Concerted evolution: molecular mechanism and biological implications

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.

Genetic variation in recalcitrant repetitive regions of the Drosophila melanogaster genome

Many essential functions of organisms are encoded in highly repetitive genomic regions, including histones involved in DNA packaging, centromeres that are core components of chromosome segregation, ribosomal RNA comprising the protein translation machinery, telomeres that ensure chromosome integrity, piRNA clusters encoding host defenses against selfish elements, and virtually the entire Y chromosome. These regions, formed by highly similar tandem arrays, pose significant challenges for experimental and informatic study, impeding sequence-level descriptions essential for understanding genetic variation. Here, we report the assembly and variation analysis of such repetitive regions in Drosophila melanogaster, offering significant improvements to the existing community reference assembly. Our work successfully recovers previously elusive segments, including complete reconstructions of the histone locus and the pericentric heterochromatin of the X chromosome, spanning the Stellate locus to the distal flank of the rDNA cluster. To infer structural changes in these regions where alignments are often not practicable, we introduce landmark anchors based on unique variants that are putatively orthologous. These regions display considerable structural variation between different D. melanogaster strains, exhibiting differences in copy number and organization of homologous repeat units between haplotypes. In the histone cluster, although we observe minimal genetic exchange indicative of crossing over, the variation patterns suggest mechanisms such as unequal sister chromatid exchange. We also examine the prevalence and scale of concerted evolution in the histone and Stellate clusters and discuss the mechanisms underlying these observed patterns.

Sequence and epigenetic landscapes of active and silent nucleolus organizer regions in Arabidopsis

Arabidopsis thaliana has two ribosomal RNA (rRNA) gene loci, nucleolus organizer regions NOR2 and NOR4, whose complete sequences are missing in current genome assemblies. Ultralong DNA sequences assembled using an unconventional approach yielded ~5.5- and 3.9-Mbp sequences for NOR2 and NOR4 in the reference strain, Col-0. The distinct rRNA gene subtype compositions of the NORs enabled the positional mapping of their active and inactive regions, using RNA sequencing to identify subtype-specific transcripts and DNA sequencing to identify subtypes associated with flow-sorted nucleoli. Comparisons of wild-type and silencing-defective plants revealed that most rRNA gene activity occurs in the central region of NOR4, whereas most, but not all, genes of NOR2 are epigenetically silenced. Intervals of low CG and CHG methylation overlap regions where gene activity and gene subtype homogenization are high. Collectively, the data reveal the genetic and epigenetic landscapes underlying nucleolar dominance (differential NOR activity) and implicate transcription as a driver of rRNA gene concerted evolution.

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.

Question of agent of camel balantidiosis solved: Molecular identity, taxonomic solution and epidemiological considerations

Domestic camels (Camelus bactrianus, the Bactrian camel; and Camelus dromedarius, the dromedary) are pseudo-ruminant herbivores kept as livestock in rural, inhospitable regions (cold deserts and dry steppes of Asia, arid to semi-arid regions of Africa, western and central Asia). Their close contact with humans makes them a potential reservoir for zoonotic parasite infections, as has been suggested for human balantidiasis. However, there is confusion about the ciliate species that infects camels: Infundibulorium cameli was originally described in dromedaries, but this name has almost never been used and most authors identified their findings as Balantioides coli and, to a lesser extent, Buxtonella sulcata, a cattle ciliate. To clarify the taxonomic status of the parasite and the corresponding zoonotic significance for camels, we performed morphological characterization of cysts and genetic analysis (SSU-rDNA and ITS markers) of B. coli-like isolates from Bactrian camels from Bulgaria and from dromedaries from Spain and the United Arab Emirates. Our results indicate that the camel ciliate is not B. coli, nor is it B. sulcata, but is a different species that should be placed in the same genus as the latter. Thus, camels are not a reservoir for human balantidiasis. Although the correct genus name would be Infundibulorium according to the principle of priority, this would lead to confusion since this name has almost fallen into disuse since its initial description, but Buxtonella is almost universally used by researchers and veterinarians for the cattle ciliate. We therefore propose to apply the reversal of precedence and use Buxtonella as the valid genus name. Consequently, we propose Buxtonella cameli n.comb. as the name for the camel ciliate.

The 18S rRNA genes of Haemoproteus (Haemosporida, Apicomplexa) parasites from European songbirds with remarks on improved parasite diagnostics

BACKGROUND

The nuclear ribosomal RNA genes of Plasmodium parasites are assumed to evolve according to a birth-and-death model with new variants originating by duplication and others becoming deleted. For some Plasmodium species, it has been shown that distinct variants of the 18S rRNA genes are expressed differentially in vertebrate hosts and mosquito vectors. The central aim was to evaluate whether avian haemosporidian parasites of the genus Haemoproteus also have substantially distinct 18S variants, focusing on lineages belonging to the Haemoproteus majoris and Haemoproteus belopolskyi species groups.

