Measuring global gene expression in polyploidy; a cautionary note from allohexaploid wheat

Association of gene expression with biomass content and composition in sugarcane

About 64% of the total aboveground biomass in sugarcane production is from the culm, of which ~90% is present in fiber and sugars. Understanding the transcriptome in the sugarcane culm, and the transcripts that are associated with the accumulation of the sugar and fiber components would facilitate the modification of biomass composition for enhanced biofuel and biomaterial production. The Sugarcane Iso-Seq Transcriptome (SUGIT) database was used as a reference for RNA-Seq analysis of variation in gene expression between young and mature tissues, and between 10 genotypes with varying fiber content. Global expression analysis suggests that each genotype displayed a unique expression pattern, possibly due to different chromosome combinations and maturation amongst these genotypes. Apart from direct sugar- and fiber-related transcripts, the differentially expressed (DE) transcripts in this study belonged to various supporting pathways that are not obviously involved in the accumulation of these major biomass components. The analysis revealed 1,649 DE transcripts between the young and mature tissues, while 555 DE transcripts were found between the low and high fiber genotypes. Of these, 151 and 23 transcripts respectively, were directly involved in sugar and fiber accumulation. Most of the transcripts identified were up-regulated in the young tissues (2 to 22-fold, FDR adjusted p-value <0.05), which could be explained by the more active metabolism in the young tissues compared to the mature tissues in the sugarcane culm. The results of analysis of the contrasting genotypes suggests that due to the large number of genes contributing to these traits, some of the critical DE transcripts could display less than 2-fold differences in expression and might not be easily identified. However, this transcript profiling analysis identified full-length candidate transcripts and pathways that were likely to determine the differences in sugar and fiber accumulation between tissue types and contrasting genotypes.

High Transferability of Homoeolog-Specific Markers between Bread Wheat and Newly Synthesized Hexaploid Wheat Lines

Bread wheat (Triticum aestivum, 2n = 6x = 42, AABBDD) has a complex allohexaploid genome, which makes it difficult to differentiate between the homoeologous sequences and assign them to the chromosome A, B, or D subgenomes. The chromosome-based draft genome sequence of the ‘Chinese Spring’ common wheat cultivar enables the large-scale development of polymerase chain reaction (PCR)-based markers specific for homoeologs. Based on high-confidence ‘Chinese Spring’ genes with known functions, we developed 183 putative homoeolog-specific markers for chromosomes 4B and 7B. These markers were used in PCR assays for the 4B and 7B nullisomes and their euploid synthetic hexaploid wheat (SHW) line that was newly generated from a hybridization between Triticum turgidum (AABB) and the wild diploid species Aegilops tauschii (DD). Up to 64% of the markers for chromosomes 4B or 7B in the SHW background were confirmed to be homoeolog-specific. Thus, these markers were highly transferable between the ‘Chinese Spring’ bread wheat and SHW lines. Homoeolog-specific markers designed using genes with known functions may be useful for genetic investigations involving homoeologous chromosome tracking and homoeolog expression and interaction analyses.

Transcriptome and metabolome of synthetic Solanum autotetraploids reveal key genomic stress events following polyploidization

Polyploids are generally classified as autopolyploids, derived from a single species, and allopolyploids, arising from interspecific hybridization. The former represent ideal materials with which to study the consequences of genome doubling and ascertain whether there are molecular and functional rules operating following polyploidization events. To investigate whether the effects of autopolyploidization are common to different species, or if species-specific or stochastic events are prevalent, we performed a comprehensive transcriptomic and metabolomic characterization of diploids and autotetraploids of Solanum commersonii and Solanum bulbocastanum. Autopolyploidization remodelled the transcriptome and the metabolome of both species. In S. commersonii, differentially expressed genes (DEGs) were highly enriched in pericentromeric regions. Most changes were stochastic, suggesting a strong genotypic response. However, a set of robustly regulated transcripts and metabolites was also detected, including purine bases and nucleosides, which are likely to underlie a common response to polyploidization. We hypothesize that autopolyploidization results in nucleotide pool imbalance, which in turn triggers a genomic shock responsible for the stochastic events observed. The more extensive genomic stress and the higher number of stochastic events observed in S. commersonii with respect to S. bulbocastanum could be the result of the higher nucleoside depletion observed in this species.

