Analysis of the repetitive component and retrotransposon population in the genome of a marine angiosperm, Posidonia oceanica (L.) Delile

Characterisation of LTR-Retrotransposons of Stevia rebaudiana and Their Use for the Analysis of Genetic Variability

Stevia rebaudiana is one of the most important crops belonging to the Asteraceae family. Stevia is cultivated all over the world as it represents a valid natural alternative to artificial sweeteners thanks to its leaves, which produce steviol glycosides that have high sweetening power and reduced caloric value. In this work, the stevia genome sequence was used to isolate and characterise full-length long-terminal repeat retrotransposons (LTR-REs), which account for more than half of the genome. The Gypsy retrotransposons were twice as abundant as the Copia ones. A disproportionate abundance of elements belonging to the Chromovirus/Tekay lineage was observed among the Gypsy elements. Only the SIRE and Angela lineages represented significant portions of the genome among the Copia elements. The dynamics with which LTR-REs colonised the stevia genome were also estimated; all isolated full-length elements turned out to be relatively young, with a proliferation peak around 1–2 million years ago. However, a different analysis conducted by comparing sequences encoding retrotranscriptase showed the occurrence of an older period in which there was a lot of LTR-RE proliferation. Finally, a group of isolated full-length elements belonging to the lineage Angela was used to analyse the genetic variability in 25 accessions of S. rebaudiana using the Inter-Retrotransposon Amplified Polymorphism (IRAP) protocol. The obtained fingerprints highlighted a high degree of genetic variability and were used to study the genomic structures of the different accessions. It was hypothesised that there are four ancestral subpopulations at the root of the analysed accessions, which all turned out to be admixed. Overall, these data may be useful for genome sequence annotations and for evaluating genetic variability in this species, which may be useful in stevia breeding.

The Singular Evolution of Olea Genome Structure

The current view of plant genome evolution proposes that genome size has mainly been determined by polyploidisation and amplification/loss of transposons, with a minor role played by other repeated sequences, such as tandem repeats. In cultivated olive (Olea europaea subsp. europaea var. europaea), available data suggest a singular model of genome evolution, in which a massive expansion of tandem-repeated sequences accompanied changes in nuclear architecture. This peculiar scenario highlights the importance of focusing on Olea genus evolution, to shed light on mechanisms that led to its present genomic structure. Next-generation sequencing technologies, bioinformatics and in situ hybridisation were applied to study the genomic structure of five related Olea taxa, which originated at different times from their last common ancestor. On average, repetitive DNA in the Olea taxa ranged from ~59% to ~73% of the total genome, showing remarkable differences in terms of composition. Among repeats, we identified 11 major families of tandem repeats, with different abundances in the analysed taxa, five of which were novel discoveries. Interestingly, overall tandem repeat abundance was inversely correlated to that of retrotransposons. This trend might imply a competition in the proliferation of these repeat classes. Indeed, O. paniculata, the species closest to the Olea common ancestor, showed very few tandem-repeated sequences, while it was rich in long terminal repeat retrotransposons, suggesting that the amplification of tandem repeats occurred after its divergence from the Olea ancestor. Furthermore, some tandem repeats were physically localised in closely related O. europaea subspecies (i.e., cultivated olive and O. europaea subsp. cuspidata), which showed a significant difference in tandem repeats abundance. For 4 tandem repeats families, a similar number of hybridisation signals were observed in both subspecies, apparently indicating that, after their dissemination throughout the olive genome, these tandem repeats families differentially amplified maintaining the same positions in each genome. Overall, our research identified the temporal dynamics shaping genome structure during Olea speciation, which represented a singular model of genome evolution in higher plants.

Phenotypic plasticity under rapid global changes: The intrinsic force for future seagrasses survival

Coastal oceans are particularly affected by rapid and extreme environmental changes with dramatic consequences for the entire ecosystem. Seagrasses are key ecosystem engineering or foundation species supporting diverse and productive ecosystems along the coastline that are particularly susceptible to fast environmental changes. In this context, the analysis of phenotypic plasticity could reveal important insights into seagrasses persistence, as it represents an individual property that allows species' phenotypes to accommodate and react to fast environmental changes and stress. Many studies have provided different definitions of plasticity and related processes (acclimation and adaptation) resulting in a variety of associated terminology. Here, we review different ways to define phenotypic plasticity with particular reference to seagrass responses to single and multiple stressors. We relate plasticity to the shape of reaction norms, resulting from genotype by environment interactions, and examine its role in the presence of environmental shifts. The potential role of genetic and epigenetic changes in underlying seagrasses plasticity in face of environmental changes is also discussed. Different approaches aimed to assess local acclimation and adaptation in seagrasses are explored, explaining strengths and weaknesses based on the main results obtained from the most recent literature. We conclude that the implemented experimental approaches, whether performed with controlled or field experiments, provide new insights to explore the basis of plasticity in seagrasses. However, an improvement of molecular analysis and the application of multi-factorial experiments are required to better explore genetic and epigenetic adjustments to rapid environmental shifts. These considerations revealed the potential for selecting the best phenotypes to promote assisted evolution with fundamental implications on restoration and preservation efforts.

