Corinne Mhiri

Transposable elements (TEs) are important components of most plant genomes. These mobile repetitive sequences are highly diverse in terms of abundance, structure, transposition mechanisms, activity and insertion specificities across plant species. This review will survey the different mechanisms that may explain the variability of TE patterns in land plants, highlighting the tight connection between TE dynamics and host genome specificities, and their co-evolution to face and adapt to a changing environment. We present the current TE classification in land plants, and describe the different levels of genetic and epigenetic controls originating from the plant, the TE itself, or external environmental factors. Such overlapping mechanisms of TE regulation might be responsible for the high diversity and dynamics of plant TEs observed in nature.

During seed germination, proteins are translated not only from mRNAs newly transcribed upon imbibition but also from long-lived mRNAs that are synthesized during seed maturation and stored in the mature dry seeds. To clarify the distinct roles of proteins translated from long-lived mRNAs and de novo transcribed mRNAs in germinating rice embryos, proteome analysis based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) combining the use of a transcriptional inhibitor was performed. We observed that α-amanitin significantly represses transcription in germinating embryos; nevertheless, the embryos could germinate, albeit slowly. The proteomic analysis revealed that a total of 109 proteins were translated from long-lived mRNAs associated with germination as well as 222 proteins whose expression were dependent on de novo transcription upon imbibition. Transcriptomic datasets available in public databases demonstrated that mRNAs of the 222 proteins notably increased during germination while those of the 109 proteins highly accumulated in dry embryos and constitutively expressed upon imbibition. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that many of the 109 proteins from long-lived mRNAs are implicated in energy production such as glycolysis or annotated as nucleotide binding proteins, while the 222 proteins are involved in pathways such as pyruvate metabolism and TCA cycle following glycolysis, and momilactones biosynthesis. We propose that long-lived mRNAs support initial energy production and activation of translational machinery upon imbibition whereas de novo transcription accelerates the energy production after glycolysis, which enables rice seeds to germinate vigorously.

Summary: The genomic shock hypothesis suggests that allopolyploidy is associated with genome changes driven by transposable elements, as a response to imbalances between parental insertion loads. To explore this hypothesis, we compared three allotetraploids, Nicotiana arentsii, N. rustica and N. tabacum, which arose over comparable time frames from hybridisation between increasingly divergent diploid species. We used sequence‐specific amplification polymorphism (SSAP) to compare the dynamics of six transposable elements in these allopolyploids, their diploid progenitors and in corresponding synthetic hybrids. We show that element‐specific dynamics in young Nicotiana allopolyploids reflect their dynamics in diploid progenitors. Transposable element mobilisation is not concomitant with immediate genome merger, but occurs within the first generations of allopolyploid formation. In natural allopolyploids, such mobilisations correlate with imbalances in the repeat profile of the parental species, which increases with their genetic divergence. Other restructuring leading to locus loss is immediate, nonrandom and targeted at specific subgenomes, independently of cross orientation. The correlation between transposable element mobilisation in allopolyploids and quantitative imbalances in parental transposable element loads supports the genome shock hypothesis proposed by McClintock.

Initial characterization of the flax genome showed that Ty1-copia retrotransposons are abundant, with several members being recently inserted, and in close association with genes. Recent insertions indicate a potential for ongoing transpositional activity that can create genomic diversity among accessions, cultivars or varieties. The polymorphisms generated constitute a good source of molecular markers that may be associated with phenotype if the insertions alter gene activity. Flax, where accessions are bred mainly for seed nutritional properties or for fibers, constitutes a good model for studying the relationship of transpositional activity with diversification and breeding. In this study, we estimated copy number and used a type of transposon display known as Sequence-Specific Amplification Polymorphisms (SSAPs), to characterize six families of Ty1-copia elements across 14 flax accessions. Polymorphic insertion sites were sequenced to find insertions that could potentially alter gene expression, and a preliminary test was performed with selected genes bearing transposable element (TE) insertions. Quantification of six families of Ty1-copia elements indicated different abundances among TE families and between flax accessions, which suggested diverse transpositional histories. SSAPs showed a high level of polymorphism in most of the evaluated retrotransposon families, with a trend towards higher levels of polymorphism in low-copy number families. Ty1-copia insertion polymorphisms among cultivars allowed a general distinction between oil and fiber types, and between spring and winter types, demonstrating their utility in diversity studies. Characterization of polymorphic insertions revealed an overwhelming association with genes, with insertions disrupting exons, introns or within 1 kb of coding regions. A preliminary test on the potential transcriptional disruption by TEs of four selected genes evaluated in three different tissues, showed one case of significant impact of the insertion on gene expression. We demonstrated that specific Ty1-copia families have been active since breeding commenced in flax. The retrotransposon-derived polymorphism can be used to separate flax types, and the close association of many insertions with genes defines a good source of potential mutations that could be associated with phenotypic changes, resulting in diversification processes.

