1 China Agricultural University, National Maize Improvement Center of China, www.cau.edu.cn.
2 China Agricultural University, National Maize Improvement Center of China, www.cau.edu.cn. Email: email@example.com
In this investigation, maize heterotic groups and patterns were analyzed based on the planting areas from 1992 to 2001 using 84 parent lines of 71 widely extended hybrids and classification results by SSR markers. The results indicated that a change occurred in the major heterotic groups of maize took place during the past decade in China. The major heterotic groups were Lancaster, Reid, Tang SPT, Zi330 and E28 in the early 1990s, while they became Reid, Tem-tropic I, Zi330, Tang SPT and Lancaster in the early 21st century. Tem-tropic I was a new heterotic group, which contained tropical maize germplasm. The changes for heterotic patterns also occurred. Some new heterotic patterns associated with Tem-tropic I appeared, such as Reid × Tem-tropic I, Zi330 × Tem-tropic I, Tang SPT × Tem-tropic I, etc. Another change was the order of heterotic patterns. In the early and middle 1990’s, the top five heterotic patterns were Reid × Tang SPT, Zi330 × Lancaster, Lancaster × Tang SPT, Lancaster × E28 and Reid × Zi330, while they became Reid × Tem-tropic I, Reid × Zi330, Reid × Tang SPT, Zi330×Tem-tropic I and Lancaster × Tang SPT in early 21 century. Reid × Tem-tropic I and Zi330 × Tem-tropic I were laid on the first and forth Chinese heterotic patterns respectively in 2001.
Maize heterotic groups and patterns with elite germ plasm in China were investigated based on classification by SSR markers and planting areas.
Maize; Heterotic group; Heterotic pattern; SSR
The studies on maize heterotic groups and patterns are very helpful to raise the breeding efficiency. Recently, bio-technolgy provided a powerful tool for the study of heterotic groups and patterns at molecular level. Melchinger (1991) demonstrated that the classification of heterotic group should be feasible using molecular markers. Mumm et al (1995) assigned 148 American maize inbreds into two major heterotic groups and eleven sub-groups with the RFLP markers. 57 European maize inbreds were also classified with RFLP markers  Dubreuil et al  classified 116 European and north American maize inbreds into the dent and flint two groups and 11 sub-groups with 63 probe/enzyme combinations of RFLP. Smith et al  indicated that the classification results of 57 maize inbreds by SSR and RFLP markers almost agreed with their pedigree. Warburton et al  demonstrated maize inbreds in CIMMYT should not be assigned into two heterotic groups (HG-A and HG-B) and should be more heterotic groups with SSR markers. Recently, there were also some reports of classification of maize heterotic groups by molecular markers [7, 8, 9, 10] However, there are few of reports focusing on trend analysis of Chinese maize heterotic groups and patterns during past decade based on classification by molecular markers.
Eighty four inbred lines involved in 71 hybrids were chosen to represent maize germplasm in China. A set of 111 SSR primers was selected, which were covered throughout the maize genome. SSR loci were individually amplified using DNA of each inbred using protocols described by Maize DB (www.agron.missouri.edu). The genetic similarity of inbred lines were estimated according to Nei and Li .Cluster analysis was carried out on the matrix of generic similarities using UPGMA by the NTSYS－PC software . The variation tendencies of hetertic groups and patterns were analyzed according to the results of the heterotic groups classified by SSR markers and the planting areas of hybrids in China from 1992 to 2001 referring to the annual reports from Agricultural Ministry of China.
Totally, 660 alleles were detected using 111 SSR markers among 84 inbreds, among which the GD varied between 0.10 and 0.77 with an average 0.59. These inbreds were clustered into seven groups at the Nei-Li distance of 0.92 through the UPGMA clustering algorithm. They were named according to their origin and germplasm resources. Reid group including B73 had 21 inbreds while Lancaster group including Mo17 were 12 inbreds. 14 inbreds consisted of Zi330 group. It should be noted that the six inbreds (Qi319, 178, P138, Shen137, Dan599 and 87-1) selected from a set of foreign hybrids were tightly clustered into one group. This group also contained 4 topical lines (M9, S37, BT1 and Nan21-3), which may proved these foreign hybrids might be contain some tropical germplasm at molecular level.
Their pedigree was expected that the classification results of heterotic groups by molecular markers were associated with their pedigree. For example, eleven inbreds originated from Huang ZS (HuangYS-3, Xi502, 5237, WenH413, H21, Chang7-2, Ji853, Q1261, K12, Shang741 and 444) were grouped into Tang SPT group together. Ji842, 4F1, HuoTH and Ji846 were clustered with Mo17, which were the 2nd-cycle lines related to Mo17, while 446, 200B, LongK11, 48-2 and S434 were clustered with Zi330, which were the 2nd-cycle lines involved in Zi330.
According to heterotic groups classified in this study and the cultivated areas of investigated hybrids, the proportion of major heterotic groups in 1992-2001 was shown in Fig 1. Reid, Lancaster, Tang SPT and Zi330 were four major heterotic groups among seven heterotic groups and their planting area proportion of total planting areas were 21.86％, 16.81%, 15.84％ and 13.73% respectively. A change in the major heterotic groups took place over the past ten years. In the early 1990s, the major heterotic groups were Lancaster, Reid, Tang SPT, Zi330 and E28, while they were Reid, Tem-tropic I, Zi330, Tang SPT and Lancaster at the beginning of 21st century.
According to the heterotic groups of 84 inbreds classified by SSR markers, fifteen heterotic patterns were concluded among the major commercial hybrids. Among the 71 hybrids investigated, the parents of all but one hybrid Bai D9, were assigned into different heterotic groups respectively, up to 98.59% of expected proportion. The result provided further evidence that information of combinations may support the classification made using molecular markers.
The fifteen heterotic patterns could be divided into the major and minor patterns when less than 5% of planting areas per year were taken as a standard. The major patterns included Reid × Tem-tropic I, Reid × Zi330, Reid × Tang ZPT, Zi330 × Tem-tropic I, Lancaster × Tang ZPT, Zi330 × Lancaster, Lancaster × E28, Reid × Lancaster and Reid × E28; while the minor were Tem-tropic I × Others, Tang SPT × Tem-tropic I, Tang ZPT × Others, Zi330×Others, Lancaster × Lancaster and Zi330×Tang SPT (Fig 2).
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Fig 1 Proportional variation of major heterotic groups by planting areas in 1992-2001
Fig 2 Proportional variation of major heterotic patterns by planting areas in 1992-2001