METHODS

The almost complete 18S rRNA genes of 19 Haemoproteus lineages of the subgenus Parahaemoproteus, which are common in passeriform birds from the Palaearctic, were sequenced. The PCR products of 20 blood and tissue samples containing 19 parasite lineages were subjected to molecular cloning, and ten clones in mean were sequenced each. The sequence features were analysed and phylogenetic trees were calculated, including sequence data published previously from eight additional Parahaemoproteus lineages. The geographic and host distribution of all 27 lineages was visualised as CytB haplotype networks and pie charts. Based on the 18S sequence data, species-specific oligonucleotide probes were designed to target the parasites in host tissue by in situ hybridization assays.

RESULTS

Most Haemoproteus lineages had two or more variants of the 18S gene like many Plasmodium species, but the maximum distances between variants were generally lower. Moreover, unlike in most mammalian and avian Plasmodium species, the 18S sequences of all but one parasite lineage clustered into reciprocally monophyletic clades. Considerably distinct 18S clusters were only found in Haemoproteus tartakovskyi hSISKIN1 and Haemoproteus sp. hROFI1. The presence of chimeric 18S variants in some Haemoproteus lineages indicates that their ribosomal units rather evolve in a semi-concerted fashion than according to a strict model of birth-and-death evolution.

CONCLUSIONS

Parasites of the subgenus Parahaemoproteus contain distinct 18S variants, but the intraspecific variability is lower than in most mammalian and avian Plasmodium species. The new 18S data provides a basis for more thorough investigations on the development of Haemoproteus parasites in host tissue using in situ hybridization techniques targeting specific parasite lineages.

Invasion success of a Lessepsian symbiont-bearing foraminifera linked to high dispersal ability, preadaptation and suppression of sexual reproduction

Among the most successful Lessepsian invaders is the symbiont-bearing benthic foraminifera Amphistegina lobifera. In its newly conquered habitat, this prolific calcifier and ecosystem engineer is exposed to environmental conditions that exceed the range of its native habitat. To disentangle which processes facilitated the invasion success of A. lobifera into the Mediterranean Sea we analyzed a ~ 1400 bp sequence fragment covering the SSU and ITS gene markers to compare the populations from its native regions and along the invasion gradient. The genetic variability was studied at four levels: intra-genomic, population, regional and geographical. We observed that the invasion is not associated with genetic differentiation, but the invasive populations show a distinct suppression of intra-genomic variability among the multiple copies of the rRNA gene. A reduced genetic diversity compared to the Indopacific is observed already in the Red Sea populations and their high dispersal potential into the Mediterranean appears consistent with a bridgehead effect resulting from the postglacial expansion from the Indian Ocean into the Red Sea. We conclude that the genetic structure of the invasive populations reflects two processes: high dispersal ability of the Red Sea source population pre-adapted to Mediterranean conditions and a likely suppression of sexual reproduction in the invader. This discovery provides a new perspective on the cost of invasion in marine protists: The success of the invasive A. lobifera in the Mediterranean Sea comes at the cost of abandonment of sexual reproduction.

A comprehensive overview of breeding strategy to improve phenotypic and quality traits in Valeriana jatamansi Jones

Valeriana jatamansi is a high value perennial herb that grows at an altitude of 1000-3000 MASL in the Indian Himalayan Region and is used in the Ayurvedic, Unani and Chinese systems of medicine. The plant extracts and essential oil (EO) obtained from its roots are used in the pharmaceutical, aromatic and flavouring industries. On account of high global annual demand and lack of organized cultivation of this herb, it is mostly collected from the wild causing depletion of the natural populations and also leading to variable produce making it unsuitable for industrial use. Therefore, to promote its cultivation, it becomes imperative to develop varieties with uniformity. Significant variations have been reported in V. jatamansi populations in terms of underground biomass accumulation, essential oil and valepotriates indicating potential of genotypic variability for effective selection. The review focuses on the diversity and variations at inter- and intra-population levels for phenotypic traits, variations for different active constituents and scope of improvement through selective breeding in V. jatamansi. The species has cross-pollinated breeding behaviour on account of floral dimorphism and presents unique opportunities for development of homozygous progeny lines through controlled self/sib-pollination by applying the breeding methods described in the review for population improvement. The germplasm resources of unique and improved selections can be maintained clonally to ensure their true-to-type identity. This review article was framed in the year 2022 after thoroughly studying the literature from the year 1919-2022. The study focuses on the variations in V. jatamansi which could be used to maximize the production through various breeding techniques for biomass and yield of different active constituents to meet the requirements of pharmaceutical and aroma industries.

