Successful exploitation of heterosis has been instrumental in quantum jump in productivity of maize, cotton and many other crops. The genetic basis of heterosis is given as HF1 = dy2 highlights the importance of enhancing genetic diversity for maximizing the heterosis. Development of hybrid oriented heterotic populations (genetically diverse populations) and application of schemes for improving combining ability is an integral part of hybrid breeding. In this regard, the concept of heterotic grouping includes the subdivision of the germplasm available into two divergent populations. The heterotic groups (hybrid oriented populations) so formed can be improved by inter population selection methods (reciprocal recurrent selection scheme).
Heterotic group refers to a group of related or unrelated genotypes from the same or different populations, which display similar combining ability and heterotic response when crossed with other genetically distinct germplasm group. Heterotic pattern refers to a specific pair of two heterotic groups, which express high heterosis and consequently high hybrid performance in their crosses (Melchinger and Gumber, 1998).
Avinash et al. (2017) classified sixty four maize inbred lines into heterotic groups using specific combining ability effects for yield. The combining ability effects and mean grain yield of the inbred lines when crossed to CML 474 and V 373 testers were used as the bases in classifying the lines into heterotic groups. The lines with positive high gca and high per-se performance can be used to develop pools. Such a pool will have accumulated desirable genes in high frequency. The lines with negative gca are discarded. The lines exhibiting contrasting specific combining ability effects (sca) with two testers were placed into separate heterotic groups. Inbred lines showing positive sca effect with CML 474 tester (A) but having negative sca effects with V 373 tester (B) were placed into the heterotic A group. Similarly, inbred lines displaying positive sca effect with V 373 tester B but having negative sca effects with CML474 tester A were put into the heterotic B group.
Badu-Apraku et al. (2015) classified inbreds into heterotic groups using the sca effects of grain yield, heterotic group specific and general combining ability (HSGCA), heterotic grouping based on general combining ability of multiple traits (HGCAMT) and the molecular based genetic distance methods. The breeding efficiency of each of these heterotic grouping methods was compared. The procedure for comparison of breeding efficiency consisted of dividing the total number of hybrids into two major group’s i.e. inter-group and intra-group crosses. Classification of inbreds into heterotic groups based on sca effects of grain yield is influenced by interaction between two inbred lines and the interaction between the hybrid and environment. This leads to classification of same inbreds into different heterotic groups in different studies. To rectify this HSGCA method has been proposed. Both HSGCA and SCA-GY are primarily based on one trait i.e. grain yield. Grain yield has low heritability under stress. HGCAMT use multiple traits of inbreds with significant gca effects across contrasting environments. The best classification method is the one in which inter group crosses produce superior hybrids than within group crosses and breeding efficiency may be defined as percentage of superior high yielding hybrids obtained across the total number of inter heterotic crosses.
Ranganatha et al. (2013) developed heterotic groups in cotton based on the genetic diversity in the form of visible differences in plant type traits viz., robust (taller plants) and compact (shorter plants) and reported that crosses involving between group genotypes (inter plant type) are highly heterotic when compared to crosses involving within group crosses (intra plant type). Patil et al. (2011) employed a modified method of reciprocal recurrent to improve combining ability of inbreds that suits the mating system of often cross pollinated. Two diverse F1’s were selected as base populations instead of random mating populations. Two diverse F1’s were selected based on predicted double cross performance were advanced to F4 generation and subjected to reciprocal selection for combining ability against these two crosses used as reciprocal testers.
References:
Avinash, S., Manivannan, A., Bilal, A., Ekta, S. and Vinay, M., 2017, Heterotic grouping in early maturing Indian maize lines. Intl. J. of Agri. Innov. and Res., 6(1):2319-1473.
Badu-Aprakua, B., Annora, B., Oyekunlec, M., Akinwaleb, R. O., Fakoredeb, M. A. B., Talabia, A.O., Akaogua, I. C., Melakua, G. and Fasanmadeaa, Y., 2015, Grouping of early maturing quality protein maize inbreds based on SNP markers and combining ability under multiple environments. Field Crops Research, 183:169–183.
Melchinger, A. E. and Gumber, R. K., 1998, Overview of heterosis and heterotic groups in agronomic crops. In Lamkey K. R. and Staub J. E. eds. Concepts and breeding of heterosis in crop plants. CSSA, Madison, WI, pp. 29-44.
Patil, S. S., Ramakrishna, V., Manjula, S. M., Swati, P., Ranganatha, H. M., Kencharaddi, H. C. and Deepakbabu, H., 2011, Deploying reciprocal selection for combining ability for improving performance of hybrids in cotton (Gossypium hirsutum). Indian J. Genet., 71(2):180-184.
Ranganatha, H. M., Patil, S. S., Swathi, P., Rajeev, S., Srivalli, P. and Venkatesh, M. K., 2013, Development of heterotic pairs or groups of cotton genotypes based on predicted double cross performance. Intl J. Agri. Crop Sci., 6(5):231-235.
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