Evaluation of Synthetic Populations Developed from a Maize Variety (BSK) by Two Methods of Recurrent Selection¹

Abstract

The effectiveness of half-sib selection with a doublecross tester and Si selection per se was evaluated following four cycles of selection in BSK (the Krug Hi I syn. 3 strain of 'Krug Yellow Dent'). Five populations, BSK CO, BSK(HT)C2, BSK(S)C2, BSK(HT)C4, and BSK(S)C4, were selfed, sibbed, and crossed in a diallel series. They were also test-crossed to four single-cross testers.The average of the four single-cross testcross yields of the BSK (S) populations increased 10.6% during the four cycles, and the BSK(HT) populations increased 5.7%. The selfing series populations performed better than the testcross series populations when selfed, as indicated by a 38.7% increase for the BSK(S) bulk S^'s compared with a 12.0% increase for the BSK(HT) bulk S/s. The populations per se increased 16.3% in the selfing series and 6.3% in the testcross series. The BSK(HT) x BSK(S) crosses exhibited heterosis, which indicated that the two methods developed populations that differed in gene frequency.This study has shown that the mean yield and the general combining ability of BSK were significantly improved by four cycles of selection with both methods of recurrent selection, but Sj selection per se was more effective. Additional index words: Zea mays L., Grain yield, Population improvement, Test-cross evaluation, Sj line per se evaluation.A BASIC concern in maize (Zea mays L.) breeding is the identification of superior genotypes. This often is difficult because of the many factors, genetic and environmental, that affect characters under selection. Various methods of visual selection during inbreeding combined with evaluation by topcrossing have lacked efficiency. Recurrent selection was de-

Comparison of half-sib and S1 recurrent selection in 'Krug Yellow Dent'

“…Second, the predicted gains based on estimates of the additive genetic variance in Krug (Wright et al, 1971) resulted in similar values to those given by Burton et al (1971) and those reported here.…”

“…S^ selection was more effective than half-sib selection, at least in cycles 0-4. Burton et al (1971) also found a larger response in the testcrosses with four single crosses for the population developed by four cycles of selection. The increased response to half-sib selection in cycles 4-8 in the population per se and in testcrosses observed in this study may be attributed to the positive effects of the changes made in the selection procedure, the changes in agronomic practices and the use of inbred testers.…”

“…In many cases, it was reported that the predicted responses were larger than the responses observed in evaluation experiments (Burton et al, 1971;Moll and Smith, 1981;Hallauer and Miranda, 1981).…”

“…Despite a decreasing correlation between Sj^ and testcross yields, they concluded that "the more productive S^ lines tended to produce the more productive crosses". Burton et al (1971) evaluated four cycles of recurrent selection in Krug (BSK) based on S^ lines per se [BSK(S)] and on testcrosses with an unrelated double-cross tester [BSK(HI)], They concluded that the mean yield and the combining ability of the Krug variety were improved significantly for both methods, but the method was more effective.…”

“…Smith (1979aSmith ( , 1979b proposed a modification of the Hammond and Gardner model that included a parameter to estimate the effects of inbreeding that accumulate in recurrent selection programs. He reanalyzed data published by Burton et al (1971), Eberhart et al (1973), andRussell et al (1973), using the proposed model. In all population improvement programs analyzed he found highly significant amounts of inbreeding depression for grain yield due to finite population size.…”