Varying strength of selection contributes to the intragenomic diversity of rRNA genes

Harmonized coexistence of intragenomic variations in diatom Skeletonema strains

Metabarcoding analysis has been demonstrated to be an effective technology for monitoring diversity and dynamics of phytoplankton including Skeletonema species. Although molecular diversity uncovered in metabarcoding projects has generally been interpreted as sum of interspecies diversity and intraspecies diversity, accumulating evidence suggests that it also harbors unprecedentedly high levels of intra-genomic variations (IGVs). As up to thousands of amplicon sequence variants (ASVs) identified in a typical metabarcoding project can be annotated to be Skeletonema species, we hypothesize that substantial portions of these ASVs are contributed by IGVs. Here, the nature of IGVs in Skeletonema species was quantitatively analyzed by carrying out single-strain metabarcoding analysis of 18S rDNA V4 in 49 strains belonging to seven Skeletonema species. Results showed that each Skeletonema strain harbored a high level of IGVs as expected. While many Skeletonema strains each contained one dominant ASV and a substantial number of ASVs displaying much lower relative abundance, other Skeletonema strains each contained multiple ASVs with comparable or nearly equally abundances. Thus the co-existence of multiple dominant ASVs in a single cell indicated a tug-of-war of these variants in evolution, which may eventually result in harmonized coexistence of multiple dominant ASVs. A total of nine dominant ASVs and 652 non-dominant ASVs were found in 49 strains of seven Skeletonema species, indicating rich interspecies and intraspecies variations, and complex evolution of IGVs in genus of Skeletonema. The results confirmed that the extensive degree of IGVs was the main contributor to the high molecular diversity revealed by metabarcoding analysis. This study highlights the importance of quantitative characterization of IGVs in Skeletonema species for accurate interpretation of species diversity in metabarcoding analysis.

Copy number and sequence variation in rDNA of Daphnia pulex from natural populations: insights from whole-genome sequencing

Ribosomal DNA (rDNA) has a vital role in ribosome biogenesis as it contains the genes that encode ribosomal RNA (rRNA) separated by intergenic spacers (IGSs). The rRNA genes occur in hundreds to tens of thousands of copies per haploid genome in eukaryotes and are generally highly conserved with low variation within species. Due to the repetitive nature and large size of rDNA arrays, detecting intraindividual variation can be difficult. In this study, we use whole-genome sequences of 169 Daphnia pulex individuals from 10 natural populations to measure the copy number and sequence variation in rDNA. This revealed that variation in rDNA copy number between individuals spans an order of magnitude. We further observed a substantial level of sequence variation within individual genomes. As expected, single-nucleotide polymorphisms occurred in regions of lower functional constraint such as the IGS and expansion segments of the rRNA genes. The presence of strong linkage disequilibrium among variants facilitated identification of haplotypes within each population. Although there was evidence of recombination among haplotypes from different populations, it is insufficient to eliminate linkage disequilibrium within populations. Estimating copy number and haplotype diversity within individuals revealed that the level of intraindividual sequence variation is not strongly correlated with copy number. The observed patterns of variation highlight a complex evolutionary history of rDNA in D. pulex. Future research should explore the functional implications of rDNA copy number and sequence variation on organismal phenotypes.

The Dynamic Interplay Between Ribosomal DNA and Transposable Elements: A Perspective From Genomics and Cytogenetics

Although both are salient features of genomes, at first glance ribosomal DNAs and transposable elements are genetic elements with not much in common: whereas ribosomal DNAs are mainly viewed as housekeeping genes that uphold all prime genome functions, transposable elements are generally portrayed as selfish and disruptive. These opposing characteristics are also mirrored in other attributes: organization in tandem (ribosomal DNAs) versus organization in a dispersed manner (transposable elements); evolution in a concerted manner (ribosomal DNAs) versus evolution by diversification (transposable elements); and activity that prolongs genomic stability (ribosomal DNAs) versus activity that shortens it (transposable elements). Re-visiting relevant instances in which ribosomal DNA-transposable element interactions have been reported, we note that both repeat types share at least four structural and functional hallmarks: (1) they are repetitive DNAs that shape genomes in evolutionary timescales, (2) they exchange structural motifs and can enter co-evolution processes, (3) they are tightly controlled genomic stress sensors playing key roles in senescence/aging, and (4) they share common epigenetic marks such as DNA methylation and histone modification. Here, we give an overview of the structural, functional, and evolutionary characteristics of both ribosomal DNAs and transposable elements, discuss their roles and interactions, and highlight trends and future directions as we move forward in understanding ribosomal DNA-transposable element associations.

