“Epistasis and its implications in Plant Breeding”

   Advances in the field of science have been more progressive wherever in the beginning only simple issues were investigated to establish basic principles which later on acclaimed general applicability to a wide range of complex situations. Genetics is not an exception to such an array of sequential developments. It was only for Gregor Johann Mendel to have envisaged the existence of factors, now termed as genes, completely independent in their transmission and action to produce easily classifiable differences among the living organisms. Initially, Mendel and other workers assumed that characters are governed by single genes but later it was discovered that many characters are governed by two or more genes and these genes interact with each other in various ways in development of a single character referred as gene interaction.

Epistasis has various implications in plant breeding. Numerous examples are available in different crop species where non-allelic interactions have contributed to the improvement of specific characters. One such example is the evolution and further improvement of fiber quality characters in new world cotton1. Epistasis reduces the classical F2 phenotypic ratio and assist in further breeding programmes by aiding in selection of parents2, it has attained considerable importance in the development of hybrid varieties of crop plants.

Various biometrical and molecular marker-based methods have been developed to estimate the epistasis. A simulated data set was used to estimate the influence of epistasis in heterosis and combining ability analyses of the populations by measuring the correlations between the average frequency for the genes that increase the trait expression of a parametric (true) variety and general combining ability (GCA) effects and found that if only a minor fraction of the genes are epistatic or if the magnitude of the epistatic effects are of reduced magnitude, the epistasis will not have any impact on the heterosis and combining ability3. The epistatic and environmental interaction quantitative trait loci (QTL) for leaf orientation-related traits in maize have indicated the importance of QTL×environmental interaction in determining the phenotypes of quantitative traits4.

Though the deoxy ribonucleic acid (DNA) marker technologies have provided the best method for measuring epistatic effects on quantitative traits, DNA markers or other genomic technologies may be required to better exploit epistasis for crop improvement. Currently, marker assisted selection methods treat QTLs as building blocks that maintain their effects in isolation or in groups. If we can reliably identify epistatic effects with markers, they shall aid in selection of multi-locus genotypes rather than specific QTL alleles in an additive fashion.

References:

1. CHAHAL, G. S., SINGH, T. H., AND VIRK, D. S., 1991, Measurement and utilization of non-allelic interactions in plant breeding, 59-63. 

2. HOLLAND, J. B., 2001, Epistasis and plant breeding. Plant Breed. Rev., 21: 27-92.

 3. SHI, Y., WANG, X., GUO, S., REN, Z., KU, L., ZHU, Y., AND CHEN, Y., 2017, Detection of epistatic and environmental interaction QTL s for leaf orientation‐related traits in maize. Plant Breed., 136(1):33-40. 

4. VIANA, J. M. S., 2023, The impact of epistasis in the heterosis and combining ability analyses. Front. Plant Scie., 14:1168419.