Targeted Recombination

 

 

 

   Evolution of concepts in plant genetics and breeding has eased in true-to-type choosing apparently s  uperior lines. Despite advances in DNA marker based selection, plant breeders have to rely upon random meiotic events. For the plethora of modus operandi in creation of variations, the need for generating larger population is inevitable to realize desirable products merely because it is probabilistic. Targeted recombination is ability to induce or select for specific recombination points so as to maximize genetic gains. In principle, it uses any of the available tools for site directed induction of double stranded breaks (or DSB) on one of the homologues. Preferably, when DSBs are repaired by homologous recombination machinery in the somatic cells, induces recombination.  In meiotic cells, though this event naturally occurs, it is required to scale up the population size to be raised to recover desired segregants.¹ Alternatively, targeted recombination can be relied upon, to direct the recombination to desirable centi-morgans.

            Despite several reports on directed mutagenesis using UV to induce double stranded breaks, advent of CRISPR-Cas9 system has added unambiguity to the site of excision. Employing site directed double strand breaks, Manganese sensitivity loci in Yeast was fine mapped to single polymorphism.³ While reducing the colonies to be screened, the study could also demonstrate increased confidence level of localizing the region on the genome. This principle was extrapolated to field crops by Rex Bernardo, who observed that the relative efficiency of targeted recombinations can theoretically be 212 per cent over natural recombinations, for even complex traits like grain yield in Maize. Similar results were published in Soybean, Wheat, Pea and Barley where targeted recombination in less than one third of all the chromosomes have led to same or higher predicted gains² than natural ones.

            To date, targeted recombination has only been conceptualized in theory. While, bringing into play requires fine tuning of the methodologies to fit into practical outfits. On the flip side are, population specificity of the targets. Further, desirable recombination for a given trait could turn out to be undesirable for another. Under such circumstances, selection indices are to be preferred to prioritize the target points of recombination. Potentially, the use of this concept can be felt in marker assisted backcross breeding to considerably reduce linkage drag and population size to be screened. In summary, targeted recombinations can serve as a potential tool in realizing higher crop genetic gains when put into practice.

Bibliography:

1. Allard, R. W., 1960, Principles of Plant Breeding. John Wiley and Sons, New York.

2. Ru, S. and Bernardo, R., 2019, Targeted recombination to increase genetic gain in self pollinated species. Theor. Appl. Genet., 132: 289-300.

3. Sadhu, M. J., Bloom, J. S., Day, L. and Kruglyak, L., 2016, CRISPR-directed mitotic recombination enables genetic mapping without crosses. Science, 352 (6289): 1113-1116.