Low Long Terminal Repeat (LTR)-Retrotransposon Expression in Leaves of the Marine Phanerogam Posidonia Oceanica L

Seagrasses as Posidonia oceanica reproduce mostly by vegetative propagation, which can reduce genetic variability within populations. Since, in clonally propagated species, insurgence of genetic variability can be determined by the activity of transposable elements, we have estimated the activity of such repeat elements by measuring their expression level in the leaves of plants from a Mediterranean site, for which Illumina complementary DNA (cDNA) sequence reads (produced from RNAs isolated by leaves of plants from deep and shallow meadows) were publicly available. Firstly, we produced a collection of retrotransposon-related sequences and then mapped Illumina cDNA reads onto these sequences. With this approach, it was evident that Posidonia retrotransposons are, in general, barely expressed; only nine elements resulted transcribed at levels comparable with those of reference genes encoding tubulins and actins. Differences in transcript abundance were observed according to the superfamily and the lineage to which the retrotransposons belonged. Only small differences were observed between retrotransposon expression levels in leaves of shallow and deep Posidonia meadow stands, whereas one TAR/Tork element resulted differentially expressed in deep plants exposed to heat. It can be concluded that, in P. oceanica, the contribution of retrotransposon activity to genetic variability is reduced, although the nine specific active elements could actually produce new structural variations.

A computational genome-wide analysis of long terminal repeats retrotransposon expression in sunflower roots (Helianthus annuus L.)

Long terminal repeats (LTR) retrotransposons have a major role in determining genome size, structure and function, thanks to their ability to transpose. We performed a meta-analysis of LTR-retrotransposon expression in roots of sunflower plantlets treated with different plant hormones, chemicals and NaCl. By using Illumina cDNA libraries, available from public repositories, we measured the number of reads matching the retrotranscriptase domains isolated from a whole genome library of retrotransposons. LTR-retrotransposons resulted in general barely expressed, except for 4 elements, all belonging to the AleII lineage, which showed high transcription levels in roots of both control and treated plants. The expression of retrotransposons in treated plants was slightly higher than in the control. Transcribed elements belonged to specific chromosomal loci and were not abundant in the genome. A few elements resulted differentially expressed depending on the treatment. Results suggest that, although most retrotransposons are not expressed, the transcription of such elements is related to their abundance, to their position in the chromosome and to their lineage.

Genome size, chromosome number determination, and analysis of the repetitive elements in Cissus quadrangularis

Cissus quadrangularis (Vitaceae) is a perennial climber endemic to Africa and is characterized by succulent angular stems. The plant grows in arid and semi-arid regions of Africa especially in the African savanna. The stem of C. quadrangularis has a wide range of applications in both human and animal medicine, but there is limited cytogenetic information available for this species. In this study, the chromosome number, genome size, and genome composition for C. quadrangularis were determined. Flow cytometry results indicated that the genome size of C. quadrangularis is approximately 2C = 1.410 pg. Fluorescence microscopy combined with DAPI stain showed the chromosome numbers to be 2n = 48. It is likely that C. quadrangularis has a tetraploid genome after considering the basic chromosome numbers in Cissus genus (n = 10, 11, or 12). A combination of low-throughput genome sequencing and bioinformatics analysis allowed identification and quantification of repetitive elements that make up about 52% of the C. quadrangularis genome, which was dominated by LTR-retrotransposons. Two LTR superfamilies were identified as Copia and Gypsy, with 24% and 15% of the annotated clusters, respectively. The comparison of repeat elements for C. quadrangularis, Vitis vinifera, and four other selected members in the Cissus genus revealed a high diversity in the repetitive element components, which could suggest recent amplification events in the Cissus genus. Our data provides a platform for further studies on the phylogeny and karyotype evolution in this genus and in the family Vitaceae.

Arbuscular mycorrhizal fungi induce the expression of specific retrotransposons in roots of sunflower (Helianthus annuus L.)

Retrotransposon expression during arbuscular mycorrhizal (AM) fungal colonisation of sunflower roots (Helianthus annuus) was analysed using Illumina RNA-Seq, in order to verify whether mycorrhizal symbiosis can activate retrotransposable elements. Illumina cDNA libraries were produced from RNAs isolated from the roots of sunflower plants at 4 and 16 days after inoculation with the AM fungus Rhizoglomus irregulare and from their respective control plants. Illumina reads were mapped to a library of reverse transcriptase-encoding sequences, putatively belonging to long terminal repeat retrotransposons of Gypsy and Copia superfamilies. Forty-six different reverse transcriptase sequences were transcribed, although at a low rate, in mycorrhizal or control roots and only four were significantly over-expressed at day 16, compared with control roots. Almost all expressed or over-expressed sequences belonged to low-copy elements, mostly, of the Copia superfamily. A meta-analysis, using publicly available Illumina cDNA libraries obtained from sunflower plants treated with different hormones and chemicals, mimicking stimuli produced by abiotic and biotic stresses, was also conducted. Such analyses indicated that the four reverse transcriptase sequences over-expressed in mycorrhizal roots were explicitly induced only by AM symbiosis, showing the specificity of AM stimuli compared to that of other fungal/plant interactions.

