Heterotic group (HG), also called as heterotic pool is defined as “a group of related or unrelated genotypes (inbred lines or populations) from the same or different populations, which display similar combining ability and heterotic response when crossed with genotypes from complementary and genetically distinct germplasm groups”. By comparison, heterotic pattern refers to “a specific pair of two HGs, which express high heterosis and consequently high hybrid performance in their cross”4.

Heterotic grouping help enhance the efficiency of hybrid breeding by (i) avoiding the development and evaluation of unnecessary hybrids thus saving time and resources of the researcher, (ii) increasing gca variance, and hence making early generation test cross evaluation and selection highly effective and (iii) streamlining the hybrid breeding programme by dictating, which parents within a HG to cross to develop breeding population (BP) and which parent from opposite to use as tester to evaluate experimental hybrids derived from BPs. The hybrids resulting from crossing inbred lines belonging to different HGs exhibit greater magnitude of heterosis1.

Different methods of heterotic grouping have been proposed. These are (i) pedigree analysis, (ii) phenotypic clustering, (iii) combined ability and (iv) marker based genetic distance. The advantages and disadvantages of different methods of heterotic grouping and for their relative efficiencies are discussed using examples2,3 in this seminar. The criteria to identify the best tester, which is a prerequisite for classification of breeding lines into HGs based on combining ability are also discussed in this seminar.

The process of heterotic grouping is the continuous process, in which newly developed inbreds are merged to enrich existing HGs or constitute a new independent HG. The individuals of HGs need to be further and continuously improved by reciprocal recurrent selection (RRS). RRS improves the genetic divergence between HGs which in turn leads to maintenance of heterotic pattern and high frequency of heterotic hybrids.

References

1. ASLAM, M. AND ZAFAR, S. A., 2021, Heterotic group theory: A thriving vitality in hybrid maize breeding. J. Agric. Basic Sci., 5(1): 45-61

2. HUSSAIN, I., ALI, S., LIU, W., AWAIS, M., LI, J., LIAO, Y., ZHU, M., FU, C., LIU, D. AND WANG, F., 2022, Identification of heterotic groups and patterns based on genotypic and phenotypic characteristics among rice accessions of diverse origins. Front. Genet., 13: 811124.

3. KUMAR, P., LONGMEI, N., JAT, B. S., CHOUDHARY, M., YATHISH, K. R., BHUSHAN, B., GOYAL, M. AND RAKSHIT, S., 2022, Heterotic grouping of Indian baby corn lines based on combining ability. Indian J. Genet. Plant Breed., 82(2): 161-166.

4. MELCHINGER, A.E. AND GUMBER, R.K., 1998, Overview of heterosis and heterotic groups in agronomic crops. Concepts and breeding of heterosis in crop plants, 25: 29-44.