Summary: Because of its highly efficient homologous recombination, the moss Physcomitrella patens is a model organism particularly suited for reverse genetics, but this inherent characteristic limits forward genetic approaches. Here, we show that the tobacco (Nicotiana tabacum) retrotransposon Tnt1 efficiently transposes in P. patens, being the first retrotransposon from a vascular plant reported to transpose in a bryophyte. Tnt1 has a remarkable preference for insertion into genic regions, which makes it particularly suited for gene mutation. In order to stabilize Tnt1 insertions and make it easier to select for insertional mutants, we have developed a two‐component system where a mini‐Tnt1 with a retrotransposition selectable marker can only transpose when Tnt1 proteins are co‐expressed from a separate expression unit. We present a new tool with which to produce insertional mutants in P. patens in a rapid and straightforward manner that complements the existing molecular and genetic toolkit for this model species.

Background and Aims Speciation in angiosperms can be accompanied by changes in floral colour that may influence pollinator preference and reproductive isolation. This study investigates whether changes in floral colour can accompany polyploid and homoploid hybridization, important processes in angiosperm evolution. Methods Spectral reflectance of corolla tissue was examined for 60 Nicotiana (Solanaceae) accessions (41 taxa) based on spectral shape (corresponding to pigmentation) as well as bee and hummingbird colour perception in order to assess patterns of floral colour evolution. Polyploid and homoploid hybrid spectra were compared with those of their progenitors to evaluate whether hybridization has resulted in floral colour shifts. Key Results Floral colour categories in Nicotiana seem to have arisen multiple times independently during the evolution of the genus. Most younger polyploids displayed an unexpected floral colour, considering those of their progenitors, in the colour perception of at least one pollinator type, whereas older polyploids tended to resemble one or both of their progenitors. Conclusions Floral colour evolution in Nicotiana is weakly constrained by phylogeny, and colour shifts do occur in association with both polyploid and homoploid hybrid divergence. Transgressive floral colour in N. tabacum has arisen by inheritance of anthocyanin pigmentation from its paternal progenitor while having a plastid phenotype like its maternal progenitor. Potentially, floral colour evolution has been driven by, or resulted in, pollinator shifts. However, those polyploids that are not sympatric (on a regional scale) with their progenitor lineages are typically not divergent in floral colour from them, perhaps because of a lack of competition for pollinators.

Evidence accumulated over the last decade has shown that allopolyploid genomes may undergo drastic reorganization. However, timing and mechanisms of structural diploidization over evolutionary timescales are still poorly known. As transposable elements (TEs) represent major and labile components of plant genomes, they likely play a pivotal role in fuelling genome changes leading to long-term diploidization. Here, we exploit the 4.5 MY old allopolyploid Nicotiana section Repandae to investigate the impact of TEs on the evolutionary dynamics of genomes. Sequence-specific amplified polymorphisms (SSAP) on seven TEs with expected contrasted dynamics were used to survey genome-wide TE insertion polymorphisms. Comparisons of TE insertions in the four allopolyploid species and descendents of the diploid species most closely related to their actual progenitors revealed that the polyploids showed considerable departure from predicted additivity of the diploids. Large numbers of new SSAP bands were observed in polyploids for two TEs, but restructuring for most TE families involved substantial loss of fragments relative to the genome of the diploid representing the paternal progenitor, which could be due to changes in allopolyploids, diploid progenitor lineages or both. The majority of non-additive bands were shared by all polyploid species, suggesting that significant restructuring occurred early after the allopolyploid event that gave rise to their common ancestor. Furthermore, several gains and losses of SSAP fragments were restricted to N. repanda, suggesting a unique evolutionary trajectory. This pattern of diploidization in TE genome fractions supports the hypothesis that TEs are central to long-term genome turnover and depends on both TE and the polyploid lineage considered.

Transposable elements are the major component of the maize genome and presumably highly polymorphic yet they have not been used in population genetics and association analyses. Using the Transposon Display method, we isolated and converted into PCR-based markers 33 Miniature Inverted Repeat Transposable Elements (MITE) polymorphic insertions. These polymorphisms were genotyped on a population-based sample of 26 American landraces for a total of 322 plants. Genetic diversity was high and partitioned within and among landraces. The genetic groups identified using Bayesian clustering were in agreement with published data based on SNPs and SSRs, indicating that MITE polymorphisms reflect maize genetic history. To explore the contribution of MITEs to phenotypic variation, we undertook an association mapping approach in a panel of 367 maize lines phenotyped for 26 traits. We found a highly significant association between the marker ZmV1-9, on chromosome 1, and male flowering time. The variance explained by this association is consistent with a flowering delay of +123 degree-days. This MITE insertion is located at only 289 nucleotides from the 3′ end of a Cytochrome P450-like gene, a region that was never identified in previous association mapping or QTL surveys. Interestingly, we found (i) a non-synonymous mutation located in the exon 2 of the gene in strong linkage disequilibrium with the MITE polymorphism, and (ii) a perfect sequence homology between the MITE sequence and a maize siRNA that could therefore potentially interfere with the expression of the Cytochrome P450-like gene. Those two observations among others offer exciting perspectives to validate functionally the role of this region on phenotypic variation.