Patterns of tandem repetition in plant whole genome assemblies

Identification and analysis of lignin biosynthesis genes related to fruit ripening and stress response in banana (Musa acuminata L. AAA group, cv. Cavendish)

BACKGROUND

Lignin is a key component of the secondary cell wall of plants, providing mechanical support and facilitating water transport as well as having important impact effects in response to a variety of biological and abiotic stresses.

RESULTS

In this study, we identified 104 genes from ten enzyme gene families related to lignin biosynthesis in Musa acuminata genome and found the number of MaCOMT gene family was the largest, while MaC3Hs had only two members. MaPALs retained the original members, and the number of Ma4CLs in lignin biosynthesis was significantly less than that of flavonoids. Segmental duplication existed in most gene families, except for MaC3Hs, and tandem duplication was the main way to expand the number of MaCOMTs. Moreover, the expression profiles of lignin biosynthesis genes during fruit development, postharvest ripening stages and under various abiotic and biological stresses were investigated using available RNA-sequencing data to obtain fruit ripening and stress response candidate genes. Finally, a co-expression network of lignin biosynthesis genes was constructed by weighted gene co-expression network analysis to elucidate the lignin biosynthesis genes that might participate in lignin biosynthesis in banana during development and in response to stresses.

CONCLUSION

This study systematically identified the lignin biosynthesis genes in the Musa acuminata genome, providing important candidate genes for further functional analysis. The identification of the major genes involved in lignin biosynthesis in banana provides the basis for the development of strategies to improve new banana varieties tolerant to biological and abiotic stresses with high yield and high quality.

Identification and characterization of a new family of long satellite DNA, specific of true toads (Anura, Amphibia, Bufonidae)

Amphibians have some of the most variable genome sizes among vertebrates. Genome size variation has been attributed to repetitive and noncoding DNA, including satellite repeats, transposable elements, introns, and nuclear insertions of viral and organelle DNA. In vertebrates, satellite DNAs have been widely described in mammals, but few molecular studies have been carried out in amphibians. Here, we provide a detailed characterization of a new family of satellite DNA, present in all 15 examined species of the family Bufonidae. Southern-blot analysis and PCR reveal that this satellite is formed by monomers of 807 bp, is organized in tandem arrays, and has an AT-content of 57.4%. Phylogenetic analyses show that most clades exhibit species-specific variances, indicating that this satellite DNA has evolved by concerted evolution. The homogenization/fixation process is heterogeneous in Bufonidae, where the genera Bufo and Bufotes do not show species-specific differences, while populations from Rhinella marina exhibit population-specific changes. Additionally, variants of this satellite DNA have been identified in Duttaphrynus melanostictus and R. marina, supporting the 'library hypothesis' (a set, 'library', of satellite DNAs is shared by a species group). Physical mapping in Bufo bufo, Bufo spinosus, Epidalea calamita and Bufotes viridis provides evidence that this repetitive DNA is not dispersed in the karyotype, but accumulated in pericentromeric regions of some chromosomal pairs. This location, together with its presence in the transcriptomes of bufonids, could indicate a role in centromere function or heterochromatin formation and maintenance.

Heat Shock Cognate 70 Functions as A Chaperone for the Stability of Kinetochore Protein CENP-N in Holocentric Insect Silkworms

The centromere, in which kinetochore proteins are assembled, plays an important role in the accurate congression and segregation of chromosomes during cell mitosis. Although the function of the centromere and kinetochore is conserved from monocentric to holocentric, the DNA sequences of the centromere and components of the kinetochore are varied among different species. Given the lack of core centromere protein A (CENP-A) and CENP-C in the lepidopteran silkworm Bombyx mori, which possesses holocentric chromosomes, here we investigated the role of CENP-N, another important member of the centromere protein family essential for kinetochore assembly. For the first time, cellular localization and RNA interference against CENP-N have confirmed its kinetochore function in silkworms. To gain further insights into the regulation of CENP-N in the centromere, we analyzed the affinity-purified complex of CENP-N by mass spectrometry and identified 142 interacting proteins. Among these factors, we found that the chaperone protein heat shock cognate 70 (HSC70) is able to regulate the stability of CENP-N by prohibiting ubiquitin-proteasome pathway, indicating that HSC70 could control cell cycle-regulated degradation of CENP-N at centromeres. Altogether, the present work will provide a novel clue to understand the regulatory mechanism for the kinetochore activity of CENP-N during the cell cycle.

Characterization of repeated DNA sequences in genomes of blue-flowered flax

BACKGROUND

Members of different sections of the genus Linum are characterized by wide variability in size, morphology and number of chromosomes in karyotypes. Since such variability is determined mainly by the amount and composition of repeated sequences, we conducted a comparative study of the repeatomes of species from four sections forming a clade of blue-flowered flax. Based on the results of high-throughput genome sequencing performed in this study as well as available WGS data, bioinformatic analyses of repeated sequences from 12 flax samples were carried out using a graph-based clustering method.

