The inheritance of resistance to Alternaria blight in sweetpotato is not very widely studied as more priority has often been given to SPVD, which so far has been the most devastating sweetpotato disease in East Africa (Gibson et al., 1998). However, Alternaria blight has gained importance in most of the sweetpotato producing areas. From previous studies, it is clear that both the landraces and improved cultivars have varying levels of resistance to Alternaria blight.
It is, however, not clear, if this resistance is durable or non-durable. Efforts are also underway at the Ugandan National Sweetpotato Program to screen the germplasm and breeding populations for resistance to the disease. Quantitative approaches have been previously used to study different sweetpotato diseases. For example, quantitative inheritance studies of resistance to Fusarium wilt disease were carried out by Jones (1969). He observed that the entire variance for resistance to Fusarium wilt was accounted for by the additive component and that heritability was high. These results were further confirmed by Collins (1977) based on a diallel analysis of sweetpotato resistance to Fusarium wilt. There is limited information in the available literature about similar studies on Alternaria blight of sweetpotato and thus its mode of inheritance is not known. However, genetic studies have been carried out on two related diseases, Alternaria leaf
blight (Alternaria dauci (Kühn) Groves and Skolko) of carrot by Simon and Strandberg (1998), and Alternaria early blight (Alternaria solani Sorauer) of diploid potato (Solanum tuberosum L.) by Christ and Haynes (2001). Simon and Strandberg (1998) used the diallel and observed highly significant genotypic differences for resistance to Alternaria blight in carrot with both general combining ability (GCA) and specific combining ability (SCA) effects contributing significantly to the variation but the GCA sum of squares (SS) were 2.2 times greater than the SCA SS. Thus, the additive variance was more important than the non-additive variance. Christ and Haynes (2001) reported narrow-sense heritability of 61% for resistance to early blight in potato indicating that additive genetic variance predominates.
1.8.2 The polycross mating design
Controlled crossing methods based on for example, diallel (Griffing, 1956) and North-Carolina II factorial (Comstock and Robinson, 1952) mating designs are very reliable for identifying superior parents and good cross-combinations. However, their use in sweetpotato and potato breeding is difficult, labour intensive and time consuming. In these methods, a set of crosses are required to be made in which selected female parents are crossed with selected male parents in a specific pattern based on the design (Gopal, 1994). With incompatibility and sterility in sweetpotato coupled with poor seed set, obtaining the required cross-combinations is usually very difficult. Therefore, controlled crossing can be avoided by exploiting random, open- pollination in a polycross design (Jones and Dukes, 1980; Stuber, 1980; Jones, 1986). A polycross is the natural inter-crossing of a group of plants in an isolated crossing block (Stuber, 1980; Nyquist and Santini, 2007). Jones (1986) recommended that a limited number of parents (not more than 30) should be used to establish a polycross and left to be randomly crossed by naturally occurring insects, usually honey bees. The parents in a polycross are arranged in such a way so as to provide an equal opportunity for each to cross with each and every other parent (Stuber, 1980; Nyquist and Santini, 2007). For a polycross arrangement to be perfect, each parent should have every other parent as the nearest neighbour once in all four compass directions i.e. south, north, east and west (Olesen and Olesen, 1973). Wright (1965) outlined a total of 12 field plans for systematically designed polycross arrangements starting with a 6 x 6 to a 46 x 46 genotype layout in which he clearly demonstrated the nearest neighbour principle. In all these arrangements, an important aspect of a polycross that can determine its success or failure is synchronisation of flowering. This may necessitate staggered planting of the parents so that they all bloom at the same time (Stuber, 1980). In addition, Tumana and Kesavan (1987) emphasised the need for self-incompatibility and cross-compatibility among parents if the polycross system of mating is to be effective. In this design, only the female parent of each family is known and the progeny are half-sibs (Stuber, 1980) and only the GCA effects can be generated (Olesen and Olesen, 1973; Saladago, 1989).
