Concept of combining ability

IN detailed explanation about combining ability, SCA, GCA, formulas to derive GCA and SCA, significance, examples to explain, case studies.

1. CONCEPT OF COMBINING ABILITY Submitted to Dr. T. Haritha Scientist Department of genetics and plant breeding Submitted by S. Ravi teja BAM 20-23

2. Introduction: Combining ability in crosses is defined as the ability of parents to combine amongst each other during the process of fertilization so that favorable genes or characters are transmitted to their progenies. Two types of combining ability, general (GCA) and specific (SCA), have been recognized in quantitative genetics. The concepts of general combining ability (GCA) and specific combining ability (SCA) were first introduced by in relation to corn breeding and have been expanded into animal breeding by and others

3. The term GCA is used to designate the average performance of an inbred line in hybrid combinations. SCA is used to designate those cases in which certain combinations do relatively better or worse than would be expected on the basis of the average performance of the lines involved. According to GCA is due to genes which are largely additive in their effects and SCA is due to genes with dominance or epistatic effect. Defined by sprauge and tatum 1942

4. Concept of combining ability Crossing a line to several others provides an additional measure of the line, i.e. the mean performance of the line in all its crosses. This mean performance, when expressed as a deviation from the mean of all crosses, is called the general combining ability of the line. It is the average value of all F 1 's having this line as one parent, the value being expressed as a deviation from the overall mean of crosses. Any particular cross, then, has an “expected” value which is the sum of the general combining abilities of its two parental lines. The cross may, however, deviate from this expected value to a greater or lesser extent. This deviation is called the specific combining ability of the two lines in combination. In statistical terms, the general combining abilities are main effects and the specific combining ability is an interaction. The true mean X of a cross between lines P and Q can thus be expressed as

5. The true mean X of a cross between lines P and Q can thus be expressed as X - x̅ = GCA P + GCA Q + SCA PQ Where x̅ is the mean of all crosses, and GCA and SCA are the general and specific combining abilities respectively. In practice another term, E, must be added to the right-hand side to represent sampling error in estimating X. The terms on the right-hand side of equation are uncorrelated with each other, so the total between-cross variance (excluding error variance) is made up as follows: The two components into which the total between-cross variance can be partitioned are the variance of general combining ability and the variance of specific combining ability. These are observational components of variance in the sense and are estimated from an analysis of variance. Their importance lies in the fact that the causal components of genetic variance contribute to them differently, as we shall now see..

6. From this it can be seen that differences of general combining ability are due to the additive variance and A x A interactions in the base population; and differences of specific combining ability are attributable to the non-additive genetic variance. Consequently the variance of general combining ability increases linearly with F (apart from the interaction component), while the variance of specific combining ability increases with higher powers of F. It is therefore the specific, and not the general, combining ability that is expected to increase in variance more rapidly as the inbreeding reaches high levels.

7. The components genetic variance in equation [15.8] are those of a random breeding population with all gene frequencies equal to those in the lines crossed and with coupling and repulsion linkages in equilibrium. This random-breeding population can be regarded as being the base population, real or hypothetical, from which the lines were derived without selection. Or, alternatively, it can be regarded as a synthetic population made by random mating among the crosses and then bred by random mating for long enough to reach linkage equilibrium. Which of these viewpoints is to be adopted affects the details of the analysis of variance. In the first case the lines are regarded as a sample of the population and are therefore random factors: in the second case the lines are the whole population and are fixed factors.

8. Feature of General Combining Ability It is due to additive genetic variance and additive x additive gene interaction. It denotes combining ability of genotype esp. inbred with various testers. Helps in identification and selection of best genotype to use it in hybridization, as a parent. Estimated by half sib mating Have relationship with narrow sense heritability.

9. Procedure for estimating and utilizing General Combining Ability (GCA): 1) First Year: Superior plants for the character under improvement are from the base population. The selected plants are selfed and also crosses heterozygous tester having broad genetic base. The selfed seed is kept storage. 2) Second Year: The crossed seed is sown and the combining ability of selected plants is evaluated and plants with good GCA are identified.