RESULTS

It was found that the genomes of closely related species, which have a similar karyotype structure, are also similar in the repeatome composition. In contrast, the repeatomes of karyologically distinct species differed significantly, and no similar tandem-organized repeats have been identified in their genomes. At the same time, many common mobile element families have been identified in genomes of all species, among them, Athila Ty3/gypsy LTR retrotransposon was the most abundant. The 30-chromosome members of the sect. Linum (including the cultivated species L. usitatissimum) differed significantly from other studied species by a great number of satellite DNA families as well as their relative content in genomes.

CONCLUSIONS

The evolution of studied flax species was accompanied by waves of amplification of satellite DNAs and LTR retrotransposons. The observed inverse correlation between the total contents of dispersed repeats and satellite DNAs allowed to suggest a relationship between both classes of repeating sequences. Significant interspecific differences in satellite DNA sets indicated a high rate of evolution of this genomic fraction. The phylogenetic relationships between the investigated flax species, obtained by comparison of the repeatomes, agreed with the results of previous molecular phylogenetic studies.

The Molecular Cytogenetic Characterization of Pistachio (Pistacia vera L.) Suggests the Arrest of Recombination in the Largest Heteropycnotic Pair HC1

This paper represents the first molecular cytogenetic characterization of the strictly dioecious pistachio tree (Pistacia vera L.). The karyotype was characterized by fluorescent in situ hybridization (FISH) with probes for 5S and 45S rDNAs, and the pistachio specific satellite DNAs PIVE-40, and PIVE-180, together with DAPI-staining. PIVE-180 has a monomeric unit of 176–178 bp and high sequence homology between family members; PIVE-40 has a 43 bp consensus monomeric unit, and is most likely arranged in higher order repeats (HORs) of two units. The P. vera genome is highly heterochromatic, and prominent DAPI positive blocks are detected in most chromosomes. Despite the difficulty in classifying chromosomes according to morphology, 10 out of 15 pairs (2n = 30) could be distinguished by their unique banding patterns using a combination of FISH probes. Significantly, the largest pair, designated HC1, is strongly heteropycnotic, shows differential condensation, and has massive enrichment in PIVE-40 repeats. There are two types of HC1 chromosomes (type-I and type-II) with differing PIVE-40 hybridization signal. Only type-I/II heterozygotes and type-I homozygotes individuals were found. We speculate that the differentiation between the two HC1 chromosomes is due to suppression of homologous recombination at meiosis, reinforced by the presence of PIVE-40 HORs and differences in PIVE-40 abundance. This would be compatible with a ZW sex-determination system in the pistachio tree.

Genome-wide identification and characterization of R2R3MYB family in Solanum lycopersicum

The R2R3MYB proteins comprise one of the largest families of transcription factors and play regulatory roles in developmental processes and defense responses in plants. However, there has been relatively little effort to systematically carry out comprehensive genomic and functional analyses of these genes in tomato (Solanum lycopersicum L.), a reference species for Solanaceae plants, and the model plant for fruit development. In this study, a total of 121 R2R3MYB genes were identified in the tomato genome released recently and further classified into 29 subgroups based on the phylogenetic analysis of the complete protein sequences. Phylogenetic comparison of the members of this superfamily among tomato, Arabidopsis, grape, rice, poplar, soybean, cucumber and apple revealed that the putative functions of some tomato R2R3MYB proteins were clustered into the Arabidopsis functional clades. The chromosome distribution pattern revealed that tomato R2R3MYB genes were enriched on several chromosomes and 52 % of the family members were tandemly duplicated genes. Tissue specificity or different expression levels of SlR2R3MYBs in different tissues suggested differential regulation of tissue development as well as metabolic regulation. The transcript abundance level analysis during abiotic conditions identified a group of R2R3MYB genes that responded to one or more treatments suggesting that the SlR2R3MYBs played major roles in the plant response to abiotic conditions and involved in signal transduction pathways. This study not only provides a solid foundation for further functional dissection of tomato R2R3MYB family genes, but may also be profitable for, in the future, the improvement of tomato stress tolerance and fruit quality.

Unique features of the loblolly pine (Pinus taeda L.) megagenome revealed through sequence annotation

The largest genus in the conifer family Pinaceae is Pinus, with over 100 species. The size and complexity of their genomes (∼20-40 Gb, 2n = 24) have delayed the arrival of a well-annotated reference sequence. In this study, we present the annotation of the first whole-genome shotgun assembly of loblolly pine (Pinus taeda L.), which comprises 20.1 Gb of sequence. The MAKER-P annotation pipeline combined evidence-based alignments and ab initio predictions to generate 50,172 gene models, of which 15,653 are classified as high confidence. Clustering these gene models with 13 other plant species resulted in 20,646 gene families, of which 1554 are predicted to be unique to conifers. Among the conifer gene families, 159 are composed exclusively of loblolly pine members. The gene models for loblolly pine have the highest median and mean intron lengths of 24 fully sequenced plant genomes. Conifer genomes are full of repetitive DNA, with the most significant contributions from long-terminal-repeat retrotransposons. In depth analysis of the tandem and interspersed repetitive content yielded a combined estimate of 82%.

