In plant breeding, different laws, models and approaches are developed that assists in cultivar development. Most such laws and models have one basic assumption of diploid segregation that is not met in all the cases. Perhaps, there is another kind of ploidy present in organisms that is known as polyploidy. Polyploids can have entirely different genetics and breeding behavior.

Polyploidy is the condition of possessing more than two complete sets of genomes or chromosomes in a cell. Polyploidy is widespread whereby, 35% of angiosperms are polyploid1. Polyploids are of three types: autopolyploids, allopolyploids and segmental allopolyploids. Polyploids can occur naturally due to formation of unreduced gametes or they can be produced artificially by treating with chemical agents such as colchicine or nitrous oxide. Polyploids, mainly autopolyploids show multivalent formation that may result in production aneuploid gametes and subsequent sterility in offsprings. Genetically, autopolyploids show different kind of genotypes including nulliplex (aaa), simplex (Aaa), duplex (AAa) and triplex (AAA) that result in entirely different polysomic ratios that are not same as mendelian 3:1 ratio for single gene. Additionally, autopolyploids have one unique phenomenon of double reduction that further complicates the ratios in polyploids. However, allopolyploids usually show normal disomic ratios same as diploids.

Due to occurrence of complex polysomic ratios and more than two copies of a single locus, there are certain unique tools and approaches that are used in polyploids. Dosage dependent SNP markers are used to genotype different individuals. Dedicated software for linkage and QTL analysis such as fitTetra and tetraploidMap are available that can handle complex polysomic ratios and dosage dependent genotyping data.

Polyploids play very important role in crop improvement as many important crop species are allopolyploids and even new allopolyploid species are being created by human, most famous being triticale. Apart from that, polyploidization has been proven successful in increasing yield, imparting tolerance to various biotic and abiotic stresses2. A study was conducted in rice whereby, tetraploid rice ‘Balilla-4x’ exhibited significant increase in plant height, spike length and yield per plant as compared to diploid ‘Balilla-2x’. Yield related genes STH1, OsYUC9 and OsDEP1 were significantly upregulated in ‘Balilla-4x’ plants3. In another experiment, QTLs associated with agronomic traits were identified in developed dihaploid potato population4.

In conclusion, there is a wide majority of polyploid crop species that show different breeding behavior and complex polysomic ratios as compared to diploids. Pertaining to their importance and wide spread occurrence some tools and software are already available for analysis in polyploid crops, however, still there is a need for developing more tools and software that can make polyploid breeding convenient.

References

 1WOOD, T. E., TAKEBAYASHI, N., BARKER, M. S., MAYROSE, I., GREENSPOON, P. B. AND RIESEBERG, L. H., 2009, The frequency of polyploid speciation in vascular plants. Proc. Natl. Acad. Sci., 106 (33) : 13875-13879.

 2TOSSI, V. E., MARTINEZ TOSAR, L. J., LAINO, L. E., IANNICELLI, J., REGALADO, J. J., ESCANDÓN, A. S., BAROLI, I., CAUSIN, H. F. AND PITTA-ÁLVAREZ, S. I., 2022, Impact of polyploidy on plant tolerance to abiotic and biotic stresses. Front. Plant Sci., 13 : 869423.

3YANG, S., HE, X., MAO, X., ZHANG, Y., ZHANG, L., AO, Y., ZHAO, X., WANG, W., CHEN, S. AND JIANG, T., 2024, Polyploidized rice improves plant height and yield through regulation of the expression of yield-related genes. Acta Soc. Bot. Pol., 93.

 4MANRIQUE-CARPINTERO, N. C., COOMBS, J. J., PHAM, G. M., LAIMBEER, F. P. E., BRAZ, G. T., JIANG, J., VEILLEUX, R. E., BUELL, C. R. AND DOUCHES, D. S., 2018, Genome reduction in tetraploid potato reveals genetic load, haplotype variation, and loci associated with agronomic traits. Front. Plant Sci., 9 : 944.