The genetic study utility of a hexaploid wheat DH population with non-recombinant A- and B-genomes

To study the D-genome of the wild wheat relative Aegilops tauschii Cosson at the hexaploid level, we developed a synthetic doubled-haploid (DH) hexaploid wheat population, SynDH3. This population was derived from the spontaneous chromosome doubling of triploid F1 hybrid plants obtained from a cross between Triticum turgidum ssp. dicoccon PI377655 and A. tauschii ssp. strangulata AS66 × ssp. tauschii AS87. SynDH3 is a diploidization-hexaploid DH population containing recombinant D chromosomes from two different A. tauschii genotypes, with A and B chromosomes from T. turgidum being homogenous across the entire population. Using this population, we constructed a genetic map. Of the 440 markers used to construct the map, 421 (96%) were assigned to 12 linkage groups; these included 346 Diversity Arrays Technology (DArT) and 75 simple sequence repeat (SSR) markers. The total map length of the seven D chromosomes spanned 916.27 cM, with an average length of 130.90 cM per chromosome and an average distance between markers of 3.47 cM. Seven segregation distortion regions were detected on seven linkage groups. Out of 50 markers shared with those on a common wheat map, 37 showed a consistent order. The utility of the diploidization-hexaploid DH population for mapping qualitative trait genes was confirmed using the dominant glaucousness-inhibiting gene W2 (I) as an example.

Storage products and transcriptional analysis of the endosperm of cultivated wheat and two wild wheat species

The starch and protein in wheat (Triticum aestivum L.) endosperm provide 20 percent of the calories eaten by humans and were heavily selected for during domestication. We examined the main storage products and gene expression patterns that may embody compositional differences between two wild species Aegilops crassa and Aegilops tauschii and cultivated bread wheat. The storage product profiles differed significantly with T. aestivum accumulating twice as much carbon as the wild species, while the latter had 1.5 to 2-fold more total nitrogen per seed. Transcriptional analyses of endosperms of similar fresh weight were compared using a cDNA macroarray. Aegilops tauschii, and especially Ae. crassa had stronger hybridizations with storage protein sequences, but while there were differences in transcripts for starch biosynthetic genes, they were less dramatic. Of these, we cloned the Starch Branching Enzymes (SBE) IIa promoter region and the genomic clone of the Brittle-1 (Bt1) ADPglucose transporter. While Ae. crassa SBEIIa sequence was more divergent than that of Ae. tauschii’s compared to bread wheat, there were no sequence polymorphisms that would explain the observed expression differences in Bt1 between these species. Furthermore, while there were nucleotide differences between Bt1 in Ae. crassa and bread wheat, they were synonymous at the amino acid level. Some of transcriptional differences identified here, however, deserve further examination as part of a strategy to manipulate wheat starch and protein composition.

Identification of suitable reference genes for normalization of qPCR data in comparative transcriptomics analyses in the Triticeae

Comparative transcriptomics are useful to determine the role of orthologous genes among Triticeae species. Thus they constitute an interesting tool to improve the use of wild relatives for crop breeding. Reverse transcription quantitative real-time PCR (qPCR) is the most accurate measure of gene expression but efficient normalization is required. The choice and optimal number of reference genes must be experimentally determined and the primers optimized for cross-species amplification. Our goal was to test the utility of wheat-reference genes for qPCR normalization when species carrying the following genomes (A, B, D, R, H ( v ) and H ( ch )) are compared either simultaneously or in smaller subsets of samples. Wheat/barley/rye consensus primers outperformed wheat-specific ones which indicate that consensus primers should be considered for data normalization in comparative transcriptomics. All genes tested were stable but their ranking in terms of stability differed among subsets of samples. CDC (cell division control protein, AAA-superfamily of ATPases, Ta54227) and RLI (68 kDa protein HP68 similar to Arabidopsis thaliana RNase L inhibitor protein, Ta2776) were always among the three most stable genes. The optimal number of reference genes varied between 2 and 3 depending on the subset of samples and the method used (geNorm vs. coefficient of determination between sequential normalization factors). In any case a maximum number of three reference genes would provide adequate normalization independent of the subset of samples considered. This work constitutes a substantial advance towards comparative transcriptomics using qPCR since it provides useful primers/reference genes.

