Abstract
Rainfed lowland rice (RLR) grown in Bangladesh often gets submerged for a period of 7-14 days after transplanting incurring substantial yield loss. Therefore, efforts were undertaken to introgress flash-flood tolerant gene called Sub1 into the popular RLR mega variety BR11. A total of four selection strategies of marker-assisted backcrossing (MABC) scheme viz. A. Foreground, Recombinant and Background Selection, B. Foreground and Phenotypic Selection, C. Foreground, Phenotypic and Background Selection, and D. Foreground, Recombinant and Phenotypic Selection were investigated under the present study. In MABC technique of Approach A, 1 tightly linked, 2 gene-based, 4 flanking and 116 background microsatellite markers were used to apply foreground, recombinant and background selection, respectively, in backcrosses between a submergence tolerant donor IR40931-33-1-3-2 and BR11. Maximum number of BC2F1 seeds produced was 676 from a single selected BC,F1 plant following high volume backcrossing technique. Comparison of selection strategies reflected that nine submergence tolerant precision introgression lines (PILs) were developed from approach A by two backcrosses in BC2F2 generation, 1 BC3F4 bulk population from approach B, 4 promising submergence tolerant fixed backcross recombinant lines (BRLs) from approach C and 14 early (around two weeks) submergence tolerant BC2F4 segregating lines from approach D. The size of the introgression from the donor parent on the carrier chromosome of the selected lines was 800 Kb, 15 Mb, 15 Mb and 6.9 Mb in approach A, B, C and D, respectively. Approach A emphasized on the production of around 1000 BC, F, plants, selection of SRP (single recombinant at proximal end) type best plant in BC,F, and deployment of additional background markers in BC2F2 generation to finish MABC by BC2F2 generation. Approach B indicated that modified backcross breeding scheme could be finished by maximum 5 backcrosses. Again, recombinant selection was found effective in minimizing linkage drag as there was huge donor introgression when this selection level was not practiced in approach C and finally phenotypic selection was found to some extent effective after recombinant selection over smaller plants in BC,F, generation from approach D. BR11-Subl was 99.8% similar to BR11. The Sub1 QTL in all the lines was confirmed by using gene-based molecular markers. BR11-Subl along with other precision introgression and recombinant lines also showed submergence tolerance similar to the tolerant parent. Successful recovery of the genetic background of BR11 was obtained in respect to yield, yield contributing parameters and grain physico-chemical properties of BR11-Subl. The BRLs viz. IR85260-66-217 and IR85260-66-258 produced 0.5 ton/ha more yield being significantly higher than BR11-Subl under non-submerged condition. The present study recommended that the PILS IR85260-391- 148 (BR11-Sub1), IR85260-391-217 and the BRLS IR85260-66-217, IR85260-66-258 and IR85260-66-654 possessed potentiality of releasing as submergence tolerant modern varieties based on their high yielding potential, acceptable plant type, grain quality as well as excellent submergence tolerance. The strategies developed in Approach A could be followed for rapid and precise introgression of Sub1 QTL into other RLR rice varieties.