Unraveling the maternal and paternal origins of allotetraploid Vigna reflexo-pilosa

The genomic structures of Vigna hirtella Ridl. and Vigna trinervia (B.Heyne ex Wight & Arn.) Tateishi & Maxted, key ancestral species of the allotetraploid Vigna reflexo-pilosa var. glabra (Roxb.) N.Tomooka & Maxted, remain poorly understood. This study presents a comprehensive genomic comparison of these species to deepen our knowledge of their evolutionary trajectories. By comparing the genomic profiles of V. hirtella and V. trinervia with those of V. reflexo-pilosa, we investigate the complex genomic mechanisms underlying allopolyploid evolution within the genus Vigna. Comparison of the chloroplast genome revealed that V. trinervia is closely related to V. reflexo-pilosa. De novo assembly of the whole genome, followed by synteny analysis and Ks value calculations, confirms that V. trinervia is closely related to the A genome of V. reflexo-pilosa, and V. hirtella to its B genome. Furthermore, the comparative analyses reveal that V. reflexo-pilosa retains residual signatures of a previous polyploidization event, particularly evident in higher gene family copy numbers. Our research provides genomic evidence for polyploidization within the genus Vigna and identifies potential donor species of allotetraploid species using de novo assembly techniques. Given the Southeast Asian distribution of both V. hirtella and V. trinervia, natural hybridization between these species, with V. trinervia as the maternal ancestor and V. hirtella as the paternal donor, seems plausible.

Repetitive DNA sequence detection and its role in the human genome

Repetitive DNA sequences playing critical roles in driving evolution, inducing variation, and regulating gene expression. In this review, we summarized the definition, arrangement, and structural characteristics of repeats. Besides, we introduced diverse biological functions of repeats and reviewed existing methods for automatic repeat detection, classification, and masking. Finally, we analyzed the type, structure, and regulation of repeats in the human genome and their role in the induction of complex diseases. We believe that this review will facilitate a comprehensive understanding of repeats and provide guidance for repeat annotation and in-depth exploration of its association with human diseases.

Intragenomic rDNA variation - the product of concerted evolution, mutation, or something in between?

The classical model of concerted evolution states that hundreds to thousands of ribosomal DNA (rDNA) units undergo homogenization, making the multiple copies of the individual units more uniform across the genome than would be expected given mutation frequencies and gene redundancy. While the universality of this over 50-year-old model has been confirmed in a range of organisms, advanced high throughput sequencing techniques have also revealed that rDNA homogenization in many organisms is partial and, in rare cases, even apparently failing. The potential underpinning processes leading to unexpected intragenomic variation have been discussed in a number of studies, but a comprehensive understanding remains to be determined. In this work, we summarize information on variation or polymorphisms in rDNAs across a wide range of taxa amongst animals, fungi, plants, and protists. We discuss the definition and description of concerted evolution and describe whether incomplete concerted evolution of rDNAs predominantly affects coding or non-coding regions of rDNA units and if it leads to the formation of pseudogenes or not. We also discuss the factors contributing to rDNA variation, such as interspecific hybridization, meiotic cycles, rDNA expression status, genome size, and the activity of effector genes involved in genetic recombination, epigenetic modifications, and DNA editing. Finally, we argue that a combination of approaches is needed to target genetic and epigenetic phenomena influencing incomplete concerted evolution, to give a comprehensive understanding of the evolution and functional consequences of intragenomic variation in rDNA.

