Miniature Inverted-Repeat Transposable Elements: Small DNA Transposons That Have Contributed to Plant MICRORNA Gene Evolution.

Angiosperms form the largest phylum within the Plantae kingdom and show remarkable genetic variation due to the considerable difference in the nuclear genome size of each species. Transposable elements (TEs), mobile DNA sequences that can amplify and change their chromosome position, account for much of the difference in nuclear genome size between individual angiosperm species. Considering the dramatic consequences of TE movement, including the complete loss of gene function, it is unsurprising that the angiosperms have developed elegant molecular strategies to control TE amplification and movement. Specifically, the RNA-directed DNA methylation (RdDM) pathway, directed by the repeat-associated small-interfering RNA (rasiRNA) class of small regulatory RNA, forms the primary line of defense to control TE activity in the angiosperms. However, the miniature inverted-repeat transposable element (MITE) species of TE has at times avoided the repressive effects imposed by the rasiRNA-directed RdDM pathway. MITE proliferation in angiosperm nuclear genomes is due to their preference to transpose within gene-rich regions, a pattern of transposition that has enabled MITEs to gain further transcriptional activity. The sequence-based properties of a MITE results in the synthesis of a noncoding RNA (ncRNA), which, after transcription, folds to form a structure that closely resembles those of the precursor transcripts of the microRNA (miRNA) class of small regulatory RNA. This shared folding structure results in a MITE-derived miRNA being processed from the MITE-transcribed ncRNA, and post-maturation, the MITE-derived miRNA can be used by the core protein machinery of the miRNA pathway to regulate the expression of protein-coding genes that harbor homologous MITE insertions. Here, we outline the considerable contribution that the MITE species of TE have made to expanding the miRNA repertoire of the angiosperms.

Mining of Miniature Transposable Elements in Brassica Species at BrassicaTED.

Miniature form transposable elements (mTEs) are ubiquitous in plant genomes and directly linked to gene regulation and evolution. With the advantage of completely sequenced genomes of Brassica rapa and Brassica oleracea, an open-source web portal called, BrassicaTED was developed. This database provides a user-friendly interface to explore invaluable information of mTEs in Brassica species and unique visualization and comparison tools. In this chapter, we describe an overview of this database construction and explain the utilities of data search, visualization, and analysis tools. In addition, we show the possible obstacles users may encounter when using this database.

Riding the Wave: The SINE-Specific V Highly-Conserved Domain Spread into Mammalian Genomes Exploiting the Replication Burst of the MER6 DNA Transposon.

Transposable elements are widely distributed within genomes where they may significantly impact their evolution and cell functions. Short interspersed elements (SINEs) are non-autonomous, fast-evolving elements, but some of them carry a highly conserved domain (HCD), whose sequence remained substantially unchanged throughout the metazoan evolution. SINEs carrying the HCD called V are absent in amniote genomes, but V-like sequences were found within the miniature inverted-repeat transposable element (MITE) MER6 in Homo sapiens. In the present work, the genomic distribution and evolution of MER6 are investigated, in order to reconstruct the origin of human V domain and to envisage its possible functional role. The analysis of 85 tetrapod genomes revealed that MER6 and its variant MER6A are found in primates, while only the MER6A variant was found in bats and eulipotyphlans. These MITEs appeared no longer active, in line with literature data on mammalian DNA transposons. Moreover, they appeared to have originated from a Mariner element found in turtles and from a V-SINE from bony fishes. MER6 insertions were found within genes and conserved in mRNAs: in line with previous hypothesis on functional role of HCDs, the MER6 V domain may be important for cell function also in mammals.

QTL Mapping Combined With Comparative Analyses Identified Candidate Genes for Reduced Shattering in Setaria italica.