Consequences of polyploidy and divergence as revealed by cytogenetic mapping of tandem repeats in African clawed frogs (Xenopus, Pipidae)

Repetitive elements have been identified in several amphibian genomes using whole genome sequencing, but few studies have used cytogenetic mapping to visualize these elements in this vertebrate group. Here, we used fluorescence in situ hybridization and genomic data to map the U1 and U2 small nuclear RNAs and histone H3 in six species of African clawed frog (genus Xenopus), including, from subgenus Silurana, the diploid Xenopus tropicalis and its close allotetraploid relative X. calcaratus and, from subgenus Xenopus, the allotetraploid species X. pygmaeus, X. allofraseri, X. laevis, and X. muelleri. Results allowed us to qualitatively evaluate the relative roles of polyploidization and divergence in the evolution of repetitive elements because our focal species include allotetraploid species derived from two independent polyploidization events - one that is relatively young that gave rise to X. calcaratus and another that is older that gave rise to the other (older) allotetraploids. Our results demonstrated conserved loci number and position of signals in the species from subgenus Silurana; allotetraploid X. calcaratus has twice as many signals as diploid X. tropicalis. However, the content of repeats varied among the other allotetraploid species. We detected almost same number of signals in X. muelleri as in X. calcaratus and same number of signals in X. pygmaeus, X. allofraseri, X. laevis as in the diploid X. tropicalis. Overall, these results are consistent with the proposal that allopolyploidization duplicated these tandem repeats and that variation in their copy number was accumulated over time through reduction and expansion in a subset of the older allopolyploids.

Polymorphisms of rDNA genes in Cyberlindnera yeast suggest birth-and-death evolution events

In eukaryotes, the ribosome machinery is encoded by repeats of the ribosomal RNA genes: 26/28S, 18S, 5.8S, and 5S, structured in tandem arrays and frequently homogenized within a genome. This homogenization is thought to be driven by concerted evolution, evolving as a unit, which contributes to its target as the species barcode in modern taxonomy. However, high heterogeneity of rDNA genes has been reported, including in Saccharomycotina yeasts. Here, we describe the polymorphisms and heterogeneity of D1/D2 domains (26S rRNA) and the intergenic transcribed spacer of a new yeast species with affinities to the genus Cyberlindnera and their evolution. Both regions are not homogenized, failing the prediction of concerted evolution. Phylogenetic network analysis of cloned sequences revealed that Cyberlindnera sp. rDNAs are diverse and evolved by reticulation rather than by bifurcating tree evolution model. Predicted rRNA secondary structures also confirmed structural differences, except for some conserved hairpin loops. We hypothesize that some rDNA is inactive within this species and evolves by birth-and-death rather than concerted evolution. Our findings propel further investigation into the evolution of rDNA genes in yeasts.

Allopatric Lineage Divergence of the East Asian Endemic Herb Conandron ramondioides Inferred from Low-Copy Nuclear and Plastid Markers

The evolutionary histories of ornamental plants have been receiving only limited attention. We examined the origin and divergence processes of an East Asian endemic ornamental plant, Conandron ramondioides. C. ramondioides is an understory herb occurring in primary forests, which has been grouped into two varieties. We reconstructed the evolutionary and population demography history of C. ramondioides to infer its divergence process. Nuclear and chloroplast DNA sequences were obtained from 21 Conandron populations on both sides of the East China Sea (ECS) to explore its genetic diversity, structure, and population differentiation. Interestingly, the reconstructed phylogeny indicated that the populations should be classified into three clades corresponding to geographical regions: the Japan (Honshu+Shikoku) clade, the Taiwan-Iriomote clade, and the Southeast China clade. Lineage divergence between the Japan clade and the Taiwan-Iriomote and Southeast China clades occured 1.14 MYA (95% HPD: 0.82-3.86), followed by divergence between the Taiwan-Iriomote and Southeast China clades approximately 0.75 MYA (95% HPD: 0.45-1.3). Furthermore, corolla traits (floral lobe length to tube length ratios) correlated with geographical distributions. Moreover, restricted gene flow was detected among clades. Lastly, the lack of potential dispersal routes across an exposed ECS seafloor during the last glacial maximum suggests that migration among the Conandron clades was unlikely. In summary, the extant Conandron exhibits a disjunct distribution pattern as a result of vicariance rather than long-distance dispersal. We propose that allopatric divergence has occurred in C. ramondioides since the Pleistocene. Our findings highlight the critical influence of species' biological characteristics on shaping lineage diversification of East Asian relic herb species during climate oscillations since the Quaternary.

Computational Docking Reveals Co-Evolution of C4 Carbon Delivery Enzymes in Diverse Plants

Proteins are modular functionalities regulating multiple cellular activities in prokaryotes and eukaryotes. As a consequence of higher plants adapting to arid and thermal conditions, C4 photosynthesis is the carbon fixation process involving multi-enzymes working in a coordinated fashion. However, how these enzymes interact with each other and whether they co-evolve in parallel to maintain interactions in different plants remain elusive to date. Here, we report our findings on the global protein co-evolution relationship and local dynamics of co-varying site shifts in key C4 photosynthetic enzymes. We found that in most of the selected key C4 photosynthetic enzymes, global pairwise co-evolution events exist to form functional couplings. Besides, protein-protein interactions between these enzymes may suggest their unknown functionalities in the carbon delivery process. For PEPC and PPCK regulation pairs, pocket formation at the interactive interface are not necessary for their function. This feature is distinct from another well-known regulation pair in C4 photosynthesis, namely, PPDK and PPDK-RP, where the pockets are necessary. Our findings facilitate the discovery of novel protein regulation types and contribute to expanding our knowledge about C4 photosynthesis.

