An RNA-seq transcriptome analysis of floral buds of an interspecific Brassica hybrid between B. carinata and B. napus

Hybridization and gene expression: Beyond differentially expressed genes

Gene expression has a key role in reproductive isolation, and studies of hybrid gene expression have identified mechanisms causing hybrid sterility. Here, we review the evidence for altered gene expression following hybridization and outline the mechanisms shown to contribute to altered gene expression in hybrids. Transgressive gene expression, transcending that of both parental species, is pervasive in early generation sterile hybrids, but also frequently observed in viable, fertile hybrids. We highlight studies showing that hybridization can result in transgressive gene expression, also in established hybrid lineages or species. Such extreme patterns of gene expression in stabilized hybrid taxa suggest that altered hybrid gene expression may result in hybridization-derived evolutionary novelty. We also conclude that while patterns of misexpression in hybrids are well documented, the understanding of the mechanisms causing misexpression is lagging. We argue that jointly assessing differences in cell composition and cell-specific changes in gene expression in hybrids, in addition to assessing changes in chromatin and methylation, will significantly advance our understanding of the basis of altered gene expression. Moreover, uncovering to what extent evolution of gene expression results in altered expression for individual genes, or entire networks of genes, will advance our understanding of how selection moulds gene expression. Finally, we argue that jointly studying the dual roles of altered hybrid gene expression, serving both as a mechanism for reproductive isolation and as a substrate for hybrid ecological adaptation, will lead to significant advances in our understanding of the evolution of gene expression.

Cytological and transcriptomic analyses provide insights into the pollen fertility of synthetic allodiploid Brassica juncea hybrids

KEY MESSAGE

Imbalanced chromosomes and cell cycle arrest, along with down-regulated genes in DNA damage repair and sperm cell differentiation, caused pollen abortion in synthetic allodiploid Brassica juncea hybrids. Interspecific hybridization is considered to be a major pathway for species formation and evolution in angiosperms, but the occurrence of pollen abortion in the hybrids is common, prompting us to recheck male gamete development in allodiploid hybrids after the initial combination of different genomes. Here, we investigated the several key meiotic and mitotic events during pollen development using the newly synthesised allodiploid B. juncea hybrids (AB, 2n = 2× = 18) as a model system. Our results demonstrated the partial synapsis and pairing of non-homologous chromosomes concurrent with chaotic spindle assembly, affected chromosome assortment and distribution during meiosis, which finally caused difference in genetic constitution amongst the final tetrads. The mitotic cell cycle arrest during microspore development resulted in the production of anucleate pollen cells. Transcription analysis showed that sets of key genes regulating cyclin (CYCA1;2 and CYCA2;3), DNA damage repair (DMC1, NBS1 and MMD1), and ubiquitin-proteasome pathway (SINAT4 and UBC) were largely downregulated at the early pollen meiosis stages, and those genes involved in sperm cell differentiation (DUO1, PIRL1, PIRL9 and LBD27) and pollen wall synthesis (PME48, VGDH11 and COBL10) were mostly repressed at the late pollen mitosis stages in the synthetic allodiploid B. juncea hybrids (AB). In conclusion, this study elucidated the related mechanisms affecting pollen fertility during male gametophyte development at the cytological and transcriptomic levels in the synthetic allodiploid B. juncea hybrids.

Genomic selection and genetic architecture of agronomic traits during modern rapeseed breeding

Rapeseed (Brassica napus L.) is an important oil-producing crop for the world. Its adaptation, yield and quality have been considerably improved in recent decades, but the genomic basis underlying successful breeding selection remains unclear. Hence, we conducted a comprehensive genomic assessment of rapeseed in the breeding process based on the whole-genome resequencing of 418 diverse rapeseed accessions. We unraveled the genomic basis for the selection of adaptation and agronomic traits. Genome-wide association studies identified 628 associated loci-related causative candidate genes for 56 agronomically important traits, including plant architecture and yield traits. Furthermore, we uncovered nonsynonymous mutations in plausible candidate genes for agronomic traits with significant differences in allele frequency distributions across the improvement process, including the ribosome recycling factor (BnRRF) gene for seed weight. This study provides insights into the genomic basis for improving rapeseed varieties and a valuable genomic resource for genome-assisted rapeseed breeding.

Development of a relevant strategy using de novo transcriptome assembly method for transcriptome comparisons between Muscovy and common duck species and their reciprocal inter-specific mule and hinny hybrids fed ad libitum and overfed

Background

Common Pekin and Muscovy ducks and their intergeneric hinny and mule hybrids have different abilities for fatty liver production. RNA-Seq analyses from the liver of these different genetic types fed ad libitum or overfed would help to identify genes with different response to overfeeding between them. However RNA-seq analyses from different species and comparison is challenging. The goal of this study was develop a relevant strategy for transcriptome analysis and comparison between different species.

Results

Transcriptomes were first assembled with a reference-based approach. Important mapping biases were observed when heterologous mapping were conducted on common duck reference genome, suggesting that this reference-based strategy was not suited to compare the four different genetic types. De novo transcriptome assemblies were then performed using Trinity and Oases. Assemblies of transcriptomes were not relevant when more than a single genetic type was considered. Finally, single genetic type transcriptomes were assembled with DRAP in a mega-transcriptome. No bias was observed when reads from the different genetic types were mapped on this mega-transcriptome and differences in gene expression between the four genetic types could be identified.

Conclusions

Analyses using both reference-based and de novo transcriptome assemblies point out a good performance of the de novo approach for the analysis of gene expression in different species. It also allowed the identification of differences in responses to overfeeding between Pekin and Muscovy ducks and hinny and mule hybrids.

Transcriptome analysis reveals hybridization-induced genome shock in an interspecific F1 hybrid from Camellia

The combination of two divergent genomes during hybridization can result in "genome shock". Although genome shock has been reported in the hybrids of some herbaceous plants, the pattern and the principle it follows are far from understood, especially in woody plants. Here, the gene expression patterns were remodeled in the F₁ hybrid from the crossing of Camellia azalea × Camellia amplexicaulis compared with the parents as revealed by RNA-seq. About 54.5% of all unigenes were differentially expressed between the F₁ hybrid and at least one of the parents, including 6404 unigenes with the highest expression level in the F₁ hybrid. A series of genes, related to flower development, essential for RNA-directed DNA methylation and histone methylation, as well as 223 transposable elements, were enriched; and most of them exhibited a higher level of expression in the F₁ hybrid. These results indicated that the genome shock induced by interspecific hybridization in Camellia could indeed result in changes of gene expression patterns, potentially through regulating DNA methylation and histone methylation which may be helpful for the maintaining of genome stability and even related to the unique phenotype of the F₁ hybrid.