Effect of S3 Recurrent Selection in Four Tropical Maize Populations on Their Selfed and Randomly Mated Generations
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Maize (Zea mays L.) breeders working with lowland tropical germplasm have experienced difficulties in developing good inbred lines. One limitation is moderate to drastic inbreeding depression (ID) in these germplasms. In the present study, two cycles of S3 recurrent selection were conducted in four white maize populations (Pop. 21, Pop. 25, Pop. 29, Pop. 32) with tropical adaptation to improve them as hybrid-oriented source germplasm, and the effect of selection was examined in the selfed (S1) and randomly mated (Syn-2) generations. In each population, 200 to 250 S3 progenies were evaluated and 15 to 20 selected and intermated to form improved versions after cycle 1 (C1) and cycle 2 (C2). Selection was practiced for high grain yield, vigor, and other agronomic traits during inbreeding generations and in the S3-progeny tests. The Si and Syn-2 generations of the C0, C1, and C2 of four populations were evaluated in three environments in Mexico, using split-split plot design with populations in main plots, generations in subplots, and cycles in sub-subplots. Selection improved the grain yield of the S1 generation, and the linear gain per cycle (Gt) was 12, 9, 4, and 4% in Pop. 21, Pop. 29, Pop. 32, and Pop. 25, respectively, being significant in Pop. 21 and Pop. 29. The C2 of Pop. 21 had the highest grain yield in both Syn-2 and S1 generations. Generally, selection did not significantly affect grain moisture, days to silk, and anthesis-silking interval, but it increased plant height in different populations. Combined over populations, however, grain moisture was significantly increased in addition to grain yield and plant height in both S1 and Syn-2 generations. The linear Gc for grain yield across populations was 7.41 and 3.20% in the S1 and Syn-2 generations, respectively. The effect of selection on grain yield and plant height was generally lower in the Syn-2 than the S1 generation of Pop. 25, Pop. 29, and Pop. 32, but was comparable in two generations of Pop. 21. The ID for grain yield across populations decreased from 39% in C0 to 35% in C2. The selection was effective in improving grain yield and reducing ID; and the improved populations would serve as better germplasm sources for the extraction of vigorous inbred lines.
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