Extensive nrDNA ITS polymorphism in Lycium: Non-concerted evolution and the identification of pseudogenes

The internal transcribed spacer (ITS) is one of the most extensively sequenced molecular markers in plant systematics due to its generally concerted evolution. While non-concerted evolution has been found in some plant taxa, such information is missing in Lycium. Molecular studies of six species and two variants of the genus Lycium revealed high levels of intra- and inter-individual polymorphism in the ITS, indicating non-concerted evolution. All genomic DNA ITS paralogues were identified as putative pseudogenes or functional paralogues through a series of comparisons of sequence features, including length and substitution variation, GC content, secondary structure stability, and the presence of conserved motifs in the 5.8S gene, and the rate of evolution. Approximately, 60% of ITS pseudogenes could be easily detected. Based on phylogenetic analysis, all pseudogenes were highly distinct from their corresponding functional copies, tended to evolve neutrally, and clustered randomly together in the evolutionary tree. The results probably suggest that this ITS non-concerted evolution is related to the recent divergence between tandem repeats within the Lycium genome and hybridization between species. Our study complements those of pseudogenes in plant taxa and provides a theoretical basis for the phylogeny and genetic origin of the genus Lycium while having important implications for the use of ITS molecular markers for phylogenetic reconstruction.

Bifurcation drives the evolution of assembly-line biosynthesis

Reprogramming biosynthetic assembly-lines is a topic of intense interest. This is unsurprising as the scaffolds of most antibiotics in current clinical use are produced by such pathways. The modular nature of assembly-lines provides a direct relationship between the sequence of enzymatic domains and the chemical structure of the product, but rational reprogramming efforts have been met with limited success. To gain greater insight into the design process, we wanted to examine how Nature creates assembly-lines and searched for biosynthetic pathways that might represent evolutionary transitions. By examining the biosynthesis of the anti-tubercular wollamides, we uncover how whole gene duplication and neofunctionalization can result in pathway bifurcation. We show that, in the case of the wollamide biosynthesis, neofunctionalization is initiated by intragenomic recombination. This pathway bifurcation leads to redundancy, providing the genetic robustness required to enable large structural changes during the evolution of antibiotic structures. Should the new product be non-functional, gene loss can restore the original genotype. However, if the new product confers an advantage, depreciation and eventual loss of the original gene creates a new linear pathway. This provides the blind watchmaker equivalent to the design, build, test cycle of synthetic biology.

Using RDNA sequences to define dinoflagellate species

Dinoflagellate species are traditionally defined using morphological characters, but molecular evidence accumulated over the past several decades indicates many morphologically-based descriptions are inaccurate. This recognition led to an increasing reliance on DNA sequence data, particularly rDNA gene segments, in defining species. The validity of this approach assumes the divergence in rDNA or other selected genes parallels speciation events. Another concern is whether single gene rDNA phylogenies by themselves are adequate for delineating species or if multigene phylogenies are required instead. Currently, few studies have directly assessed the relative utility of multigene versus rDNA-based phylogenies for distinguishing species. To address this, the current study examined D1-D3 and ITS/5.8S rDNA gene regions, a multi-gene phylogeny, and morphological characters in Gambierdiscus and other related dinoflagellate genera to determine if they produce congruent phylogenies and identify the same species. Data for the analyses were obtained from previous sequencing efforts and publicly available dinoflagellate transcriptomic libraries as well from the additional nine well-characterized Gambierdiscus species transcriptomic libraries generated in this study. The D1-D3 and ITS/5.8S phylogenies successfully identified the described Gambierdiscus and Alexandrium species. Additionally, the data showed that the D1-D3 and multigene phylogenies were equally capable of identifying the same species. The multigene phylogenies, however, showed different relationships among species and are likely to prove more accurate at determining phylogenetic relationships above the species level. These data indicated that D1-D3 and ITS/5.8S rDNA region phylogenies are generally successful for identifying species of Gambierdiscus, and likely those of other dinoflagellates. To assess how broadly general this finding is likely to be, rDNA molecular phylogenies from over 473 manuscripts representing 232 genera and 863 described species of dinoflagellates were reviewed. Results showed the D1-D3 rDNA and ITS phylogenies in combination are capable of identifying 97% of dinoflagellate species including all the species belonging to the genera Alexandrium, Ostreopsis and Gambierdiscus, although it should be noted that multi-gene phylogenies are preferred for inferring relationships among these species. A protocol is presented for determining when D1-D3, confirmed by ITS/5.8S rDNA sequence data, would take precedence over morphological features when describing new dinoflagellate species. This protocol addresses situations such as: a) when a new species is both morphologically and molecularly distinct from other known species; b) when a new species and closely related species are morphologically indistinguishable, but genetically distinct; and c) how to handle potentially cryptic species and cases where morphotypes are clearly distinct but have the same rDNA sequence. The protocol also addresses other molecular, morphological, and genetic approaches required to resolve species boundaries in the small minority of species where the D1-D3/ITS region phylogenies fail.