Ultra-deep sequencing of 45S rDNA to discern intragenomic diversity in three Chrysodeixis species for molecular identification

Species identification is necessary to prevent introductions of exotic plant pests through global trade. Many of these pests are understudied and lack publicly available DNA sequence data on which rapid molecular identification methods can be based. One such lineage is the genus Chrysodeixis, which includes three species of potential concern for United States trade initiatives: C. includens, C. chalcites, and C. eriosoma. Here we describe a method to generate robust 45S rDNA profiles using long read sequencing in order to clarify evolutionary relationships and develop a real-time PCR identification technique. Such an identification tool will be useful in rapidly differentiating between Chrysodeixis species of quarantine concern where traditional morphological identification methods are insufficient. Molecular methods such as this greatly reduce the time spent identifying each specimen, allow for detection of eDNA, vastly increase throughput, and increase the probability of detection. The methods presented here will be generally adaptable to any understudied lepidopteran taxa that necessitates a molecular diagnostic assay and, with adjustment or testing of the primers, could be applied to any group for which development of rDNA profiles in a benchtop setting would prove useful.

Substantial rDNA copy number reductions alter timing of development and produce variable tissue-specific phenotypes in C. elegans

The genes that encode ribosomal RNAs are present in several hundred copies in most eukaryotes. These vast arrays of repetitive ribosomal DNA (rDNA) have been implicated not just in ribosome biogenesis, but also aging, cancer, genome stability, and global gene expression. rDNA copy number is highly variable among and within species; this variability is thought to associate with traits relevant to human health and disease. Here we investigate the phenotypic consequences of multicellular life at the lower bounds of rDNA copy number. We use the model Caenorhabditis elegans, which has previously been found to complete embryogenesis using only maternally provided ribosomes. We find that individuals with rDNA copy number reduced to ∼5% of wild type are capable of further development with variable penetrance. Such individuals are sterile and exhibit severe morphological defects, particularly in post-embryonically dividing tissues such as germline and vulva. Developmental completion and fertility are supported by an rDNA copy number ∼10% of wild type, with substantially delayed development. Worms with rDNA copy number reduced to ∼33% of wild type display a subtle developmental timing defect that was absent in worms with higher copy numbers. Our results support the hypothesis that rDNA requirements vary across tissues and indicate that the minimum rDNA copy number for fertile adulthood is substantially less than the lowest naturally observed total copy number. The phenotype of individuals with severely reduced rDNA copy number is highly variable in penetrance and presentation, highlighting the need for continued investigation into the biological consequences of rDNA copy number variation.

Nematode gene annotation by machine-learning-assisted proteotranscriptomics enables proteome-wide evolutionary analysis

Nematodes encompass more than 24,000 described species, which were discovered in almost every ecological habitat, and make up >80% of metazoan taxonomic diversity in soils. The last common ancestor of nematodes is believed to date back to ∼650-750 million years, generating a large and phylogenetically diverse group to be explored. However, for most species high-quality gene annotations are incomprehensive or missing. Combining short-read RNA sequencing with mass spectrometry-based proteomics and machine-learning quality control in an approach called proteotranscriptomics, we improve gene annotations for nine genome-sequenced nematode species and provide new gene annotations for three additional species without genome assemblies. Emphasizing the sensitivity of our methodology, we provide evidence for two hitherto undescribed genes in the model organism Caenorhabditis elegans Extensive phylogenetic systems analysis using this comprehensive proteome annotation provides new insights into evolutionary processes of this metazoan group.

Varying strength of selection contributes to the intragenomic diversity of rRNA genes

Ribosome biogenesis in eukaryotes is supported by hundreds of ribosomal RNA (rRNA) gene copies that are encoded in the ribosomal DNA (rDNA). The multiple copies of rRNA genes are thought to have low sequence diversity within one species. Here, we present species-wide rDNA sequence analysis in Saccharomyces cerevisiae that challenges this view. We show that rDNA copies in this yeast are heterogeneous, both among and within isolates, and that many variants avoided fixation or elimination over evolutionary time. The sequence diversity landscape across the rDNA shows clear functional stratification, suggesting different copy-number thresholds for selection that contribute to rDNA diversity. Notably, nucleotide variants in the most conserved rDNA regions are sufficiently deleterious to exhibit signatures of purifying selection even when present in only a small fraction of rRNA gene copies. Our results portray a complex evolutionary landscape that shapes rDNA sequence diversity within a single species and reveal unexpectedly strong purifying selection of multi-copy genes.