mPing: The bursting transposon

Comprehensive survey of transposon mPing insertion sites and transcriptome analysis for identifying candidate genes controlling high protein content of rice

Rice is the most important crop species in the world, being staple food of more than 80% of people in Asia. About 80% of rice grain is composed of carbohydrates (starch), with its protein content as low as 7-8%. Therefore, increasing the protein content of rice offers way to create a stable protein source that contributes to improving malnutrition and health problems worldwide. We detected two rice lines harboring a significantly higher protein content (namely, HP5-7 and HP7-5) in the EG4 population. The EG4 strain of rice is a unique material in that the transposon mPing has high transpositional activity and high copy numbers under natural conditions. Other research indicated that mPing is abundant in the gene-rich euchromatic regions, suggesting that mPing amplification should create new allelic variants, novel regulatory networks, and phenotypic changes in the EG4 population. Here, we aimed to identify the candidate genes and/or mPing insertion sites causing high protein content by comprehensively identifying the mPing insertion sites and carrying out an RNA-seq-based transcriptome analysis. By utilizing the next-generation sequencing (NGS)-based methods, ca. 570 mPing insertion sites were identified per line in the EG4 population. Our results also indicated that mPing apparently has a preference for inserting itself in the region near a gene, with 38 genes in total found to contain the mPing insertion in the HP lines, of which 21 and 17 genes were specific to HP5-7 and HP7-5, respectively. Transcriptome analysis revealed that most of the genes related to protein synthesis (encoding glutelin, prolamin, and globulin) were up-regulated in HP lines relative to the control line. Interestingly, the differentially expressed gene (DEG) analysis revealed that the expression levels of many genes related to photosynthesis decreased in both HP lines; this suggests the amount of starch may have decreased, indirectly contributing to the increased protein content. The high-protein lines studied here are expected to contribute to the development of high protein-content rice by introducing valuable phenotypic traits such as high and stable yield, disease resistance, and abundant nutrients.

Fundamentally different repetitive element composition of sex chromosomes in Rumex acetosa

BACKGROUND AND AIMS

Dioecious species with well-established sex chromosomes are rare in the plant kingdom. Most sex chromosomes increase in size but no comprehensive analysis of the kind of sequences that drive this expansion has been presented. Here we analyse sex chromosome structure in common sorrel (Rumex acetosa), a dioecious plant with XY1Y2 sex determination, and we provide the first chromosome-specific repeatome analysis for a plant species possessing sex chromosomes.

METHODS

We flow-sorted and separately sequenced sex chromosomes and autosomes in R. acetosa using the two-dimensional fluorescence in situ hybridization in suspension (FISHIS) method and Illumina sequencing. We identified and quantified individual repeats using RepeatExplorer, Tandem Repeat Finder and the Tandem Repeats Analysis Program. We employed fluorescence in situ hybridization (FISH) to analyse the chromosomal localization of satellites and transposons.

KEY RESULTS

We identified a number of novel satellites, which have, in a fashion similar to previously known satellites, significantly expanded on the Y chromosome but not as much on the X or on autosomes. Additionally, the size increase of Y chromosomes is caused by non-long terminal repeat (LTR) and LTR retrotransposons, while only the latter contribute to the enlargement of the X chromosome. However, the X chromosome is populated by different LTR retrotransposon lineages than those on Y chromosomes.

CONCLUSIONS

The X and Y chromosomes have significantly diverged in terms of repeat composition. The lack of recombination probably contributed to the expansion of diverse satellites and microsatellites and faster fixation of newly inserted transposable elements (TEs) on the Y chromosomes. In addition, the X and Y chromosomes, despite similar total counts of TEs, differ significantly in the representation of individual TE lineages, which indicates that transposons proliferate preferentially in either the paternal or the maternal lineage.

Structural variants in 3000 rice genomes

Investigation of large structural variants (SVs) is a challenging yet important task in understanding trait differences in highly repetitive genomes. Combining different bioinformatic approaches for SV detection, we analyzed whole-genome sequencing data from 3000 rice genomes and identified 63 million individual SV calls that grouped into 1.5 million allelic variants. We found enrichment of long SVs in promoters and an excess of shorter variants in 5′ UTRs. Across the rice genomes, we identified regions of high SV frequency enriched in stress response genes. We demonstrated how SVs may help in finding causative variants in genome-wide association analysis. These new insights into rice genome biology are valuable for understanding the effects SVs have on gene function, with the prospect of identifying novel agronomically important alleles that can be utilized to improve cultivated rice.

Impact of Repetitive Elements on the Y Chromosome Formation in Plants

In contrast to animals, separate sexes and sex chromosomes in plants are very rare. Although the evolution of sex chromosomes has been the subject of numerous studies, the impact of repetitive sequences on sex chromosome architecture is not fully understood. New genomic approaches shed light on the role of satellites and transposable elements in the process of Y chromosome evolution. We discuss the impact of repetitive sequences on the structure and dynamics of sex chromosomes with specific focus on Rumex acetosa and Silene latifolia. Recent papers showed that both the expansion and shrinkage of the Y chromosome is influenced by sex-specific regulation of repetitive DNA spread. We present a view that the dynamics of Y chromosome formation is an interplay of genetic and epigenetic processes.

DNA transposon activity is associated with increased mutation rates in genes of rice and other grasses

DNA (class 2) transposons are mobile genetic elements which move within their ‘host' genome through excising and re-inserting elsewhere. Although the rice genome contains tens of thousands of such elements, their actual role in evolution is still unclear. Analysing over 650 transposon polymorphisms in the rice species Oryza sativa and Oryza glaberrima, we find that DNA repair following transposon excisions is associated with an increased number of mutations in the sequences neighbouring the transposon. Indeed, the 3,000 bp flanking the excised transposons can contain over 10 times more mutations than the genome-wide average. Since DNA transposons preferably insert near genes, this is correlated with increases in mutation rates in coding sequences and regulatory regions. Most importantly, we find this phenomenon also in maize, wheat and barley. Thus, these findings suggest that DNA transposon activity is a major evolutionary force in grasses which provide the basis of most food consumed by humankind.

DNA transposons are numerous in plant genomes. Here, Wicker et al. analyse transposon polymorphisms in rice and other grasses and show that sequences flanking excision sites contain up to 10 times more mutations than average, suggesting transposons are a major factor shaping the evolution of grass genomes.