The Importance of Gene Duplication and Domain Repeat Expansion for the Function and Evolution of Fertilization Proteins

The process of gene duplication followed by gene loss or evolution of new functions has been studied extensively, yet the role gene duplication plays in the function and evolution of fertilization proteins is underappreciated. Gene duplication is observed in many fertilization protein families including Izumo, DCST, ZP, and the TFP superfamily. Molecules mediating fertilization are part of larger gene families expressed in a variety of tissues, but gene duplication followed by structural modifications has often facilitated their cooption into a fertilization function. Repeat expansions of functional domains within a gene also provide opportunities for the evolution of novel fertilization protein. ZP proteins with domain repeat expansions are linked to species-specificity in fertilization and TFP proteins that experienced domain duplications were coopted into a novel sperm function. This review outlines the importance of gene duplications and repeat domain expansions in the evolution of fertilization proteins.

Template switching in DNA replication can create and maintain RNA hairpins

The evolutionary origin of RNA stem structures and the preservation of their base pairing under a spontaneous and random mutation process have puzzled theoretical evolutionary biologists. DNA replication-related template switching is a mutation mechanism that creates reverse-complement copies of sequence regions within a genome by replicating briefly along either the complementary or nascent DNA strand. Depending on the relative positions and context of the four switch points, this process may produce a reverse-complement repeat capable of forming the stem of a perfect DNA hairpin or fix the base pairing of an existing stem. Template switching is typically thought to trigger large structural changes, and its possible role in the origin and evolution of RNA genes has not been studied. Here, we show that the reconstructed ancestral histories of RNA genes contain mutation patterns consistent with the DNA replication-related template switching. In addition to multibase compensatory mutations, the mechanism can explain complex sequence changes, although mutations breaking the structure rarely get fixed in evolution. Our results suggest a solution for the long-standing dilemma of RNA gene evolution and demonstrate how template switching can both create perfect stems with a single mutation event and help maintaining the stem structure over time. Interestingly, template switching also provides an elegant explanation for the asymmetric base pair frequencies within RNA stems.

Polymorphism of 16s rRNA Gene: Any Effect on the Biomolecular Quantitation of the Honey Bee (Apis mellifera L., 1758) Pathogen Nosema ceranae?

The microsporidian Nosema ceranae is a severe threat to the western honey bee Apis mellifera, as it is responsible for nosemosis type C, which leads the colonies to dwindle and collapse. Infection quantification is essential to clinical and research aims. Assessment is made often with molecular assays based on rRNA genes, which are present in the N. ceranae genome as multiple and polymorphic copies. This study aims to compare two different methods of Real-Time PCR (qPCR), respectively relying on the 16S rRNA and Hsp70 genes, the first of which is described as a multiple and polymorphic gene. Young worker bees, hatched in the laboratory and artificially inoculated with N. ceranae spores, were incubated at 33 °C and subject to different treatment regimens. Samples were taken post-infection and analyzed with both qPCR methods. Compared to Hsp70, the 16S rRNA method systematically detected higher abundance. Straightforward conversion between the two methods is made impossible by erratic 16s rRNA/Hsp70 ratios. The 16s rRNA polymorphism showed an increase around the inoculated dose, where a higher prevalence of ungerminated spores was expected due to the treatment effects. The possible genetic background of that irregular distribution is discussed in detail. The polymorphic nature of 16S rRNA showed to be a limit in the infection quantification. More reliably, the N. ceranae abundance can be assessed in honey bee samples with methods based on the single-copy gene Hsp70.

Genome of the world's smallest flowering plant, Wolffia australiana, helps explain its specialized physiology and unique morphology

Watermeal, Wolffia australiana, is the smallest known flowering monocot and is rich in protein. Despite its great potential as a biotech crop, basic research on Wolffia is in its infancy. Here, we generated the reference genome of a species of watermeal, W. australiana, and identified the genome-wide features that may contribute to its atypical anatomy and physiology, including the absence of roots, adaxial stomata development, and anaerobic life as a turion. In addition, we found evidence of extensive genome rearrangements that may underpin the specialized aquatic lifestyle of watermeal. Analysis of the gene inventory of this intriguing species helps explain the distinct characteristics of W. australiana and its unique evolutionary trajectory.

Halim Park and Jin Hwa Park et al. report the nuclear genome sequence of the duckweed Wolffia australiana, the smallest known flowering plant. The genome assembly represents an improvement over a recently published genome and highlights genome rearrangements that may be linked to its specialized aquatic adaptations.