Evolution and homoplasy at the Bem6 microsatellite locus in three sweetpotato whitefly (Bemisia tabaci) cryptic species

BACKGROUND

The evolution of individual microsatellite loci is often complex and homoplasy is common but often goes undetected. Sequencing alleles at a microsatellite locus can provide a more complete picture of the common evolutionary mechanisms occurring at that locus and can reveal cases of homoplasy. Within species homoplasy can lead to an underestimate of differentiation among populations and among species homoplasy can produce a misleading interpretation regarding shared alleles and hybridization. This is especially problematic with cryptic species.

RESULTS

By sequencing alleles from three cryptic species of the sweetpotato whitefly (Bemisia tabaci), designated MEAM1, MED, and NW, the evolution of the putatively dinucleotide Bem6 (CA₈)imp microsatellite locus is inferred as one of primarily stepwise mutation occurring at four distinct heptaucleotide tandem repeats. In two of the species this pattern yields a compound tandem repeat. Homoplasy was detected both among species and within species.

CONCLUSIONS

In the absence of sequencing, size homoplasious alleles at the Bem6 locus lead to an overestimate of alleles shared and hybridization among cryptic species of Bemisia tabaci. Furthermore, the compound heptanucleotide motif structure of a putative dinucleotide microsatellite has implications for the nomenclature of heptanucleotide tandem repeats with step-wise evolution.

Comparative analysis of tandem repeats from hundreds of species reveals unique insights into centromere evolution

Background

Centromeres are essential for chromosome segregation, yet their DNA sequences evolve rapidly. In most animals and plants that have been studied, centromeres contain megabase-scale arrays of tandem repeats. Despite their importance, very little is known about the degree to which centromere tandem repeats share common properties between different species across different phyla. We used bioinformatic methods to identify high-copy tandem repeats from 282 species using publicly available genomic sequence and our own data.

Results

Our methods are compatible with all current sequencing technologies. Long Pacific Biosciences sequence reads allowed us to find tandem repeat monomers up to 1,419 bp. We assumed that the most abundant tandem repeat is the centromere DNA, which was true for most species whose centromeres have been previously characterized, suggesting this is a general property of genomes. High-copy centromere tandem repeats were found in almost all animal and plant genomes, but repeat monomers were highly variable in sequence composition and length. Furthermore, phylogenetic analysis of sequence homology showed little evidence of sequence conservation beyond approximately 50 million years of divergence. We find that despite an overall lack of sequence conservation, centromere tandem repeats from diverse species showed similar modes of evolution.

Conclusions

While centromere position in most eukaryotes is epigenetically determined, our results indicate that tandem repeats are highly prevalent at centromeres of both animal and plant genomes. This suggests a functional role for such repeats, perhaps in promoting concerted evolution of centromere DNA across chromosomes.

Whole genome duplication of intra- and inter-chromosomes in the tomato genome

Whole genome duplication (WGD) events have been proven to occur in the evolutionary history of most angiosperms. Tomato is considered a model species of the Solanaceae family. In this study, we describe the details of the evolutionary process of the tomato genome by detecting collinearity blocks and dating the WGD events on the tree of life by combining two different methods: synonymous substitution rates (Ks) and phylogenetic trees. In total, 593 collinearity blocks were discovered out of 12 pseudo-chromosomes constructed. It was evident that chromosome 2 had experienced an intra-chromosomal duplication event. Major inter-chromosomal duplication occurred among all the pseudo-chromosome. We calculated the Ks value of these collinearity blocks. Two peaks of Ks distribution were found, corresponding to two WGD events occurring approximately 36-82 million years ago (MYA) and 148-205 MYA. Additionally, the results of phylogenetic trees suggested that the more recent WGD event may have occurred after the divergence of the rosid-asterid clade, but before the major diversification in Solanaceae. The older WGD event was shown to have occurred before the divergence of the rosid-asterid clade and after the divergence of rice-Arabidopsis (monocot-dicot).

[Microsynteny analysis of tomato and peach genome]

Tomato and peach are two important model species belonging to the families Solanaceae and Rosaceae, respectively. Recently, more and more sequence data generated from their whole genome sequencing projects can be used for bioinformatic analysis. Microsynteny analysis for tomato and peach were conducted to detect conserved syntenic blocks using high quality genetic and physical maps. A large number of microsyntenic regions were detected through three com-parisons: comparison between tomato genetic map and peach physical map, between tomato physical map and peach ge-netic map, and between tomato physical map and peach physical map. Most of the syntenic blocks were short, and each block contained a small number of conserved gene pairs (261 syntenic blocks with only two homolog pairs, and 36 syntenic blocks with more than two homolog pairs). Tomato and peach had noncontinuous fragmentary microsynteny and some syntenic groups composed of complex networks among different chromosomes or BAC contigs. After comparing the ho-mologous proteins with tomato fruit-related EST libraries, a total of 9 proteins were found in different syntenic groups re-lating to fruit development and ripening. Microsynteny identified in this study could facilitate further investigation of fruit development and ripening in these two distantly related species.