Effects Of Population Improvement By Mass Selection In The Tropical Maize Population MW964

A tropical population MW964 was under bi-parental mass selection for 4 cycles aiming at improvement of their adaptability to the local day conditions in Sichuan. Mass selection is fairly effective in improving the adaptation of the exotic maize population to the local subtropical conditions. The mean days to silking and pollen shedding in MW964 after 4 cycles of selection were significantly reduced. They are, respectively, 0.71 days shorter per cycle for flowering, 0.53 days shorter per cycle for pollen shedding. This shortened the ASI (anther to silking interval) by 0.17 days per cycle. Significant correlated responses were found to plant height, tassel main aixs , barrent, eardiameta;kernel number per row,ear weight,yield per plant and yield per population. The plant height was decreased by 3.5cm per cycle. The variation coefficients of many traits decreased with the selection.Comparisons were also made between CK and four test-crosses ( C0/C1/C2/C3xtester). It was shown that many traits of MW964 displayed significant indirect responses to mass selection. The mean and the uniformity of these traits have reached the level of CK(single-cross) .The yield increased by 210.16kg/hm2 per cycle. This indicated that the combining ability of MW964 was significantly improved.Meanwhile we screened 9 pairs of SSR Primers out of 30 pairs to detect the genetic diversity of the 3 populations (C0,C2,C4). The results were: 0.5545 (C0), 0.4823 (C2), 0.2969 (C4), respectively. The distribution ratios were 86.86% among the 3 populations and 13.14% within populations. The genetic distances of C0 ,C2 and C4 were 0.114, 0.106, 0.065,respectively.In the 3 populations, the genotype of Pop.Co was much higher than C4,the frequency of Co was dispersed,and that of C4 was concentrated. Therefore it was suggested that the genetic diversities of C0 and C2 were abundant, compared to C4 .The genetic diversity among the populations was bigger than that within populations. The genetic diversity within populations declined along with the selection process. This might be due to the continuous, oriented selection for the flowering traits and might also be due to the genetic drift induced by selection intensity and effective population size.