Recurrent selection aims at improving quantitatively inherited traits in a systematic manner by gradually increasing the frequency of favorable alleles within the population. Recurrent selection effective in improving breeding populations and gives the best long-term response to selection. Association between alleles at molecular marker loci and alleles at quantitative trait loci (QTL) can be used to select indirectly for favorable QTL alleles.

Marker-assisted selection can be used to substantially increase breeding efficiency. With a relatively small number of genes from a few elite parental lines, Marker-Assisted Backcrossing and pedigree selection can be powerful means of improving the breeding value of lines by selecting different combinations of favorable QTL alleles. But as the number of QTL allele increases, this soon becomes infeasible. In most cases, the number of genes that control quantitative traits is expected to be large, especially in experiments that are designed to improve multiple characters. As such, marker-aided recurrent selection (MARS) is an appropriate breeding strategy for increasing the frequency of favorable QTL alleles in the population.

Marker assisted recurrent selection (MARS) was used to more effectively pyramid minor genes controlling plant response to crown rot disease. Two populations comprising 475 genotypes were phenotyped for adult plant resistance and genotyped using a SNP’s. A genome-wide association study (GWAS) was used to estimate quantitative trait locus (QTL) effects as more than two parents were recombined in each population. A total of 23 significant and unique marker-trait associations (MTAs) were identified in both populations. A Kompetitive allelespecific PCR (KASP) assay was developed and used to genotype the progeny and parents of each recombination cycle. A significant and positive response to each cycle of recurrent selection was observed. MARS was an effective breeding strategy for combining many QTLs of small effect for crown rot resistance in wheat breeding strategy (Rahman et al. 2020).

To improve the grain yield of the lowland-adapted popular rice variety Samba Mahsuri under reproductive-stage drought (RS) and to understand the interactions between drought QTLs, mapping populations were developed using marker-assisted recurrent selection (MARS). Among MARS PLs, lines with four qDTYs (qDTY1.1+qDTY2.1+qDTY3.1+qDTY11.1) yielded higher than PLs with other qDTY combinations. The positive interactions among QTLs, effectiveness of QTLs in different backgrounds, introgression of DTY QTLs together with resistance to biotic stresses shall help enhance grain yield under RS (Sandu et al. 2018).

Beyene et al. (2015) estimated genetic gains in grain yield (GY) using MARS in 10 biparental tropical maize (Zea mays L.) populations. In each population, 148 to 184 F2:3 (defined as C0) progenies were derived, crossed with a single-cross tester, and evaluated under water-stressed (WS) and well-watered (WW) environments in Sub-Saharan Africa (SSA). The C0 populations were genotyped with 190 to 225 Single-nucleotide polymorphism (SNP) markers. The overall gain for GY using MARS across the 10 populations was 105 kg ha−1 yr−1 under WW and 51 kg ha−1 yr−1 under WS.

under WS. Across WW environments, GY of C1S2-derived hybrids were 8.7, 5.9, and 16.2% significantly greater than those of C0, founder parents, and commercial checks, respectively.

The advantage of MARS scheme is that the complex traits viz., grain yield, biotic and abiotic characters which are controlled by polygenes or QTLs with small but additive genetic effects, it is difficult to assemble all the favorable alleles in one genotype through MABC. It aims to accumulate a large number of QTLs in a given population using a subset of markers that are significantly associated with target traits. As a result, the genetic gain achieved through MARS is higher as compared to the MABC program. MARS has the advantage over GS when one is having information about small subset markers rather than the molecular markers covering across the whole genome.

Marker-aided recurrent selection is especially valuable in speeding the breeding process, in selection of immature individuals and in characteristics that are difficult or expensive to measure such as drought and pest resistance. In this regard, the choice of a MARS strategy strongly depends on the breeding purpose, the available resources of the breeder, the genome of the species, and the nature of the trait (or traits) to be improved.

References:

Beyene, Y., Semagn, K., Crossa, J., Mugo, S., Atlin, G.N., Tarekegne, A., Meisel, B., Sehabiague, P., Vivek, B.S., Oikeh, S. and Alvarado, G., 2016, Improving maize grain yield under drought stress and non-stress environments in Sub-Saharan Africa using marker-assisted recurrent selection. Crop Sci., 56(1): 344-353.

Rahman, M., Davies, P., Bansal, U., Pasam, R., Hayden, M. and Trethowan, R., 2020, Marker-assisted recurrent selection improves the crown rot resistance of bread wheat. Mol. Breed., 40(3): 1-14.

Sandhu, N., Dixit, S., Swamy, B.M., Vikram, P., Venkateshwarlu, C., Catolos, M. and Kumar, A., 2018, Positive interactions of major-effect QTLs with genetic background that enhances rice yield under drought. Sci. Rep., 8(1): 1-13.