certain unique characteristics of the plant such as canopy vigour, including larger and thicker leaves and good leaf retention (Fukuda et al., 2002; Lebot, 2009). Leaves of polyploids are distinctly large even at the seedling stage. Their leaf stomata are generally larger and fewer per unit of the area of lamina. Also, their pollen grains are large (Lebot, 2009). Triploidy, as an effective tool in cassava improvement, especially for the production of high starch varieties for industrial use, was first realised in Kerala, India (Lebot, 2009). The triploids produced in India have been reported to be more vigorous than tetraploids, have stout stems, high leaf retention capacity, high percentage dry mass content (above 45%), and high starch content (Sreekumari et al., 2000) and high early bulking capacity (Suja et al., 2009). However, the method has been not been commonly used (Fukuda et al., 2002).
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