Histological and microarray analysis of the direct effect of water shortage alone or combined with heat on early grain development in wheat (Triticum aestivum)

Based on the in silico analysis of the representation of expressed sequence tags (ESTs) in wheat grain-related cDNA libraries, a specific 15k oligonucleotide microarray has been developed in order to monitor environmental stress-dependent gene expression changes in the wheat caryopses. Using this array, the effect of water withdrawal, with and without additional heat stress, has been investigated during the first five days of kernel development on two wheat cultivars differing in their drought sensitivity. Water shortage affected (more than twofold change) the expression of only 0.5% of the investigated genes. A parallel heat treatment increased the ratio of responding genes to 5-7% because of the temperature stress and/or the increased water deficit because of enhanced evaporation. It could be established that the two cultivars, differing in their long-term adaptation capabilities to drought, responded to the short and direct stress treatments on the same way. In response to the combined drought and heat treatment, the coordinately altered expression of genes coding for storage proteins, enzymes involved in sugar/starch metabolism, histone proteins, heat shock proteins, proteases, tonoplast aquaporins as well as several transcription factors has been observed. These gene expression changes were in agreement with histological data that demonstrated the accelerated development of the embryo as well as the endosperm.

Homoeolog-specific transcriptional bias in allopolyploid wheat

Background

Interaction between parental genomes is accompanied by global changes in gene expression which, eventually, contributes to growth vigor and the broader phenotypic diversity of allopolyploid species. In order to gain a better understanding of the effects of allopolyploidization on the regulation of diverged gene networks, we performed a genome-wide analysis of homoeolog-specific gene expression in re-synthesized allohexaploid wheat created by the hybridization of a tetraploid derivative of hexaploid wheat with the diploid ancestor of the wheat D genome Ae. tauschii.

Results

Affymetrix wheat genome arrays were used for both the discovery of divergent homoeolog-specific mutations and analysis of homoeolog-specific gene expression in re-synthesized allohexaploid wheat. More than 34,000 detectable parent-specific features (PSF) distributed across the wheat genome were used to assess AB genome (could not differentiate A and B genome contributions) and D genome parental expression in the allopolyploid transcriptome. In re-synthesized polyploid 81% of PSFs detected mid-parent levels of gene expression, and only 19% of PSFs showed the evidence of non-additive expression. Non-additive expression in both AB and D genomes was strongly biased toward up-regulation of parental type of gene expression with only 6% and 11% of genes, respectively, being down-regulated. Of all the non-additive gene expression, 84% can be explained by differences in the parental genotypes used to make the allopolyploid. Homoeolog-specific co-regulation of several functional gene categories was found, particularly genes involved in photosynthesis and protein biosynthesis in wheat.

Conclusions

Here, we have demonstrated that the establishment of interactions between the diverged regulatory networks in allopolyploids is accompanied by massive homoeolog-specific up- and down-regulation of gene expression. This study provides insights into interactions between homoeologous genomes and their role in growth vigor, development, and fertility of allopolyploid species.