Complete Characterization of Incorrect Orthology Assignments in Best Match Graphs

Genome-scale orthology assignments are usually based on reciprocal best matches. In the absence of horizontal gene transfer (HGT), every pair of orthologs forms a reciprocal best match. Incorrect orthology assignments therefore are always false positives in the reciprocal best match graph. We consider duplication/loss scenarios and characterize unambiguous false-positive (u-fp) orthology assignments, that is, edges in the best match graphs (BMGs) that cannot correspond to orthologs for any gene tree that explains the BMG. Moreover, we provide a polynomial-time algorithm to identify all u-fp orthology assignments in a BMG. Simulations show that at least [Formula: see text] of all incorrect orthology assignments can be detected in this manner. All results rely only on the structure of the BMGs and not on any a priori knowledge about underlying gene or species trees.

Rates of gene conversions between Escherichia coli ribosomal operons

Due to their universal presence and high sequence conservation, ribosomal RNA (rRNA) sequences are used widely in phylogenetics for inferring evolutionary relationships between microbes and in metagenomics for analyzing the composition of microbial communities. Most microbial genomes encode multiple copies of rRNA genes to supply cells with sufficient capacity for protein synthesis. These copies typically undergo concerted evolution that keeps their sequences identical, or nearly so, due to gene conversion, a type of intragenomic recombination that changes one copy of a homologous sequence to exactly match another. Widely varying rates of rRNA gene conversion have previously been estimated by comparative genomics methods and using genetic reporter assays. To more directly measure rates of rRNA intragenomic recombination, we sequenced the seven Escherichia coli rRNA operons in 15 lineages that were evolved for ∼13,750 generations with frequent single-cell bottlenecks that reduce the effects of selection. We identified 38 gene conversion events and estimated an overall rate of intragenomic recombination within the 16S and 23S genes between rRNA copies of 3.6 × 10⁻⁴ per genome per generation or 8.6 × 10⁻⁶ per rRNA operon per homologous donor operon per generation. This rate varied only slightly from random expectations at different sites within the rRNA genes and between rRNA operons located at different positions in the genome. Our accurate estimate of the rate of rRNA gene conversions fills a gap in our quantitative understanding of how ribosomal sequences and other multicopy elements diversify and homogenize during microbial genome evolution.

Programmed DNA Elimination in Vertebrates

Over the last few decades, an increasing number of vertebrate taxa have been identified that undergo programmed genome rearrangement, or programmed DNA loss, during development. In these organisms, the genome of germ cells is often reproducibly different from the genome of all other cells within the body. Although we clearly have not identified all vertebrate taxa that undergo programmed genome loss, the list of species known to undergo loss now represents ∼10% of vertebrate species, including several basally diverging lineages. Recent studies have shed new light on the targets and mechanisms of DNA loss and their association with canonical modes of DNA silencing. Ultimately, expansion of these studies into a larger collection of taxa will aid in reconstructing patterns of shared/independent ancestry of programmed DNA loss in the vertebrate lineage, as well as more recent evolutionary events that have shaped the structure and content of eliminated DNA.

Hybridization and recurrent evolution of left-right reversal in the land snail genus Schileykula (Orculidae, Pulmonata)

The land snail genus Schileykula Gittenberger, 1983 is distributed in arid limestone areas from western Turkey to north-western Iran. It comprises eight species, which display high variation in shell size and morphology. The cylindrical shells are 5-12 mm in height and the last shell whorls bear several inner lamellae and plicae. Two taxa differ in their chirality having sinistral shells, while all the others are dextrals such as the vast majority of orculids. The aim of this study was to establish a molecular genetic phylogeny of Schileykula and to test whether it conforms to the current morphology-based classification. Furthermore, we were interested in the phylogenetic position of the two sinistral forms in order to assess whether one or two reversals happened in the evolution of the genus. Nine out of ten species, including all four subspecies of Schileykula trapezensis and three of six subspecies of Schileykula scyphus, were investigated. A section of the mitochondrial cytochrome c oxidase subunit I gene was analyzed in 54 specimens of Schileykula and from a subsample, partial sequences of the mitochondrial genes for the 12S rRNA and the 16S rRNA, and a section of the nuclear H4/H3 histone gene cluster were obtained. The phylogenetic trees based on the mitochondrial sequences feature high support values for most nodes, and the species appear well differentiated from each other. The two chiral forms evolved independently and are not sister lineages. However, some groupings disagree with the present morphology-based classification and taxonomical conclusions are drawn. Schileykula trapezensis is polyphyletic in the molecular genetic trees; therefore, three of its subspecies are elevated to species level: Schileykula acampsis Hausdorf, 1996 comb. nov., Schileykula neuberti Hausdorf, 1996 comb. nov., and Schileykula contraria Neubert, 1993 comb. nov. Furthermore, Schileykula sigma is grouped within S. scyphus in the mitochondrial and nuclear trees and consequently treated as a subspecies of the latter (Schileykula scyphus sigma Hausdorf, 1996 comb. nov.). Schileykula nordsiecki, whose shell morphology is indistinguishable from that of the neighboring Schileykula scyphus lycaonica, but who differs in its genital anatomy, was confirmed to represent a distinct lineage. The phylogenies produced by the mitochondrial and nuclear data sets are to some extent conflicting. The patterns differ concerning the grouping of some specimens, suggesting at least two independent hybridization events involving S. contraria, S. scyphus and S. trapezensis. The results exemplify the importance of integrating both mitochondrial and nuclear sequence data in order to complement morphology-based taxonomy, and they provide further evidence for hybridization across distantly related lineages in land snails.

