| Many maize breeders have been paying attention and efforts to solve the problem ofnarrow genetic diversity in maize. In order to broaden the genetic basis in maize breeding,it is necessary to know the genetic diversity in the current maize hybrids. And the frist taskof the breeders is to improve the yild of maize. In order to study the two problems above,in present study, analyses of phenotypic characters, SSR molecular markers and pedigreeswere made to study the genetic diversity in 186 maize hybrids that were tested in theregional trials of Sichuan and Southwest China and the countermeasures for maizebreeding. The main results are as follows:1. The results showed that the variation coefficients of plant height, ear height, days tosilking, pollen shedding, ASI, ear length, fertile kernel, kernel depth, cob diameter,rows per ear, kemels per row, yield per plant, water content of ear, kernel produced percent, 100-kernel weight and kernel grivity among 186 hybrids were 5.74%, 12.06%,2.63%, 2.36%, 8.50%, 9.68%, 57.99%, 10.57%, 9.32%, 7.90%, 10.54%, 21.51%,16.19%, 10.35%, 2.46%, 10.92% and 4.93%, respectively. And the Shannon-Weavercoefficient of plant height, ear height, days to silking, pollen shedding, ASI, ear length,fertile kernel, kernel depth, cob diameter, rows per ear, kernels per row, yield per plant,water content of ear, kernel produced per cent, 100-kernel weight and kernel grivityamong 186 hybrids were 1.9927, 1.7249, 2.0226, 2.0003, 1.8708, 1.8859, 1.8226,1.1190, 1.7470, 2.0044, 1.7383, 1.9126, 1.8187, 1.5733, 2.1623 and 2.0939,respectively.There were differences in both variation coefficients and the Shannon-Weaver coefficient of different character, but all of the variations changed in a smallrange.2. The results of analysis of 17 phenotypic characters showed that similarities among the186 hybrids ranged from 0.0000 to 1.0000, with an average of 0.6508. There were 894pairs similarities between 0.0000～0.4000 with 5.2% of all similarities ,4354 pairsbetween 0.4000-0.6000 with 25.3% of all similarities, 11957 pairs between 0.6000～1.0000 with 69.5% of all similarities. Cluster analysis indicated that all the hybridscould be divided into eight groups, Cluster 1 includes 161 hybreds with 86.% of allhybrids, Cluster 2 includes 2 hybrids with 1.1% of all hybrids, Cluster 4 includes 10hybrids with 5.4% of all hybrids, Cluster 6 includes 7 hybrids with 3.8% of all hybrids,and Cluster 3, 5 and 8 only include 1 hybrid each. The results show that the similaritieswere high among the 186 hybrids.3. Sixty pairs of SSR primer distributed on the ten chromosomes of maize producedstable amplified bands and 608 alleles were detected among the hybrids. The averagenumber of alleles per locus was 10.1 with a range from 3 to 23. The values ofpolymorphism information content (PIC) for each SSR locus varied from 0.5179 to0.9256 with an average of 0.7826. The genetic similarities of SSR marker patternamong the 186 hybrids ranged from 0.6067 to 0.9162, with an average of0.7722. There were 16499 pairs of genetic similarity; in which 96.9% were 0.70000 to0.9256. The cluster analysis showed that the hybrids could be classified into tenclusters, Cluster 1 consists of 15 hybrids with 8.1% of all hybrids, Cluster 4 consists of88 hybrids with 47.3% of all hybrids, Cluster 8 consists of 47 hybrids with 25.3% of allhybrids, Cluster 9 consists of 2 hybrids with 1.1% of all hybrids, Cluster 10 consists of29 hybrids with 15.6% of all hybrids, Cluster 2, 3, 5, 6 and 7 only consists of 1 hybrideach, and 88.2% of the hybrids included in Cluster 4, Cluster 8 and Cluster 10. It wasindicated that the genetic diversity was narrow among the 186 hybrids.4. The analysis of pedigree sources of 51 hybrids showed that 36 hybrids had closegenetic relationships with the hybrids of Pioneer Company developed in late 1980s and early 1990s in the United States, such as "Y78599", "Y7865", "Y78698", accountingfor 70.58%. The results of analysis of 17 phenotypic characters showed that similaritiesamong the 51 hybrids ranged from 0.0928 to 0.9721, with an average of 0.6734. Therewere 960 pairs similarities between 0.6000～0.9721 with 75.29% of all similarities,and 94.12% of the 51 hybrids included in Cluster 1. The genetic similarities of SSRmarker pattern among the 51 hybrids ranged from 0.66192 to 0.8799, with an averageof 0.7686. There were 1196 pairs of genetic similarities ranged between 0.7000 to0.8796, accounting for 93.80% of all the genetic similarity pairs. The cluster analysisshowed that 82.25% of the 51 hybrids were in Cluster 4 ,Cluster 8 and Cluster 10. Itwas also indicated that the genetic diversity was narrow among the hybrids.5. The result showed that 17 hybrids showed significantly higher yield than the checkvariety, Chuandan 13. The variation coefficients of plant height, ear height, ear length,kernels per row, kernel depth, rows per ear and kernel grivity among 17 high yieldhybrids were 3.89%, 5.92%, 6.01%, 7.58%, 7.02%, 10.17% and 2.57%, respectively,which varied in a narrow range. The average phenotypic similarities among the 17hight yield hybrids was 0.7228, and there were 122 pairs similarities between 0.6000～0.9244 with 89.712% of all similarities. The average genetic similarities of SSRmarker pattern among the 17 hight yield hybrids was 0.7708, and there were 132 pairsof genetic similarity, in which 97.05% were between 0.70000 to 0.8809. The resultsshowed that the genetic diversity may becom narrower after the chose of regional tests.6. The result showed that the ear length, kernels per row, kernel depth, rows per ear andkernel grivity in these high yield hybrids were better than those of the check variety,Chuandan 13. Correlation analysis indicated that only ear length and kernels per rowshowed significantly higher correlation to plant yield than the other characters. Pathanalysis indicated that the direct path coefficients to plant yield were, in order, earlength, rows per ear, kernel depth, kernels per row, plant height, ear height and kernelgrivity. Ear length and rows per ear showed significant positive path coefficients toplant yield. It is, therefore, necessary to improve the germplsm and to create newgermplasm for breeding of.high yield hybrids in maize, and more attention should bepaid to selection of ear length, rows per ear, kernel depth, kernels per row and kernelrate. Special emphasis should be put on ear length.
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