Multilevel regulation and signalling processes associated with adaptation to terminal drought in wild emmer wheat

Low water availability is the major environmental factor limiting crop productivity. Transcriptome analysis was used to study terminal drought response in wild emmer wheat, Triticum dicoccoides, genotypes contrasting in their productivity and yield stability under drought stress. A total of 5,892 differentially regulated transcripts were identified between drought and well-watered control and/or between drought resistant (R) and drought susceptible (S) genotypes. Functional enrichment analyses revealed that multilevel regulatory and signalling processes were significantly enriched among the drought-induced transcripts, in particular in the R genotype. Therefore, further analyses were focused on selected 221 uniquely expressed or highly abundant transcripts in the R genotype, as potential candidates for drought resistance genes. Annotation of the 221 genes revealed that 26% of them are involved in multilevel regulation, including: transcriptional regulation, RNA binding, kinase activity and calcium and abscisic acid signalling implicated in stomatal closure. Differential expression patterns were also identified in genes known to be involved in drought adaptation pathways, such as: cell wall adjustment, cuticular wax deposition, lignification, osmoregulation, redox homeostasis, dehydration protection and drought-induced senescence. These results demonstrate the potential of wild emmer wheat as a source for candidate genes for improving drought resistance.

wDBTF: an integrated database resource for studying wheat transcription factor families

BACKGROUND

Transcription factors (TFs) regulate gene expression by interacting with promoters of their target genes and are classified into families based on their DNA-binding domains. Genes coding for TFs have been identified in the sequences of model plant genomes. The rice (Oryza sativa spp. japonica) genome contains 2,384 TF gene models, which represent the mRNA transcript of a locus, classed into 63 families.

RESULTS

We have created an extensive list of wheat (Triticum aestivum L) TF sequences based on sequence homology with rice TFs identified and classified in the Database of Rice Transcription Factors (DRTF). We have identified 7,112 wheat sequences (contigs and singletons) from a dataset of 1,033,960 expressed sequence tag and mRNA (ET) sequences available. This number is about three times the number of TFs in rice so proportionally is very similar if allowance is made for the hexaploidy of wheat. Of these sequences 3,820 encode gene products with a DNA-binding domain and thus were confirmed as potential regulators. These 3,820 sequences were classified into 40 families and 84 subfamilies and some members defined orphan families. The results were compiled in the Database of Wheat Transcription Factor (wDBTF), an inventory available on the web http://wwwappli.nantes.inra.fr:8180/wDBFT/. For each accession, a link to its library source and its Affymetrix identification number is provided. The positions of Pfam (protein family database) motifs were given when known.

CONCLUSIONS

wDBTF collates 3,820 wheat TF sequences validated by the presence of a DNA-binding domain out of 7,112 potential TF sequences identified from publicly available gene expression data. We also incorporated in silico expression data on these TFs into the database. Thus this database provides a major resource for systematic studies of TF families and their expression in wheat as illustrated here in a study of DOF family members expressed during seed development.

Comparative transcriptomics in the Triticeae

Background

Barley and particularly wheat are two grass species of immense agricultural importance. In spite of polyploidization events within the latter, studies have shown that genotypically and phenotypically these species are very closely related and, indeed, fertile hybrids can be created by interbreeding. The advent of two genome-scale Affymetrix GeneChips now allows studies of the comparison of their transcriptomes.

Results

We have used the Wheat GeneChip to create a "gene expression atlas" for the wheat transcriptome (cv. Chinese Spring). For this, we chose mRNA from a range of tissues and developmental stages closely mirroring a comparable study carried out for barley (cv. Morex) using the Barley1 GeneChip. This, together with large-scale clustering of the probesets from the two GeneChips into "homologous groups", has allowed us to perform a genomic-scale comparative study of expression patterns in these two species. We explore the influence of the polyploidy of wheat on the results obtained with the Wheat GeneChip and quantify the correlation between conservation in gene sequence and gene expression in wheat and barley. In addition, we show how the conservation of expression patterns can be used to elucidate, probeset by probeset, the reliability of the Wheat GeneChip.

