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Line x tester analysis was carried out in groundnut (Arachis hypogaea L.) to estimate the gca (general combining ability) effects of parents (6 lines and 4 testers) and the SCA (specific combining ability) effects of the crosses for pod yield and three other quantitative traits. Analysis of variance revealed highly significant differences among treatments, parents, parents vs. crosses and crosses for pod and haulm yield. The estimates of variance of specific combining ability effects, ratio of variance of general combining ability to specific combining ability and degree of dominance indicated preponderance of additive gene effects for each trait. On over all bases, role of testers in the expression of the traits was more than lines and line - tester interaction. However, lines contributed more than testers and lines x tester interaction for pod and haulm yield. Based on mean performance and GCA effects for yield and its various traits, three lines; Samnut 2, Samnut 23 and ICIAR19BT were identified as potential parents to be used for development of high yielding genotype. Hybrids like Samnut 23 X ICGV-IS 07820, Samnut 23 X ICGV-IS 07837 and Samnut 21 X ICGV-IS 07828 showed high mean performance, SCA effects for pod and haulm yield and are proposed for heterosis breeding.
Key: Line x tester, general and specific combining ability, heterosis breeding
Groundnut (Arachis hypogaea L.) is grown by small holder farmers under rain fed conditions with limited inputs. Asia and Africa account for a major portion of the world's groundnut production. Production is lower than expected in both Asia and Africa due to a number of biotic and abiotic factors that reduce yield that are often beyond the reach of resource poor farmers. Groundnut is the principal source of human dietary protein, oil,fat and vitamins such as thiamine, riboflavin and niacin in parts of Asia and Africa (Savage and Keenan 1994). Groundnut paste is an important source of calories for small children, particularly those being weaned. These children cannot obtain the calories they require from high bulk cereal grains and depend on groundnut for energy as well as vitamins. Groundnut cake and haulms (straw stems) are used as livestock feed and help to maintain livestock productivity. The crop also contributes up to 60 kg/ha nitrogen to the soil, benefiting crops subsequently planted in the same field (Sprent 1994).
Breeding strategies for developing hybrids with high yield potential and better grain quality require the expected level of heterosis and combining ability. In breeding for high yield crop plants, the breeders often face with the problem of selecting parents and crosses. Combining ability analysis is one of the effective approaches available for estimating the combining ability effects that help in selecting desirable parents and crosses for the exploitation of heterosis.
The study was under taken to determine the nature and magnitude of gene action for yield and yield components to explore the best combination of male and female. The information obtained thus will be used in selection of suitable parents and choice of appropriate breeding methods to develop high yielding groundnut variety.
Materials and Methods
Six lines of groundnut viz., Samnutn21, Samnutn22, Samnutn23, ICIAR 19BT, ICIAR 7B and ICIAR 6AT (female parents) and four testers ICGV-IS 07820, ICGV-IS07821, ICGV-IS 07828 and ICGV-IS02158 (male parents) were crossed in July, 2010 at the Institute for Agricultural Research, Ahmadu Bello University, Zaria. Necessary precautions were taken to avoid the contamination of genetic material at the time of crossing. Emasculation of the flower buds was done and sufficient hybrid seeds for each cross were produced through hand pollination. 24 F1 seeds along with their parents were planted in a 9 x 4 rectangular lattice design with three replications during 2011 raining season. The genotypes were assigned at random to experimental unit in each block and each row contained 20 plants. Inter-plant and inter-row spacing were maintained at 0.15 and 0.75 m, respectively. Two seeds per hole were sown. The experimental population was kept under normal agronomic care from sowing to maturity following IAR recommendations. At maturity, data were recorded on agronomic traits as plant height, days to maturity, pod yield (ton -1 ha), and haulm yield (ton -1ha). The data for all the traits were analyzed using an Interactive SAS Macro Program for Line - Tester analysis described by Bartolome and Gregorio, 2000 in order to examine the combining ability of the parental lines. The sum of squares for hybrids was further partitioned into variation due to lines, testers and lines-testers interactions. The mean squares due to lines and testers were tested against the mean squares due to line- tester, and the latter were tested against the pooled error. Standard errors for GCA effects of females (lines) and males (testers) and the SCA effects were also estimated. Two-tailed t-tests were used to test the significance of the GCA and SCA effects. Estimate of GCA variances () and SCA variances () were obtained as suggested by Singh and Chaudhary (1985). Ratios of mean square components associated with variance of GCA and SCA effects were computed as suggested by Baker (1978) to estimate the relative importance of GCA in explaining performance. The closer the ratio is to unity, the greater the predictability of progeny performance based on GCA effects alone. Statistical model for the combined analysis of parents and crosses as suggested by Arunachalam (1985) as:
where, = value of the observation recorded on the ( cross in the replication; is the general effect; is the effect of the line; is the effect of the tester; is the specific combining ability (sca) effect of the cross; is the block effect and is the environmental effect associated with the observation which is assumed to be normally and independently distributed .