10. Cont.. 3) Third Year: The progeny of selected plants with good GCA are grown from their selfed seed kept in cold storage. These progenies are inter mated in all possible combinations and their crossed seed is composited to form a new source of population for further selection. This complete original selection cycle. Many such cycles may be made to obtain desired results. Main features of this method to use for genetic improvement of quantitative characters in which selection is made or the basis of test crosses performance. This method also used for providing good of population and required three season or year for completion of each cycle.

11. Features of Specific Combining Ability It represents deviation from gca . It is due to dominance genetic variance and all the three types of gene interactions. Helps into identification and hence selection of best cross combinations i.e. those with the desired output. When we see that a inbred line combines well in any cross, it is due to specific combining ability. Estimated by full sib mating. Have relationship with heterosis.

12. Procedure for estimating and utilizing specific combining activity(SCA) 1) First Year: Several plants are selected and selfed . The selected plant ( male) are also crossed to an tester( female). 2) Second Year: R.Y.T is conducted using test cross progeny. Outstanding progenies are identified. 3) Third Year: Selfed seed from the progeny are planted in separate progeny row in a crossing block. All possible inter cross are made by hand among progeny. Equal amount of seed is composited, this complete the original selection cycle

13. Cont.., 4) Fourth Year: The combination seed is planted and operation of 1st year repeated. 5) Fifth Year: Second year operation repeated. 6) Sixth Year: Third year operation repeated.

14. ESTIMATION OF COMBINING ABILITIES A method of estimating general combining abilities that is convenient for use with plants is known as the polycross method. A number of plants from all the lines to be tested are grown together and allowed to pollinate naturally, self-pollination being prevented by the natural mechanism for cross-pollination, or by the arrangement of the plants in the plot. The seed from the plants of one line are therefore a mixture of random crosses with other lines, and their performance when grown tests the general combining ability of that line. The general combining abilities measured are those of lines used as female parents.

15. If the variances of general combining ability are assumed to be the same for male and female parents, the variances of general and specific combining ability can be estimated and interpreted as in equation [15.8]. The general combining ability of a line can be estimated by crossing it with individuals from the base population instead of with other inbred lines. This method is known as top-crossing. It is equivalent to crossing with a random set of lines inbred from the base population without selection because, as noted earlier, the gametes from inbred lines are not different in genetic content from those of the base population. A commonly used experimental design for crossing inbred lines is the diallel cross, in which each line is crossed with every other line. The analysis of a diallel cross for the purpose of estimating variances is complicated because it depends on whether reciprocal crosses are included, and on the assumption made about the population to which the genetic components of variance refer.

16. PROBLEM

17. CASE STUDIES Analysis of Combining Ability for Early Maturity and Yield in Rice (Genus: Oryza) at the Kenyan Coast. Al-Imran Dianga ,Kamau W. Joseph and Ruth N. Musila

18. Combining ability analysis is one of the most valuable tools used to ascertain gene action effects and help in selecting desirable parents for making crosses and coming up with high yielding and early maturing lines. Combining ability for early maturity and yield has not been studied at the Kenyan coast. (is study aimed at determining and identifying good, general, and specific combiners for selecting better parents and better cross combinations in rice crops for developing high yield and short duration lines in rain-fed rice farming. Seven lines were subjected to half-diallel mating design at the Kenya Agricultural and Livestock Research Organization (KALRO), Mtwapa , and at Bahari in Kilifi town. Evaluation for general combining ability (GCA) and specific combining ability (SCA) analysis was done. Combining ability variance and GCA and SCA effects were determined. Based on GCA effects, best parent for early maturity was Dourado Precoce , while for yield, Supaa , Komboka , and NERICA 10. SCA estimates indicated that best crosses for yield were D/S, D/N1, and K/N10, while the best performing cross for early maturity was D/N1.

19. Materials and Methods:- Seven rice lines ( Supaa , Komboka , Dourado Precoce , NERICA 1, NERICA 2, NERICA 4, and NERICA 10) were planted following half-diallel mating design during the long rain season of 2017 at KALRO, Mtwapa . Crosses were done in all possible combinations between each two of the seven parents without reciprocals to produce 21 F1 hybrids. The general combining ability (GCA) and specific combining ability (SCA) for yield and maturity among the seven lines and crosses were estimated. The F1 crosses were advanced to F2. In 2018 long rain season, the F2 crosses together with the parent lines were evaluated at KALRO, Mtwapa , and Bahari in Kilifi. Seeds of the twenty-one F2 hybrids and the seven parents were sown in a randomized complete block design (RCBD) with three replications. Each genotype was planted in a plot of 1.4 m by 1 m replicate with 20 cm × 20 cm spacing within the row and 20 cm between the rows. Two seedlings were sown per hill that was later thinned to one.