Conclusion

While there are many differences in expression on the level of individual genes and tissues, we demonstrate that the wheat and barley transcriptomes appear highly correlated. This finding is significant not only because given small evolutionary distance between the two species it is widely expected, but also because it demonstrates that it is possible to use the two GeneChips for comparative studies. This is the case even though their probeset composition reflects rather different design principles as well as, of course, the present incomplete knowledge of the gene content of the two species. We also show that, in general, the Wheat GeneChip is not able to distinguish contributions from individual homoeologs. Furthermore, the comparison between the two species leads us to conclude that the conservation of both gene sequence as well as gene expression is positively correlated with absolute expression levels, presumably reflecting increased selection pressure on genes coding for proteins present at high levels. In addition, the results indicate the presence of a correlation between sequence and expression conservation within the Triticeae.

MicroRNA expression profiles in conventional and micropropagated strawberry (Fragaria x ananassa Duch.) plants

MicroRNAs (miRNAs) are a class of small non-coding RNAs which play a critical role in plant growth and development. To detect strawberry miRNAs and discover the expression difference between conventional and micropropagated strawberry plants, we carried out the detection and quantification of strawberry miRNAs by microarray. The main findings were that 74 miRNAs were checked in strawberry plants and four miRNA genes displayed clear expression difference between conventional and micropropagated strawberry plants, including two up-regulated genes (miR535 and miR390) and two down-regulated genes (miR169a and miR169d). The ratios of conventionally propagated strawberry plant/micropropagated strawberry plant for miR535, miR390, miR169a and miR169d were 2.6884, 2.2673, 0.2496 and 0.3814, respectively. Quantitative reverse transcription polymerase chain reaction applied to the two up-regulated genes (miR535 and miR390) validated the microarray result. This is the first report on differential expression of miRNAs in conventional and micropropagated plants.

Multiplex single nucleotide polymorphism (SNP)-based genotyping in allohexaploid wheat using padlock probes

Single nucleotide polymorphisms are the most common polymorphism in plant and animal genomes and, as such, are the logical choice for marker-assisted selection. However, many plants are also polyploid, and marker-assisted selection can be complicated by the presence of highly similar, but non-allelic, homoeologous sequences. Despite this, there is practical and academic demand for high-throughput genotyping in several polyploid crop species, such as allohexaploid wheat. In this paper, we present such a system, which utilizes public single nucleotide polymorphisms previously identified in both agronomically important genes and in randomly selected, mapped, expressed sequence tags developed by the wheat community. To achieve relatively high levels of multiplexing, we used non-amplified genomic DNA and padlock probe pairs, together with high annealing temperatures, to differentiate between similar sequences in the wheat genome. Our results suggest that padlock probes are capable of discriminating between homoeologous sequences and hence can be used to efficiently genotype wheat varieties.

The identification of candidate genes associated with Pch2 eyespot resistance in wheat using cDNA-AFLP

Eyespot is a fungal disease of the stem base of cereal crops and causes lodging and the premature ripening of grain. Wheat cultivar Cappelle Desprez contains a highly durable eyespot resistance gene, Pch2 on the long arm of chromosome 7A. A cDNA-amplified fragment length polymorphism (AFLP) platform was used to identify genes differentially expressed between the eyespot susceptible variety Chinese Spring (CS) and the CS chromosome substitution line Cappelle Desprez 7A (CS/CD7A) which contains Pch2. Induced and constitutive gene expression was examined to compare differences between non-infected and plants infected with Oculimacula acuformis. Only 34 of approximately 4,700 cDNA-AFLP fragments were differentially expressed between CS and CS/CD7A. Clones were obtained for 29 fragments, of which four had homology to proteins involved with plant defence responses. Fourteen clones mapped to chromosome 7A and three of these mapped in the region of Pch2 making them putative candidates for involvement in eyespot resistance. Of particular importance are two fragments; 4CD7A8 and 19CD7A4, which have homology to an Oryza sativa putative callose synthase protein and a putative cereal cyst nematode NBS-LRR disease resistance protein (RCCN) respectively. Differential expression associated with Pch2 was examined by semi-quantitative RT-PCR. Of those genes tested, only four were differentially expressed at 14 days post inoculation. We therefore suggest that a majority of the differences in the cDNA-AFLP profiles are due to allelic polymorphisms between CS and CD alleles rather than differences in expression.

