An Atlas of Plant Transposable Elements

Advances in genomic sequencing have recently offered vast opportunities for biological exploration, unraveling the evolution and improving our understanding of Earth biodiversity. Due to distinct plant species characteristics in terms of genome size, ploidy and heterozygosity, transposable elements (TEs) are common characteristics of many genomes. TEs are ubiquitous and dispersed repetitive DNA sequences that frequently impact the evolution and composition of the genome, mainly due to their redundancy and rearrangements. For this study, we provided an atlas of TE data by employing an easy-to-use portal (APTE website). To our knowledge, this is the most extensive and standardized analysis of TEs in plant genomes. We evaluated 67 plant genomes assembled at chromosome scale, recovering a total of 49,802,023 TE records, representing a total of 47,992,091,043 (~47,62%) base pairs (bp) of the total genomic space. We observed that new types of TEs were identified and annotated compared to other data repositories. By establishing a standardized catalog of TE annotation on 67 genomes, new hypotheses, exploration of TE data and their influences on the genomes may allow a better understanding of their function and processes. All original code and an example of how we developed the TE annotation strategy is available on GitHub (Extended data).

Blm Helicase Facilitates Rapid Replication of Repetitive DNA Sequences in early Drosophila Development

The absence of functional BLM DNA helicase, a member of the RecQ family of helicases, is responsible for the rare human disorder Bloom Syndrome, which results in developmental abnormalities, DNA repair defects, genomic instability, and a predisposition to cancer. In Drosophila melanogaster, the orthologous Blm protein is essential during early development when the embryo is under the control of maternal gene products. A lack of functional maternal Blm during the syncytial cell cycles of Drosophila embryonic development results in severe nuclear defects and lethality. Amongst the small fraction of embryos from Blm mutant mothers that survive to adulthood, a prominent sex-bias favors the class of flies that inherits less repetitive DNA content, which serves as an endogenous source of replication stress. This selection against repetitive DNA content reflects a role for Blm in facilitating replication through repetitive sequences during the rapid S-phases of syncytial cell cycles. During these syncytial cycles, Blm is not required for complex DNA repair; however, the progeny sex-bias resulting from the absence of maternal Blm is exacerbated by repetitive DNA sequences and by the slowing of replication fork progression, suggesting that the essential role for Blm during this stage is to manage replication fork stress brought about by impediments to fork progression. Additionally, our data suggest that Blm is only required to manage this replication stress during embryonic development, and likely only during the early, rapid syncytial cell cycles, and not at later developmental stages. These results provide novel insights into Blm function throughout development.

LTR-retrotransposon dynamics in common fig (Ficus carica L.) genome

Background Long Terminal Repeat retrotransposons (LTR-REs) are repetitive DNA sequences that constitute a large part of the genome. The improvement of sequencing technologies and sequence assembling strategies has achieved genome sequences with much greater reliability than those of the past, especially in relation to repetitive DNA sequences. Results In this study, we analysed the genome of Ficus carica L., obtained using third generation sequencing technologies and recently released, to characterise the complete complement of full-length LTR-REs to study their dynamics during fig genome evolution. A total of 1867 full-length elements were identified. Those belonging to the Gypsy superfamily were the most abundant; among these, the Chromovirus/Tekay lineage was the most represented. For the Copia superfamily, Ale was the most abundant lineage. Measuring the estimated insertion time of each element showed that, on average, Ivana and Chromovirus/Tekay were the youngest lineages of Copia and Gypsy superfamilies, respectively. Most elements were inactive in transcription, both constitutively and in leaves of plants exposed to an abiotic stress, except for some elements, mostly belonging to the Copia/Ale lineage. A relationship between the inactivity of an element and inactivity of genes lying in close proximity to it was established. Conclusions The data reported in this study provide one of the first sets of information on the genomic dynamics related to LTR-REs in a plant species with highly reliable genome sequence. Fig LTR-REs are highly heterogeneous in abundance and estimated insertion time, and only a few elements are transcriptionally active. In general, the data suggested a direct relationship between estimated insertion time and abundance of an element and an inverse relationship between insertion time (or abundance) and transcription, at least for Copia LTR-REs.

