Heterosis and heterotic patterns of maize germplasm revealed by a multiple-hybrid population under well-watered and drought-stressed conditions

Understanding the heterosis in multiple environments between different heterotic groups is of fundamental importance in successful maize breeding. A total of 737 hybrids derived from 41 maize inbreds were evaluated over two years, with the aim of assessing the genetic diversity and their performance between heterotic groups under drought-stressed(DS) and well-watered(WW) treatments. A total of 38 737 SNPs were employed to assess the genetic diversity. The genetic distance(GD) between the parents ranged from 0.05 to 0.74, and the 41 inbreds were classified into five heterotic groups. According to the hybrid performance(high yield and early maturity between heterotic groups), the heterosis and heterotic patterns of Iowa Stiff Stalk Synthetic(BSSS)×Non-Stiff Stalk(NSS), NSS×Sipingtou(SPT) and BSSS×SPT were identified to be useful options in China’s maize breeding. The relative importance of general and specific combining abilities(GCA and SCA) suggests the importance of the additive genetic effects for grain yield traits under the WW treatment, but the non-additive effects under the DS treatment. At least one of the parental lines with drought tolerance and a high GCA effect would be required to achieve the ideal hybrid performance under drought conditions. GD showed a positive correlation with yield and yield heterosis in within-group hybrids over a certain range of GD. The present investigation suggests that the heterosis is due to the combined accumulation of superior genes/alleles in parents and the optimal genetic distance between parents, and that yield heterosis under DS treatment was mainly determined by the non-additive effects.

Analysis of Maize Heterotic Groups and Patterns During Past Decade in China

In this investigation, maize heterotic groups and patterns were analyzed based on theplanting areas from 1992 to 2001 using 84 parent lines of 71 widely extended hybrids andclassification results by SSR markers, in which these lines were assigned into sevenheterotic groups based on Ni-LIs genetic distances. The results indicated that acertain extent change for major heterotic groups of maize took place during past decadein China. The major heterotic groups were Lancaster, Reid, Tang SPT, Zi330 and E28 in theearly 1990s, while they became Reid, Tem-tropicⅠ, Zi330, Tang SPT and Lancaster in theearly 21st century. Tem-tropicⅠwas a new heterotic group, which contained tropic maizegermplasm. The changes for heterotic patterns also occurred. Some new heterotic patternscombining with Tem-tropicⅠappeared, such as ReidTem-tropicⅠ, Zi330Tem-tropicⅠ,Tang SPTTem-tropicⅠ, etc.. Another change was the order of heterotic patterns. In theearly and middle 1990s, the top five heterotic patterns were ReidTang SPT, Zi330Lancaster, LancasterTang SPT, LancasterE28 and ReidZi330, while they became ReidTem-tropicⅠ, ReidZi330, ReidTang SPT, Zi330Tem-tropicⅠand LancasterTang SPT inthe early 21 century. ReidTem-tropicⅠand Zi330Tem-tropicⅠwere laid on the firstand forth Chinese heterotic patterns respectively in 2001. These results providedsignificant information to understand the maize heterotic groups and patterns in Chinaat molecular level.