Analysis of gene expression in resynthesized Brassica napus Allopolyploids using arabidopsis 70mer oligo microarrays

Background

Studies in resynthesized Brassica napus allopolyploids indicate that homoeologous chromosome exchanges in advanced generations (S_5∶6) alter gene expression through the loss and doubling of homoeologous genes within the rearrangements. Rearrangements may also indirectly affect global gene expression if homoeologous copies of gene regulators within rearrangements have differential affects on the transcription of genes in networks.

Methodology/Principal Findings

We utilized Arabidopsis 70mer oligonucleotide microarrays for exploring gene expression in three resynthesized B. napus lineages at the S_0∶1 and S_5∶6 generations as well as their diploid progenitors B. rapa and B. oleracea. Differential gene expression between the progenitors and additive (midparent) expression in the allopolyploids were tested. The S_5∶6 lines differed in the number of genetic rearrangements, allowing us to test if the number of genes displaying nonadditive expression was related to the number of rearrangements. Estimates using per-gene and common variance ANOVA models indicated that 6–15% of 26,107 genes were differentially expressed between the progenitors. Individual allopolyploids showed nonadditive expression for 1.6–32% of all genes. Less than 0.3% of genes displayed nonadditive expression in all S_0∶1 lines and 0.1–0.2% were nonadditive among all S_5∶6 lines. Differentially expressed genes in the polyploids were over-represented by genes differential between the progenitors. The total number of differentially expressed genes was correlated with the number of genetic changes in S_5∶6 lines under the common variance model; however, there was no relationship using a per-gene variance model, and many genes showed nonadditive expression in S_0∶1 lines.

Conclusions/Significance

Few genes reproducibly demonstrated nonadditive expression among lineages, suggesting few changes resulted from a general response to polyploidization. Furthermore, our microarray analysis did not provide strong evidence that homoeologous rearrangements were a determinant of genome-wide nonadditive gene expression. In light of the inherent limitations of the Arabidopsis microarray to measure gene expression in polyploid Brassicas, further studies are warranted.

Analysis of wheat SAGE tags reveals evidence for widespread antisense transcription

Background

Serial Analysis of Gene Expression (SAGE) is a powerful tool for genome-wide transcription studies. Unlike microarrays, it has the ability to detect novel forms of RNA such as alternatively spliced and antisense transcripts, without the need for prior knowledge of their existence. One limitation of using SAGE on an organism with a complex genome and lacking detailed sequence information, such as the hexaploid bread wheat Triticum aestivum, is accurate annotation of the tags generated. Without accurate annotation it is impossible to fully understand the dynamic processes involved in such complex polyploid organisms. Hence we have developed and utilised novel procedures to characterise, in detail, SAGE tags generated from the whole grain transcriptome of hexaploid wheat.

Results

Examination of 71,930 Long SAGE tags generated from six libraries derived from two wheat genotypes grown under two different conditions suggested that SAGE is a reliable and reproducible technique for use in studying the hexaploid wheat transcriptome. However, our results also showed that in poorly annotated and/or poorly sequenced genomes, such as hexaploid wheat, considerably more information can be extracted from SAGE data by carrying out a systematic analysis of both perfect and "fuzzy" (partially matched) tags. This detailed analysis of the SAGE data shows first that while there is evidence of alternative polyadenylation this appears to occur exclusively within the 3' untranslated regions. Secondly, we found no strong evidence for widespread alternative splicing in the developing wheat grain transcriptome. However, analysis of our SAGE data shows that antisense transcripts are probably widespread within the transcriptome and appear to be derived from numerous locations within the genome. Examination of antisense transcripts showing sequence similarity to the Puroindoline a and Puroindoline b genes suggests that such antisense transcripts might have a role in the regulation of gene expression.

Conclusion

Our results indicate that the detailed analysis of transcriptome data, such as SAGE tags, is essential to understand fully the factors that regulate gene expression and that such analysis of the wheat grain transcriptome reveals that antisense transcripts maybe widespread and hence probably play a significant role in the regulation of gene expression during grain development.

