[show abstract][hide abstract] ABSTRACT: Hybrid vigor or heterosis results from the combination of genetically distant genomes at fertilization, and as well as being of major commercial importance, it is held to contribute significantly to fitness . Activation of the paternal genome marks the transition from maternal to zygotic control of development, but a reported delay of paternal-genome activation in flowering plants [2-4] and animals [5, 6] excludes heterosis from impacting on very early development. We have analyzed the allele-specific expression of 25 genes after fertilization of the egg in maize and show immediate equivalent parental genomic contribution to the zygote. Every gene expressed before the first cell division of the zygotes showed paternal transcripts. Sequence comparisons indicate that these genes are involved in a range of processes and are distributed throughout the genome. Our findings confirm that some plant species have evolved a strategy to activate the paternal genome immediately after fertilization, in contrast to the situation in other plants and in animals. Such an extensive activation of the paternal genome very early in development is consonant with observations of high levels of heterosis in early hybrid maize embryos [7, 8], indicating a significant impact of this sexual strategy on fitness.
Current Biology 11/2007; 17(19):1686-91. · 9.49 Impact Factor
[show abstract][hide abstract] ABSTRACT: Heterosis is important for conventional plant breeding and is intensively used to increase the productivity of crop plants. Genetic processes shortly after fertilization might be of particular importance with respect to heterosis, because coordination of the diverse genomes establishes a basis for future performance of the sporophyte. Here we demonstrate a strong crossbreeding advantage of hybrid maize embryos as early as 6 days after fertilization in a modern maize hybrid and provide the first embryo specific analysis of associated gene expression pattern at this early stage of development. We identified differentially expressed genes between hybrid embryos and the parental genotypes by a combined approach of suppression subtractive hybridization and differential screening by microarray hybridizations. Association of heterosis in embryos with genes related to signal transduction and other regulatory processes was implied by the enrichment of these functional classes among the identified gene set. Quantitative RT-PCR analysis validated the expression pattern of 7 of 12 genes analysed and revealed predominantly additive, but also dominant and overdominant expression patterns in hybrid embryos. These patterns indicate that gene regulatory interactions among parental alleles act at this early developmental stage and the genes identified provide entry points for the exploration of gene regulatory networks associated with the specification of the phenomenon heterosis in the plant life cycle.
[show abstract][hide abstract] ABSTRACT: The analysis of cell type-specific gene expression is an essential step in understanding certain biological processes during plant development, such as differentiation. Although methods for isolating specific cell types have been established, the application of cDNA subtraction to small populations of isolated cell types for direct identification of specific or differentially expressed transcripts has not yet been reported. As a first step in the identification of genes expressed differentially between maize egg cells and central cells, we have manually isolated these types of cell, and applied a suppression-subtractive hybridization (SSH) strategy. After microarray screening of 1030 cDNAs obtained from the subtracted libraries, we identified 340 differentially expressed clones. Of these, 142 were sequenced, which resulted in the identification of 62 individual cDNAs. The expression patterns of 20 cDNAs were validated by quantitative RT-PCR, through which we identified five transcripts with cell type-specific expression. The specific localization of some of these transcripts was also confirmed by in situ hybridization on embryo sac sections. Taken together, our data demonstrate the effectiveness of our approach in identifying differentially expressed and cell type-specific transcripts of relatively low abundance. This was also confirmed by the identification of previously reported egg cell- and central cell-specific genes in our screen. Importantly, from our analysis we identified a significant number of novel sequences not present in other embryo sac or, indeed, in other plant expressed sequence tag (EST) databases. Thus, in combination with standard EST sequencing and microarray hybridization strategies, our approach of differentially screening subtracted cDNAs will add substantially to the expression information in spatially highly resolved transcriptome analyses.
The Plant Journal 11/2005; 44(1):167-78. · 6.58 Impact Factor
[show abstract][hide abstract] ABSTRACT: Heterosis, or hybrid vigour, describes the superior performance of heterozygous hybrid individuals compared with their homozygous
parental inbred lines. Although heterosis has been intensively used in plant breeding, the molecular and genetic mechanism
underlying the phenomenon remains largely unknown. During the last years numerous laboratories initiated genomic approaches
with new, often genome wide tools toward the elucidation of the molecular basis of heterosis. Various studies described differences
in genome organization and gene expression of hybrids and their parental inbred lines. In maize, a considerable loss of co-linearity
at many loci between different inbred lines was observed. Expression profile comparisons between inbred lines and hybrids
revealed complex transcriptional networks specific for different developmental stages and tissues mainly in maize (Zea mays), rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana). Allele-specific expression data resolved the relative parental contributions and allowed figuring out the regulatory basis
for expression variation in hybrids. Integrating all these complex expression data might help to get an idea about the molecular
basis of heterosis. Thereby molecular processes during early seed development shortly after fertilization might be of particular
importance, because the allelic interplay has to be coordinated after the unification of two diverse genomes and these processes
might contribute to establish the basis for future performance of the sporophyte. Besides these fundamental interests in the
molecular basis and manifestation of hybrid vigour applied aspects of the phenomenon are of high importance to support plant
breeding and agriculture. Prediction methods are of special interest to identify the most promising parental lines of hybrid
varieties, greatly reducing the financial effort and increasing the efficiency to develop new hybrid cultivars. Until now,
most prediction approaches were based on genomic markers. The available heterosis associated expression data provide evidence
to support the idea that gene regulatory networks at the level of transcription are involved in the control of hybrid vigour.
If so, the transcriptome characteristics of inbred lines resulting from their individual genomic constitution should be useful
as quantitative markers to predict the performance of hybrids generated by crosses of these inbred lines. Additionally, expression
based prediction models promise to contribute substantially to the understanding of the genetic causes of heterosis by establishing
a direct link between the transcriptome and hybrid performance.
Chinese Science Bulletin 54(14):2363-2375. · 1.32 Impact Factor