Results and Discussion
The analysis of variance involving ten parental lines and twenty four crosses for four characters (Table 1) revealed highly significant differences among parents and the crosses. The mean squares due to parents, crosses, and line x tester interactions were significant for pod and haulm yield. The significant component of variance due to parents vs. crosses indicated prevalence of heterosis for these characters. The mean squares due to lines and testers were also highly significant for pod and haulm yield indicating the importance of additive gene effects as the main cause of the observed genetic variation for these traits. Also, the mean squares due to line x tester was significant for these characters indicating that both additive and dominance or non-additive variances were important for those characters. Both additive and non-additive gene actions appeared to play a significant role in controlling the expression of these traits, but nonadditive gene action seemed to be more important. These types of gene action appear to be playing a considerable role in groundnut breeding.
The importance of the source of variation is indicated by the relative magnitude of variance components. The variance component estimates of SCA were greater than that of GCA for plant height, pod yield and haulm yield (Table 1). In addition, the ratio of the mean square components associated with variance of GCA and SCA was much less than the theoretical maximum of unity for all traits except maturity time. These results tend to suggest that genetic variation among crosses was primarily of the non-additive type. In addition, the highly significant variance due to parent versus hybrid, which is a measure of average heterosis, also points to the importance of non-additive genetic effect in determining these traits
The selection of parental lines for hybrid programs was one of the objectives of this study. Thus, the estimates of the general combining ability of a parent provide important indicators of its potentials for generating superior lines. A low gi estimate, whether positive or negative, indicates that the mean of a parent in crossing with the other, does not differ greatly from the general mean of the crosses. On the other hand, a high estimate indicates that the parental mean is superior or inferior to the general mean. This gives information about the concentration of predominant genes with additive effects (Cruz and Regazzi, 1994).
Estimates of GCA effects for plant height, maturity time pod and haulm yield for the nine parents used in this study are presented in Table 2. The female Samnut 23 and ICIAR19BT were the best general combiner for maturity time, pod and haulm yield with highly significant and positive GCA effects. On the other hand, the female Samnut 21 showed significant and negative GCA effects for pod and haulm yield. The female ICIAR6AT showed no significant effect for the trait studied.
Among the male parents, the testers ICGV-IS07828 is the best combiner for pod and haulm yield with significant GCA effects. Since, high effect is attributed to additive and additive x additive type of gene actions, these parents could be used in breeding programme for yield improvement through pedigree breeding. The tester ICGV-IS07821 had negative effect for both pod and haulm yield, its breeding value for hybrid production was rather low. Considering the material used in this study, parents that showed negative GCA for both grain yield and seed weight probably should not be hybridized inter se, since genetic gain would not be expected for these traits in these materials, due to insufficient additive genetic variances.
The estimates of provide important information about the hybrid performance as related to its parents, showing the importance of non-additive interaction due to large or minor gene effects in particular hybrid combination. The estimates of SCA effects of selected crosses are presented in Table 3. The magnitude of SCA estimates varied among crosses. High positive SCA estimates for grain yield were obtained from the following crosses Samnut 23 X ICGV-IS 07820, Samnut 21 X ICGV-IS 07828 and ICIAR7B X ICGV-IS 07821 It was observed that large positive SCA effect for pod and haulm yield was obtained by the cross Samnut 21 X ICGV-IS 07828 and at least one parent of this mating had positive GCA effect for this trait.
The crosses that recorded high SCA effects, coupled with high per se performance for yield involved either one or both of the parents with good GCA for the trait being considered. The parents that were the best general combiners did not always produce the best hybrid combinations. This may have been expected because of lack of higher order additive interactions. This difficulty in predicting the productivity level of the hybrid, on the basis of GCA alone should necessitate testing of specific male-female combination. However, in all high yielding hybrids at least a good general combiner was involved. According to Marilia et al. (2001), the SCA effect alone has limited value for parental choice in breeding programs. The SCA effects should be used in combination with other parameters, such as hybrid means and the GCA of the respective parents. Thus, hybrid combination with high mean, with favorable SCA estimate and involving at least one of the parents with high GCA, would tend to increase the concentration of favorable alleles; an appreciable situation to any breeder.
These crosses also showed significant heterosis for yield and its contributing characters. It was observed that crosses involving, one high and the lower low, medium or high general combiner parents indicating additive as well as non additive genetic interactions are operating in these crosses. The best combiners for more than one trait could be utilized in further breeding programme. The hybrids Samnut 23 X ICGV-IS 07820 and Samnut 21 X ICGV-IS 07828 could be used for the exploitation of heterosis for yield and related characters. Biparental mating followed by selection might be worthwhile for fostering greater recombination in these crosses (Mothilal and Ezhil, 2010).