Functional Significance of Satellite DNAs: Insights From Drosophila

Since their discovery more than 60 years ago, satellite repeats are still one of the most enigmatic parts of eukaryotic genomes. Being non-coding DNA, satellites were earlier considered to be non-functional “junk,” but recently this concept has been extensively revised. Satellite DNA contributes to the essential processes of formation of crucial chromosome structures, heterochromatin establishment, dosage compensation, reproductive isolation, genome stability and development. Genomic abundance of satellites is under stabilizing selection owing of their role in the maintenance of vital regions of the genome – centromeres, pericentromeric regions, and telomeres. Many satellites are transcribed with the generation of long or small non-coding RNAs. Misregulation of their expression is found to lead to various defects in the maintenance of genomic architecture, chromosome segregation and gametogenesis. This review summarizes our current knowledge concerning satellite functions, the mechanisms of regulation and evolution of satellites, focusing on recent findings in Drosophila. We discuss here experimental and bioinformatics data obtained in Drosophila in recent years, suggesting relevance of our analysis to a wide range of eukaryotic organisms.

Species detection and delineation in the marine planktonic diatoms Chaetoceros and Bacteriastrum through metabarcoding: making biological sense of haplotype diversity

High-throughput sequencing (HTS) metabarcoding is commonly applied to assess phytoplankton diversity. Usually, haplotypes are grouped into operational taxonomic units (OTUs) through clustering, whereby the resulting number of OTUs depends on chosen similarity thresholds. We applied, instead, a phylogenetic approach to infer taxa among 18S rDNA V4-metabarcode haplotypes gathered from 48 time-series samples using the marine planktonic diatoms Chaetoceros and Bacteriastrum as test case. The 73 recovered taxa comprised both solitary haplotypes and polytomies, the latter composed each of a highly abundant, dominant haplotype and one to several minor, peripheral haplotypes. The solitary and dominant haplotypes usually matched reference sequences, enabling species assignation of taxa. We hypothesise that the super-abundance of reads in dominant haplotypes results from the homogenization effect of concerted evolution. Reads of populous peripheral haplotypes and dominant haplotypes show comparable distribution patterns over the sample dates, suggesting that they are part of the same population. Many taxa revealed marked seasonality, with closely related ones generally showing distinct periodicity, whereas others occur year-round. Phylogenies inferred from metabarcode haplotypes enable delineation of biologically meaningful taxa, whereas OTUs resulting from clustering algorithms often deviate markedly from such taxa.

The draft genome of Actinia tenebrosa reveals insights into toxin evolution

Sea anemones have a wide array of toxic compounds (peptide toxins found in their venom) which have potential uses as therapeutics. To date, the majority of studies characterizing toxins in sea anemones have been restricted to species from the superfamily, Actinioidea. No highly complete draft genomes are currently available for this superfamily, however, highlighting our limited understanding of the genes encoding toxins in this important group. Here we have sequenced, assembled, and annotated a draft genome for Actinia tenebrosa. The genome is estimated to be approximately 255 megabases, with 31,556 protein-coding genes. Quality metrics revealed that this draft genome matches the quality and completeness of other model cnidarian genomes, including Nematostella, Hydra, and Acropora. Phylogenomic analyses revealed strong conservation of the Cnidaria and Hexacorallia core-gene set. However, we found that lineage-specific gene families have undergone significant expansion events compared with shared gene families. Enrichment analysis performed for both gene ontologies, and protein domains revealed that genes encoding toxins contribute to a significant proportion of the lineage-specific genes and gene families. The results make clear that the draft genome of A. tenebrosa will provide insight into the evolution of toxins and lineage-specific genes, and provide an important resource for the discovery of novel biological compounds.

The nuclear 18S ribosomal DNAs of avian haemosporidian parasites

Background

Plasmodium species feature only four to eight nuclear ribosomal units on different chromosomes, which are assumed to evolve independently according to a birth-and-death model, in which new variants originate by duplication and others are deleted throughout time. Moreover, distinct ribosomal units were shown to be expressed during different developmental stages in the vertebrate and mosquito hosts. Here, the 18S rDNA sequences of 32 species of avian haemosporidian parasites are reported and compared to those of simian and rodent Plasmodium species.