Cloning and physical localization of male-biased repetitive DNA sequences in Spinacia oleracea (Amaranthaceae)

Spinach (Spinacia oleracea Linnaeus, 1753) is an ideal material for studying molecular mechanisms of early-stage sex chromosome evolution in dioecious plants. Degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR) technique facilitates the retrotransposon-relevant studies by enriching specific repetitive DNA sequences from a micro-dissected single chromosome. We conducted genomic subtractive hybridization to screen sex-biased DNA sequences by using the DOP-PCR amplification products of micro-dissected spinach Y chromosome. The screening yielded 55 male-biased DNA sequences with 30 576 bp in length, of which, 32 DNA sequences (12 049 bp) contained repeat DNA sequences, including LTR/Copia, LTR/Gypsy, simple repeats, and DNA/CMC-EnSpm. Among these repetitive DNA sequences, four DNA sequences that contained a fragment of Ty3-gypsy retrotransposons (SP73, SP75, SP76, and SP77) were selected as fluorescence probes to hybridization on male and female spinach karyotypes. Fluorescence in situ hybridization (FISH) signals of SP73 and SP75 were captured mostly on the centromeres and their surrounding area for each homolog. Hybridization signals primarily appeared near the putative centromeres for each homologous chromosome pair by using SP76 and SP77 probes for FISH, and sporadic signals existed on the long arms. Results can be served as a basis to study the function of repetitive DNA sequences in sex chromosome evolution in spinach.

Cloning and physical localization of male-biased repetitive DNA sequences in Spinacia oleracea (Amaranthaceae)

Spinach (Spinacia oleracea Linnaeus, 1753) is an ideal material for studying molecular mechanisms of early-stage sex chromosome evolution in dioecious plants. Degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR) technique facilitates the retrotransposon-relevant studies by enriching specific repetitive DNA sequences from a micro-dissected single chromosome. We conducted genomic subtractive hybridization to screen sex-biased DNA sequences by using the DOP-PCR amplification products of micro-dissected spinach Y chromosome. The screening yielded 55 male-biased DNA sequences with 30 576 bp in length, of which, 32 DNA sequences (12 049 bp) contained repeat DNA sequences, including LTR/Copia, LTR/Gypsy, simple repeats, and DNA/CMC-EnSpm. Among these repetitive DNA sequences, four DNA sequences that contained a fragment of Ty3-gypsy retrotransposons (SP73, SP75, SP76, and SP77) were selected as fluorescence probes to hybridization on male and female spinach karyotypes. Fluorescence in situ hybridization (FISH) signals of SP73 and SP75 were captured mostly on the centromeres and their surrounding area for each homolog. Hybridization signals primarily appeared near the putative centromeres for each homologous chromosome pair by using SP76 and SP77 probes for FISH, and sporadic signals existed on the long arms. Results can be served as a basis to study the function of repetitive DNA sequences in sex chromosome evolution in spinach.

Mitotic R-loops direct Aurora B kinase to maintain centromeric cohesion

Recent work has shown that R-loops exist at mitotic centromeres, but the function of these R-loops is not well understood. Here, we report that mitotic R-loops arise in distinct locations from those formed during interphase. They accumulate on chromosome arms in prophase, where they are quickly resolved and continue to be produced at repetitive sequences including centromeres during a mitotic stall. Aurora B kinase activity is required to resolve R-loops during prophase and R-loops promote the localization of the Chromosome Passenger Complex (CPC) to the inner centromere. CPC purified from mitotic chromosomes interacts with thirty-two proteins involved with R-loop biology. One of these, the RNA regulator RBMX, controls Aurora B localization and activity in vivo. Perturbations in R-loop homeostasis or RBMX cause defects in the maintenance of centromeric cohesion due to the mislocalization of the CPC. We conclude that R-loops are generated by mitotic processes in repetitive DNA sequences, they play important roles in mitotic fidelity, and we have identified a set of mitotic R-loop regulators including the CPC and RBMX that will enable future studies of mitotic R-loops.

