Many maize breeders have been paying attention and efforts to solve the problem of narrow 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. There may be two ways to study the genetic diversity in maize hybrids. One is to analyze the hybrids which have been widely applied in production and those in regional tests. Another is to analyze the parents of maize hybrids. Most former studies were directed to evaluate the genetic diversity of the parents and few directed to study the hybrids. In present study, analyses of phenotypic characters, SSR molecular markers and pedigrees were made to study the genetic diversity in 186 maize hybrids that were tested in the regional trials of Sichuan and Southwest China.
The results showed that the variation coefficients of plant height, ear height, days to silking, pollen shedding, ASI, ear length, fertile kernel, kernel depth, cob diameter, ear diameter, rows per ear, kernels per row, ear weight, cob weight, yield per plant, fresh weight per ear, water content, kernel rate, 100-kernel weight and test weight among 186 hybrids were 5.74%, 12.06%, 2.63%, 2.36%, 8.50%, 9.68%, 5.80%, 10.57%, 9.32%, 6.02%, 7.90%, 10.54%, 15.95%, 21.51%, 16.19%, 13.85%, 10.35%, 2.46%, 10.92%, and 4.93%, respectively. There were differences in the variation coefficients of different character, but all of the variation coefficients changed in a small range. Sixty pairs of SSR primer distributed on the ten chromosomes of maize produced stable amplified bands and 608 alleles were detected among the hybrids. The average number of alleles per locus was 10.1 with a range from 3 to 23. The values of polymorphism information content (PIC) for each SSR locus varied from 0.5179 to 0.9256 with an average of 0.7826. The genetic similarities of SSR marker pattern among the 186 hybrids ranged from 0.6067 to 0.9162, with an average of 0.7722. There were 16499 pairs of genetic similarity, in which 96.9% were 0.7000 to 0.9256. The cluster analysis showed that the hybrids could be classified into ten clusters, with 88.2% of the hybrids included in Cluster 4, Cluster 8 and Cluster 10. The analysis of pedigree sources of 51 hybrids showed that 36 hybrids had close genetic relationships with the hybrids of Pioneer Company developed in late 1980s and early 1990s in the United States, such as “Y78599”, “Y7865”, “Y78698”, accounting for 70.58%. Meanwhile 13 hybrids had close genetic relationship with “Y78599", accounting for 8.66%. The genetic similarities of SSR marker pattern among the 51 hybrids ranged from 0.661 92 to 0.8799, with an average of 0.7686. There were 1 196 pairs of genetic similarities ranged between 0.7000 and 0.8796, accounting for 93.80% of all the genetic similarity pairs. The cluster analysis showed that 88.2% of the 51 hybrids were in Cluster 4, Cluster 8, and Cluster 10. It was indicated that the similarity was high and the genetic diversity was narrow among the 186 hybrids. It is necessary to broaden the genetic basis of breeding germplasm in maize.
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