| Maize is an important grain and forage crop in China. Total planting area and output is ranked in the second place just behind USA. Heterosis is widely used in maize and other crops, but the molecular mechanism under it is still elusive. This paper studied the manifestation and mechanism of heterosis through quantitative genetics and gene expression.1. A set of 20 hybrids and its 32 parent inbred lines that were widely used in Chinese maize production since the 1960’s was selected and planted in Beijing and Dezhou. Five maize ear characters including volume-weight, hundred-grain weight, ear length, ear diameter and ear rows were studied. Analyses of variance were done for ear characters of inbred lines and hybrids respectively. Genetic variance components and heritability were dissected by using the genetic model of additive-dominance and their interactions with environments and MINQUE(1) approaches. Heterosis analysis were carried out for ear characters of the twenty hybrids by using the new method for analyzing heterosis presented by ZhuJun. The main results were as follows:The average performance of hybrids of ear traits was higher than inbreds at the two test sites, highly significant difference were detected among both inbreds and hybrids for each traits; The dominance effects were highly significant for all five ear traits and the heritability values in broad sense were also highly significant. Both additive and dominance effects were detected for volume-weight and ear rows, while additive effects were not detected for hundred-grain weight, ear length and ear diameter. Heterosis analysis indicated that population mean heterosis of F1 and F2 reached significant or highly significant level, and only ear diameter of F1 reached significant level for the population better heterosis.2. Transcriptome of ZD808 and its parent lines CL11 and NG5 were analyzed by RNA-seq during ear spikelet and floret differentiation stages. The main results are summarized as follows:1) Through Illumina sequencing, 43.9 and 63.5 million 100-bp pair-end reads were obtained for each of the replicates and genotypes. Among these, 76.06%~79.65% could be aligned to the unique position of B73 reference genome. A transcript was considered to be positively expressed only if its RPKM≥1. Based on the criterion, 20,292 genes were transcribed in at least one sample.2) During the spikelet and floret development stage, we identified 12,550 and 10,120 genes differentially expressed between the parental lines CL11 and NG5, which account 54.5% and 43.9% of analyzed genes. Between hybrid and CL11, 8,290 and 7,315 differential expressed genes were identified at two stages, between hybrid and NG5, fewer DEGs were discovered with 4,309 at first stage and 2,696 at second stage. The prevalence of gene expression divergence of the three genotypes could be the bases of heterosis formation.Expression pattern analysis revealed that additive expression was the dominant pattern(3.8% ~79.8% of analyzed genes), and very small proportion of genes show over dominance or under- over dominance. The results suggested that a balanced gene expression status attribute to complementary effect mainly influence the formation of heterosis.3) Allelic-specific expression(ASE) can affect the level of gene expression. ASE analysis indicates: In the spikelet and floret development stage of maize ear, 7,126 and 6,625 genes were identified with significant allele bias respectively, which account for 59.5% and 58.45 of analyzed genes. NG5 HYB allele bias in hybrid was more than CL11 HYB bias during both developing stages. Alleles from NG5 may contribute more in maintaining the hybrid expression activity. Cis- and trans- regulation were responsible for varied parental allele expression patterns in hybrid, our research revealed that cis- regulatory effects played a larger role than trans effects in parental allele expression divergence in hybrid and suggested a mechanism for the high frequency of additive expression patterns in the hybrid. |
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