New evidence confirming the CD genomic constitutions of the tetraploid Avena species in the section Pachycarpa Baum

The tetraploid Avena species in the section Pachycarpa Baum, including A. insularis, A. maroccana, and A. murphyi, are thought to be involved in the evolution of hexaploid oats; however, their genome designations are still being debated. Repetitive DNA sequences play an important role in genome structuring and evolution, so understanding the chromosomal organization and distribution of these sequences in Avena species could provide valuable information concerning genome evolution in this genus. In this study, the chromosomal organizations and distributions of six repetitive DNA sequences (including three SSR motifs (TTC, AAC, CAG), one 5S rRNA gene fragment, and two oat A and C genome specific repeats) were investigated using non-denaturing fluorescence in situ hybridization (ND-FISH) in the three tetraploid species mentioned above and in two hexaploid oat species. Preferential distribution of the SSRs in centromeric regions was seen in the A and D genomes, whereas few signals were detected in the C genomes. Some intergenomic translocations were observed in the tetraploids; such translocations were also detected between the C and D genomes in the hexaploids. These results provide robust evidence for the presence of the D genome in all three tetraploids, strongly suggesting that the genomic constitution of these species is DC and not AC, as had been thought previously.

Comparative analysis of morabine grasshopper genomes reveals highly abundant transposable elements and rapidly proliferating satellite DNA repeats

Background Repetitive DNA sequences, including transposable elements (TEs) and tandemly repeated satellite DNA (satDNAs), collectively called the “repeatome”, are found in high proportion in organisms across the Tree of Life. Grasshoppers have large genomes, averaging 9 Gb, that contain a high proportion of repetitive DNA, which has hampered progress in assembling reference genomes. Here we combined linked-read genomics with transcriptomics to assemble, characterize, and compare the structure of repetitive DNA sequences in four chromosomal races of the morabine grasshopper Vandiemenella viatica species complex and determine their contribution to genome evolution. Results We obtained linked-read genome assemblies of 2.73–3.27 Gb from estimated genome sizes of 4.26–5.07 Gb DNA per haploid genome of the four chromosomal races of V. viatica. These constitute the third largest insect genomes assembled so far. Combining complementary annotation tools and manual curation, we found a large diversity of TEs and satDNAs, constituting 66 to 75% per genome assembly. A comparison of sequence divergence within the TE classes revealed massive accumulation of recent TEs in all four races (314–463 Mb per assembly), indicating that their large genome sizes are likely due to similar rates of TE accumulation. Transcriptome sequencing showed more biased TE expression in reproductive tissues than somatic tissues, implying permissive transcription in gametogenesis. Out of 129 satDNA families, 102 satDNA families were shared among the four chromosomal races, which likely represent a diversity of satDNA families in the ancestor of the V. viatica chromosomal races. Notably, 50 of these shared satDNA families underwent differential proliferation since the recent diversification of the V. viatica species complex. Conclusion This in-depth annotation of the repeatome in morabine grasshoppers provided new insights into the genome evolution of Orthoptera. Our TEs analysis revealed a massive recent accumulation of TEs equivalent to the size of entire Drosophila genomes, which likely explains the large genome sizes in grasshoppers. Despite an overall high similarity of the TE and satDNA diversity between races, the patterns of TE expression and satDNA proliferation suggest rapid evolution of grasshopper genomes on recent timescales.

New evidence concerning the genome designations of the AC(DC) tetraploid Avena species

The tetraploid Avena species in the section Pachycarpa Baum, including A. insularis, A. maroccana, and A. murphyi, are thought to be involved in the evolution of hexaploid oats; however, their genome designations are still being debated. Repetitive DNA sequences play an important role in genome structuring and evolution, so understanding the chromosomal organization and distribution of these sequences in Avena species could provide valuable information concerning genome evolution in this genus. In this study, the chromosomal organizations and distributions of six repetitive DNA sequences (including three SSR motifs (TTC, AAC, CAG), one 5S rRNA gene fragment, and two oat A and C genome specific repeats) were investigated using non-denaturing fluorescence in situ hybridization (ND-FISH) in the three tetraploid species mentioned above and in two hexaploid oat species. Preferential distribution of the SSRs in centromeric regions was seen in the A and D genomes, whereas few signals were detected in the C genomes. Some intergenomic translocations were observed in the tetraploids; such translocations were also detected between the C and D genomes in the hexaploids. These results provide robust evidence for the presence of the D genome in all three tetraploids, strongly suggesting that the genomic constitution of these species is DC and not AC, as had been thought previously.