Single-species microarrays and comparative transcriptomics

BACKGROUND

Prefabricated expression microarrays are currently available for only a few species but methods have been proposed to extend their application to comparisons between divergent genomes.

METHODOLOGY/PRINCIPAL FINDINGS

Here we demonstrate that the hybridization intensity of genomic DNA is a poor basis on which to select unbiased probes on Affymetrix expression arrays for studies of comparative transcriptomics, and that doing so produces spurious results. We used the Affymetrix Xenopus laevis microarray to evaluate expression divergence between X. laevis, X. borealis, and their F1 hybrids. When data are analyzed with probes that interrogate only sequences with confirmed identity in both species, we recover results that differ substantially analyses that use genomic DNA hybridizations to select probes.

CONCLUSIONS/SIGNIFICANCE

Our findings have implications for the experimental design of comparative expression studies that use single-species microarrays, and for our understanding of divergent expression in hybrid clawed frogs. These findings also highlight important limitations of single-species microarrays for studies of comparative transcriptomics of polyploid species.

Partitioned expression of duplicated genes during development and evolution of a single cell in a polyploid plant

Polyploidy is an important driver of eukaryotic evolution, evident in many animals, fungi, and plants. One consequence of polyploidy is subfunctionalization, in which the ancestral expression profile becomes partitioned among duplicated genes (termed homoeologs). Subfunctionalization appears to be a common phenomenon insofar as it has been studied, at the scale of organs. Here, we use a high-resolution methodology to investigate the expression of thousands of pairs of homoeologs during the development of a single plant cell, using as a model the seed trichomes ("cotton fiber") of allopolyploid (containing "A" and "D" genomes) cotton (Gossypium). We demonstrate that approximately 30% of the homoeologs are significantly A- or D-biased at each of three time points studied during fiber development. Genes differentially biased toward the A or D genome belong to different biological processes, illustrating the functional partitioning of genomic contributions during cellular development. Interestingly, expression of the biased genes was shifted strongly toward the agronomically inferior D genome. Analyses of homoeologous gene expression during development of this cell showed that one-fifth of the genes exhibit changes in A/D ratios, indicating that significant alteration in duplicated gene expression is fairly frequent even at the level of development and maturation of a single cell. Comparing changes in homoeolog expression in cultivated versus wild cotton showed that most homoeolog expression bias reflects polyploidy rather than domestication. Evidence suggests, however, that domestication may increase expression bias in fibers toward the D genome, potentially implicating D-genome recruitment under human selection during domestication.

Transcriptome analysis of grain development in hexaploid wheat

Background

Hexaploid wheat is one of the most important cereal crops for human nutrition. Molecular understanding of the biology of the developing grain will assist the improvement of yield and quality traits for different environments. High quality transcriptomics is a powerful method to increase this understanding.

Results

The transcriptome of developing caryopses from hexaploid wheat (Triticum aestivum, cv. Hereward) was determined using Affymetrix wheat GeneChip^® oligonucleotide arrays which have probes for 55,052 transcripts. Of these, 14,550 showed significant differential regulation in the period between 6 and 42 days after anthesis (daa). Large changes in transcript abundance were observed which were categorised into distinct phases of differentiation (6–10 daa), grain fill (12–21 daa) and desiccation/maturation (28–42 daa) and were associated with specific tissues and processes. A similar experiment on developing caryopses grown with dry and/or hot environmental treatments was also analysed, using the profiles established in the first experiment to show that most environmental treatment effects on transcription were due to acceleration of development, but that a few transcripts were specifically affected. Transcript abundance profiles in both experiments for nine selected known and putative wheat transcription factors were independently confirmed by real time RT-PCR. These expression profiles confirm or extend our knowledge of the roles of the known transcription factors and suggest roles for the unknown ones.

Conclusion

This transcriptome data will provide a valuable resource for molecular studies on wheat grain. It has been demonstrated how it can be used to distinguish general developmental shifts from specific effects of treatments on gene expression and to diagnose the probable tissue specificity and role of transcription factors.