Ligulate inflorescence of Helianthus × multiflorus, cv. Soleil d'Or, correlates with a mis-regulation of a CYCLOIDEA gene characterised by insertion of a transposable element

Members of CYCLOIDEA (CYC)/TEOSINTE BRANCHED1 (TB1) transcription factor family are essential to control flower symmetry and inflorescence architecture. In the Helianthus annuus genome, ten CYC/TB1 genes have been identified. Studies performed on mutants recognised HaCYC2c as one of the key players controlling zygomorphism in sunflower. We identified CYC2c genes in the diploid Helianthus decapetalus (HdCYC2c) and in the interspecific hybrid Helianthus × multiflorus (H × mCYC2cA and H × mCYC2cB), a triploid (2n = 3× = 51), originated from unreduced eggs of H. decapetalus fertilised by reduced H. annuus male gametes. Phylogenetic analysis showed that HdCYC2c and H × mCYC2c were placed within a CYC2 subclade together with HaCYC2c but distinct from it. The present data showed that in H. × multiflorus the allele derived from H. annuus is deleted or highly modified. The H. × multiflorus taxon exists as radiate and ligulate inflorescence types. We analysed CYC2c expression in H. decapetalus and in the cultivar 'Soleil d'Or' of H. × multiflorus, a ligulate inflorescence type with actinomorphic corolla of disk flowers transformed into a zygomorphic ray-like corolla. In H. decapetalus, the HdCYC2c gene showed differential expression between developing flower types, being up-regulated in the corolla of ray flowers in comparison to the disk flower corolla. In H. × multiflorus, an insertion of 865 bp, which is part of a CACTA transposable element, was found in the 5'-untranslated region (5'-UTR) of H × mCYC2cB. This insertion could promote, even with epigenetic mechanisms, ectopic expression of the gene throughout the inflorescence, resulting in the observed loss of actinomorphy and originating a ligulate head.

Different histories of two highly variable LTR retrotransposons in sunflower species

In the Helianthus genus, very large intra- and interspecific variability related to two specific retrotransposons of Helianthus annuus (Helicopia and SURE) exists. When comparing these two sequences to sunflower sequence databases recently produced by our lab, the Helicopia family was shown to belong to the Maximus/SIRE lineage of the Sirevirus genus of the Copia superfamily, whereas the SURE element (whose superfamily was not even previously identified) was classified as a Gypsy element of the Ogre/Tat lineage of the Metavirus genus. Bioinformatic analysis of the two retrotransposon families revealed their genomic abundance and relative proliferation timing. The genomic abundance of these families differed significantly among 12 Helianthus species. The ratio between the abundance of long terminal repeats and their reverse transcriptases suggested that the SURE family has relatively more solo long terminal repeats than does Helicopia. Pairwise comparisons of Illumina reads encoding the reverse transcriptase domain indicated that SURE amplification may have occurred more recently than that of Helicopia. Finally, the analysis of population structure based on the SURE and Helicopia polymorphisms of 32 Helianthus species evidenced two subpopulations, which roughly corresponded to species of the Helianthus and Divaricati/Ciliares sections. However, a number of species showed an admixed structure, confirming the importance of interspecific hybridisation in the evolution of this genus. In general, these two retrotransposon families differentially contributed to interspecific variability, emphasising the need to refer to specific families when studying genome evolution.

Identification and characterisation of Short Interspersed Nuclear Elements in the olive tree (Olea europaea L.) genome

Short Interspersed Nuclear Elements (SINEs) are nonautonomous retrotransposons in the genome of most eukaryotic species. While SINEs have been intensively investigated in humans and other animal systems, SINE identification has been carried out only in a limited number of plant species. This lack of information is apparent especially in non-model plants whose genome has not been sequenced yet. The aim of this work was to produce a specific bioinformatics pipeline for analysing second generation sequence reads of a non-model species and identifying SINEs. We have identified, for the first time, 227 putative SINEs of the olive tree (Olea europaea), that constitute one of the few sets of such sequences in dicotyledonous species. The identified SINEs ranged from 140 to 362 bp in length and were characterised with regard to the occurrence of the tRNA domain in their sequence. The majority of identified elements resulted in single copy or very lowly repeated, often in association with genic sequences. Analysis of sequence similarity allowed us to identify two major groups of SINEs showing different abundances in the olive tree genome, the former with sequence similarity to SINEs of Scrophulariaceae and Solanaceae and the latter to SINEs of Salicaceae. A comparison of sequence conservation between olive SINEs and LTR retrotransposon families suggested that SINE expansion in the genome occurred especially in very ancient times, before LTR retrotransposon expansion, and presumably before the separation of the rosids (to which Oleaceae belong) from the Asterids. Besides providing data on olive SINEs, our results demonstrate the suitability of the pipeline employed for SINE identification. Applying this pipeline will favour further structural and functional analyses on these relatively unknown elements to be performed also in other plant species, even in the absence of a reference genome, and will allow establishing general evolutionary patterns for this kind of repeats in plants.