| Twenty-five species and hybrids in populus were selected as parents, and hybrid populations were obtained through artificial control-pollination. Genetic variation of quantitative traits in progenies was analysed for selection of superior cross combinations and elit hybrids at phenotype level. Genetic variation among parents was studied by Amplified Fragment Length Polymorphism (AFLP) at DNA level, the feasibility of selecting and mating parents and forecasting heterosis at genome level was also discussed based on the analysis of correlation and regression between molecular genetic distance among parents and growth traits of progenies. Moreover, in order to accumulate data to investigate molecular mechanism of heterosis, differential gene expression between hybrids and their parents was studied by DDRT-PCR at RNA level, and differential expressed cDNAs were cloned and sequenced. The major research conclusions were summarized as follows:1. Eleven clones in Section Leuce, seven in Tacamahaca, and seven in Aigeiros of Populus were used as parents, and thirty-two cross combinations among them were made. More than five thousand seedlings, which came from twenty-six crossings, were gained. Quantitative traits genetic analysis of these seedlings indicated that considerable genetic variation existed among and within crossing combinations. Variance analysis showed that the effect of crossing combination was significant or highly significant for traits of seed, growth, branch and leaf. Their family heritabilities ranged from 0.571 to 0.999, which belonged to high heritability, and individual heritabilities for fifteen traits in sixteen traits changed from 0.249 to 1.000, which belonged to middle or high heritability. Therefore, the variation among cross-combinations and individuals in the same cross combination was caused by genetic factor, and selecting superior cross combinations and plus seedlingscould be considerable.2. Combining the results of main component analysis and cluster analysis revealed that P. ussuriensis cl. 'U4'× P. deltoids cv. 'T66' , P. ussuriensis c1. 'U4' × P. deltoids cv.'T26' , P. deltoids cv. 'Lux' × P. ussuriensis c1. 'U3', P. deltoids cv. 'Lux' ×P. deltoids cv. 'D324' , (P. tomentosa×P. bolleana) ×(P. alba×P glandulosa),( P. alba×P. tomentosa) ×(P. alba×P. glandulosa) and (P. alba×P. glandulosa) ×P. tomentosa cv. 'LM50' were superior cross combinations, which had higher growth and larger leaves. Among these seven cross combinations, P. ussuriensis cl. 'U4'× P. deltoids cv. 'T66' , P. ussuriensis cl. 'U4' ×P. deltoids cv. 'T26' , P. deltoids cv. 'Lux' × P. ussuriensis cl. 'U3', (P. tomentosaxP. bolleana) ×(P alba×P glandulosa),( P. alba×p tomentosa) ×(p alba×P. glandulosa) were selected for the first time. One hundred and twenty-three plus seedlings were selected out from hybrid population according to growth and branching traits.3. Thirteen pairs of AFLP primers were screened out and used for the analysis of fifty-five clones, which came from twenty-one species and hybrids of four sections in Populus. Thirteen pairs of AFLP primers generated 858 fragments, 771 polymorphism fragments among them. Percentage of polymorphism fragments produced by each AFLP primer pair was 89.9% in average, and varied from 80.7% to 98.1%. The range of fragments size was 15bp-1000bp, and 245bp in average.4. Genetic distances (GD) among sections, species intrasections, clones intraspecies were calculated, and cluster analysis was also conducted. The result showed that GD among sections was the maximum, GD among clones intraspecies was the minimum, while GD among species intrasections was in the middle. The array order of the GD among sections was GD (0.5314) between Leuce and Turanga, GD (0.4781) between Leuce and Aigeiros, GD(0.4732) between Leuce and Tacamahaca ,GD(0.4706) between Aigeiros and Turanga, GD(0.4379) between Tacamahaca and Turanga, GD(0.3263) between Aigeiros and Tacamahaca in turn. The GD among species of Tacamahaca, Aigeiros, Leuce were 0.2482, 0.2435 and 0.1856 respectively. Compa...
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