1.8.3 North Carolina mating designs
Comstock and Robinson (1952) suggested three mating designs, North Carolina mating design I, II and III (or simply Design I, II and III), and described their statistical analyses to study gene action affecting quantitative traits. In the North Carolina I mating design or hierarchical design, the non-common parents are divided into sets. Each set is mated to one common parent, which is the common parent for the progeny from that set. That is, each member of a group of parents used as males is mated to a different group of parents used as females and no female is involved in more than one mating with the pollen parents (Dabholkar, 1992). This design is useful in generating and evaluating half-sib and full-sib families for recurrent selection and also estimating additive and dominance variances (Acquaah, 2009).
North Carolina II design is a factorial mating design where each member of a group of parents used as males is mated to each member of another group of parents used as females. It is useful in estimating genetic variance and combining ability as well degree of dominance (Stuber, 1980). This method is more applicable to plants that produce multiple flowers and each plant can be used repeatedly both as a female and male (Stuber, 1980; Lynch and Walsh, 1998; Acquaah, 2009). Every male is mated to each female following a two-way analysis of variance, in which the variation can be partitioned into differences between males (σ2m) and females (σ2f) and the interaction between them (σ2m x f) (Hill et al., 1998; Acquaah, 2009).
Table 1.1: ANOVA for the North Carolina II design repeated over environments
Source df Mean squares E(MS)
Environments e-1
Replications /E e(r-1)
Males (GCA) (m-1) M7 σ2 + rσ2mfe + rfσ2me+ reσ2mf + refσ2m
Females (GCA) (f-1) M6 σ2 + rσ2mfe + rmσ2fe + reσ2mf + remσ2f
Males x females (SCA) (m-1)(f-1) M5 σ2 + rσmfe + reσ2mf
Males x E (m-1)(e-1) M4 σ2 + rσ2mfe + rfσ2me
Females x E (f-1)(e-1) M3 σ2 + rσ2mfe + rmσ2fe
Males x females x E (m-1)(f-1)(e-1) M2 σ2 + rσ2mfe
Pooled error e(r-1)(mf-1) M1 σ2
Total ermf-1
Where: σ2 = Variance within full-sibs = environmental variance; σ2m = Variation between males = GCAm
variance; σ2f = Variation between females = GCAf variance; σ2mf = Variation due to interaction between males and female = SCA variation; σ2me = Variation due to interaction between males and the environment; σ2fe = Variation due to interaction between females and the environment; σ2mfe = Variation due to SCA interaction with the environment.
Source: Hallauer and Miranda (1988)
In the North Carolina II design, the mean square (MS) for males and MS for females provide direct estimates of the GCA for males and GCA for females, respectively. The male x female interaction MS estimates the SCA (Hallauer and Miranda, 1988).
North Carolina III was developed by Comstock and Robinson (1948). In this design, two parents (s and m) are hybridised to produce the hybrids (sm) which becomes the reference population.
Random selection from these hybrids is done and those that are selected are backcrossed to the two parents. At this level, the two parents become the females (seed parents) and the selected hybrids are the males (pollen parents). This generates a new population 2sm. The 2sm progeny families are divided into n sets for field planting. Each set comprises of p pairs of progeny families. In this design, members of each pair have the male parent in common but the female parents are different. The female parents are fixed while the male parents are randomly selected from the sm. Thus, the effects of the females are regarded as fixed (Dabholkar, 1992).
The advantages of this design are that it estimates: the average level of dominance of genes affecting the evaluated traits; the additive and dominance variance for sm population assuming no linkage and epistasis; and heritability of the traits evaluated (Hallauer and Miranda, 1988).
North Carolina mating designs I, II and III provide plant breeders with information regarding the inheritance of traits being investigated for a reference population. This knowledge allows plant breeders to determine whether selection aimed at cultivar development will be feasible from this source population and what breeding method could be the best for such a goal (Ortiz and Golmirzaie, 2002).