20. Results and Discussion General Combining Ability (GCA) Effects. High positive GCA effects values would be of great interest in all characters except plant height, days to 50% flowering, unfertile spikelets , and unproductive tillers for which negative values become more useful from the breeder’s point of view. Most parents showed positive GCA effects for days to 50% flowering except Dourado Precoce that showed significant negative GCA effects emerging as the best combiner (Table 3). Supaa was the poorest combiner in terms of days to 50% flowering because of the positive significant GCA effects. Significant GCA effects for grain yield were recorded for Supaa , Komboka , and NERICA 1. Komboka and Supaa were found to be best combiner parents for yield, while NERICA 1 was the poorest parent combiner with significant negative GCA effects.

21. Specific Combining Ability (SCA) Effects. A significant deviation from zero in SCA of across would indicate high or low specific combining ability (SCA) depending on weather the sign is positive or negative. Significant negative SCA is desirable for all traits under study except plant height, days to 50% flowering, unfertile spikelet's, and unproductive tillers. The positive SCA effects for days to 50% flowering were significant for S/K, N1/S, and K/N10 crosses, thus emerging as the poorest performing crosses based on SCA effects (Table 4), while cross D/N1 with significant positive SCA effects was the best performer. The best yielding crosses were D/S and N10/S crosses with high significant negative SCA effects, while N1/N2 was the poorest combiner with significant positive SCA effects. Plants from K/N10 and D/S crosses were the best crosses in terms of plant height as they had positive significant SCA effects. N10/K and K/N2 crosses performed better in relation to unproductive tillers, while N1/N2 cross performed poorly as it had significant negative SCA effects. Performance based on panicle length indicated that K/N4 and D/N10 were the best crosses, while K/N2 was the poorest cross. Panicles from K/N2 cross were the best in terms of SCA effects, as K/N2, N1/N2, and N10/S crosses were best for unfertile spikelet's. For 1000 grain weight, N10/N2 cross was the best, as K/S, N1/S, and D/S crosses emerged as the best crosses in terms of biomass. Results from this study reveal that SCA effects and cross performance might closely relate.

22. Combining ability analysis for yield and yield components in basmati rice Aditya Kumar 1 , Surendra Singh 2 and Santosh Kumar Magadum 2 1 Institute of Forest Productivity, Lalgutwa , Ranchi-835303, Jharkhand, India. 2 Gobind Ballabh Pant University of Agriculture and Technology, Pantnagar - 263145, Uttarakhand, India.

23. MATERIALS AND METHODS The material comprised of 8 basmati rice varieties namely Pant Sugandh Dhan 15, Basmati 370, Type 3, Pant Sugandh Dhan 17, Pusa Basmati 1, Pusa Sugandh 4, UPR 2845-6-3-1, UPR 3003-11-1-1 were crossed in half diallel fashion during wet season 2006. Next year during wet season 2007, 36 entries (28 crosses and 8 parents) were grown in a randomised block design with two replications at N.E. Borlaug Crop Research Centre of G. B. Pant University of Agriculture and Technology, Pantnagar (Uttarakhand).

24. The genotype UPR 2845-6-3-1 was found to be a good general combiner for DF, PH, FL, PL, GP, GY, BY, HI, KL, KB and AC (Table 3). The crosses Pant Sugandh Dhan 15/Pant ugandh Dhan 17, Pant Sugandh Dhan 15/ Pusa Basmati 1, Pant Sugandh Dhan 15/UPR 2845-6-3-1, Pant Sugandh Dhan 15/UPR 3003-11-1-1, Pant Sugandh Dhan 17/UPR 284-6-3-1, Pant Sugandh Dhan 17/UPR 3003-11-1-1 and UPR 2845-6-3-1/UPR 3003-11-1-1 for GY showing high sca effects were in the category of high x high general combiner cross combinations.

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