Effect of natural selection on multilocation bulk breeding populations of rice (Oryza sativa L.)

The effect of natural selection was evaluated in two rice populations derived using the MBB [multilocation bulk breeding] procedure and the conventional bulk method. The populations derived from two crosses, namely, between Yamayutaka (Y) and Hwaseongbyeo (H) and between Yamayutaka (Y) and Suwon 342 (S). The MBB procedure involved advancing the two populations as bulk under natural selection for four generations (F2 to F5) in Suwon and Unbong in Korea and evaluating the subsequent generations (F6 to F8) in Cheolwon, a site that represented the target area of any new variety to be produced. The same populations were advanced in Cheolwon to represent the conventional bulk method. F2 to F5 generations grown in the three locations were evaluated in Cheolwon for days to heading, culm and panicle lengths, number of panicles per plant, number of grains per panicle, 1000-grain weight, spikelet fertility and grain yield. In both breeding methods, the effect of natural selection was noted among locations and generations, and between cross combinations. Among locations, the lines selected from the populations advanced in Unbong yielded the highest and the frequency of high yielding lines was also highest when advanced in Unbong. The lines derived from YS cross performed better than those from YH cross. They matured earlier, had longer panicles with more grains per panicle, heavier grains and higher grain yield. Across generations, days to heading was most affected by natural selection. With advancing generations, days to heading increased. Comparison of the efficiency of the MBB with the conventional bulk method indicated that MBB resulted in increased grain yield, number of grains per panicle and panicle length, and high frequency of high yielding lines. The study revealed further that when using the bulk method, advancing generations with natural selection in a site representing the target environment do not necessarily produced the most desirable genotypes. The MBB procedure appeared to increase the probability of deleting the bad genotypes