Setaria (L.) P. Beauv is a genus of grasses that belongs to the Poaceae (grass) family, subfamily Panicoideae. Two members of the Setaria genus, Setaria italica (foxtail millet) and S. viridis (green foxtail), have been studied extensively over the past few years as model species for C4-photosynthesis and to facilitate genome studies in complex Panicoid bioenergy grasses. We exploited the available genetic and genomic resources for S. italica and its wild progenitor, S. viridis, to study the genetic basis of seed shattering. Reduced shattering is a key trait that underwent positive selection during domestication. Phenotyping of F2:3 and recombinant inbred line (RIL) populations generated from a cross between S. italica accession B100 and S. viridis accession A10 identified the presence of additive main effect quantitative trait loci (QTL) on chromosomes V and IX. As expected, enhanced seed shattering was contributed by the wild S. viridis. Comparative analyses pinpointed Sh1 and qSH1, two shattering genes previously identified in sorghum and rice, as potentially underlying the QTL on Setaria chromosomes IX and V, respectively. The Sh1 allele in S. italica was shown to carry a PIF/Harbinger MITE in exon 2, which gave rise to an alternatively spliced transcript that lacked exon 2. This MITE was universally present in S. italica accessions around the world and absent from the S. viridis germplasm tested, strongly suggesting a single origin of foxtail millet domestication. The qSH1 gene carried two MITEs in the 5'UTR. Presence of one or both MITEs was strongly associated with cultivated germplasm. If the MITE insertion(s) in qSH1 played a role in reducing shattering in S. italica accessions, selection for the variants likely occurred after the domestication of foxtail millet.

MUSTv2: An Improved De Novo Detection Program for Recently Active Miniature Inverted Repeat Transposable Elements (MITEs).

Background Miniature inverted repeat transposable element (MITE) is a short transposable element, carrying no protein-coding regions. However, its high proliferation rate and sequence-specific insertion preference renders it as a good genetic tool for both natural evolution and experimental insertion mutagenesis. Recently active MITE copies are those with clear signals of Terminal Inverted Repeats (TIRs) and Direct Repeats (DRs), and are recently translocated into their current sites. Their proliferation ability renders them good candidates for the investigation of genomic evolution. Results This study optimizes the C++ code and running pipeline of the MITE Uncovering SysTem (MUST) by assuming no prior knowledge of MITEs required from the users, and the current version, MUSTv2, shows significantly increased detection accuracy for recently active MITEs, compared with similar programs. The running speed is also significantly increased compared with MUSTv1. We prepared a benchmark dataset, the simulated genome with 150 MITE copies for researchers who may be of interest. Conclusions MUSTv2 represents an accurate detection program of recently active MITE copies, which is complementary to the existing template-based MITE mapping programs. We believe that the release of MUSTv2 will greatly facilitate the genome annotation and structural analysis of the bioOMIC big data researchers.

Both maternally and paternally imprinted genes regulate seed development in rice.

Genetic imprinting refers to the unequal expression of paternal and maternal alleles of a gene in sexually reproducing organisms, including mammals and flowering plants. Although many imprinted genes have been identified in plants, the functions of these imprinted genes have remained largely uninvestigated. We report genome-wide analysis of gene expression, DNA methylation and small RNAs in the rice endosperm and functional tests of five imprinted genes during seed development using Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated gene9 (CRISPR/Cas9) gene editing technology. In the rice endosperm, we identified 162 maternally expressed genes (MEGs) and 95 paternally expressed genes (PEGs), which were associated with miniature inverted-repeat transposable elements, imprinted differentially methylated loci and some 21-22 small interfering RNAs (siRNAs) and long noncoding RNAs (lncRNAs). Remarkably, one-third of MEGs and nearly one-half of PEGs were associated with grain yield quantitative trait loci. Most MEGs and some PEGs were expressed specifically in the endosperm. Disruption of two MEGs increased the amount of small starch granules and reduced grain and embryo size, whereas mutation of three PEGs reduced starch content and seed fertility. Our data indicate that both MEGs and PEGs in rice regulate nutrient metabolism and endosperm development, which optimize seed development and offspring fitness to facilitate parental-offspring coadaptation. These imprinted genes and mechanisms could be used to improve the grain yield of rice and other cereal crops.