Methods

Almost the entire 18S rDNAs of avian haemosporidians belonging to the genera Plasmodium (7), Haemoproteus (9), and Leucocytozoon (16) were obtained by PCR, molecular cloning, and sequencing ten clones each. Phylogenetic trees were calculated and sequence patterns were analysed and compared to those of simian and rodent malaria species. A section of the mitochondrial CytB was also sequenced.

Results

Sequence patterns in most avian Plasmodium species were similar to those in the mammalian parasites with most species featuring two distinct 18S rDNA sequence clusters. Distinct 18S variants were also found in Haemoproteus tartakovskyi and the three Leucocytozoon species, whereas the other species featured sets of similar haplotypes. The 18S rDNA GC-contents of the Leucocytozoon toddi complex and the subgenus Parahaemoproteus were extremely high with 49.3% and 44.9%, respectively. The 18S sequences of several species from all three genera showed chimeric features, thus indicating recombination.

Conclusion

Gene duplication events leading to two diverged main sequence clusters happened independently in at least six out of seven avian Plasmodium species, thus supporting evolution according to a birth-and-death model like proposed for the ribosomal units of simian and rodent Plasmodium species. Patterns were similar in the 18S rDNAs of the Leucocytozoon toddi complex and Haemoproteus tartakovskyi. However, the 18S rDNAs of the other species seem to evolve in concerted fashion like in most eukaryotes, but the presence of chimeric variants indicates that the ribosomal units rather evolve in a semi-concerted manner. The new data may provide a basis for studies testing whether differential expression of distinct 18S rDNA also occurs in avian Plasmodium species and related haemosporidian parasites.

Revisiting the evolution of the somatostatin family: Already five genes in the gnathostome ancestor

The somatostatin (SST) family members are a group of neuropeptides that are best known for their role in the regulation of growth, development and metabolism. The occurrence of six paralogous SST genes named SST1, SST2, SST3, SST4, SST5 and SST6 has been reported in vertebrates. It has been proposed that SST1, SST2 and SST5 arose in 2R from a common ancestral gene. SST3 and SST6 would have been subsequently generated by tandem duplications of the SST1 and SST2 genes respectively, at the base of the actinopterygian lineage. SST4 is thought to have appeared more recently from SST1, in teleost-specific 3R. In order to gain more insights into the SST gene family in vertebrates, we sought to identify which paralogs of this family are present in cartilaginous fish. For this purpose, we first searched the recently available genome and transcriptome databases from the catshark Scyliorhinus canicula. In a previous study, three S. canicula SST genes, called at that time SSTa, SSTb and SSTc, were identified and proposed to correspond to SST1, SST5 and SST2 respectively. In the present work, two additional SST genes, called SSTd and SSTe, were found in S. canicula plus two other chondrichtyan species, elephant shark (Callorhinchus milii) and whale shark (Rhincodon typus). Phylogeny and synteny analyses were then carried out in order to reveal the evolutionary relationships of SSTd and SSTe with other vertbrates SSTs. We showed that SSTd and SSTe correspond to SST2 and SST3 respectively, while SSTc corresponds to SST6 and not to SST2 as initially proposed. Our investigations in other vertebrate species also led us to find that the so-called SST2 gene in chicken, lungfish, sturgeons and teleosts actually corresponds to SST6. Conversely, the so-called SST6 gene in actinopterygians corresponds to SST2. Taken together, our results suggest that: i) SST3 and SST6 were already present in the gnathostome ancestor, much earlier than previously thought; ii) SST6 was also present in the tetrapod ancestor and still occurs in living birds; with this respect, it is likely that SST6 was independently lost several times during evolution: in amphibians, squamates and mammals; iii) SST2, SST3 and SST5 were probably lost in euteleosts, sarcopterygians and tetrapods, respectively.

The rpl23 gene and pseudogene are hotspots of illegitimate recombination in barley chloroplast mutator seedlings

Previously, through a TILLING (Targeting Induced Local Lesions in Genomes) approach applied on barley chloroplast mutator (cpm) seedlings a high frequency of polymorphisms in the rpl23 gene was detected. All the polymorphisms corresponded to five differences already known to exist in nature between the rpl23 gene located in the inverted repeats (IRs) and the rpl23 pseudogene located in the large single copy region (LSC). In this investigation, polymorphisms in the rpl23 gene were verified and besides, a similar situation was found for the pseudogene in cpm seedlings. On the other hand, no polymorphisms were found in any of those loci in 40 wild type barley seedlings. Those facts and the independent occurrence of polymorphisms in the gene and pseudogene in individual seedlings suggest that the detected polymorphisms initially arose from gene conversion between gene and pseudogene. Moreover, an additional recombination process involving small recombinant segments seems to occur between the two gene copies as a consequence of their location in the IRs. These and previous results support the hypothesis that the CPM protein is a component of the plastome mismatch repair (MMR) system, whose failure of the anti-recombination activity results in increased illegitimate recombination between the rpl23 gene and pseudogene.

The extremely halotolerant black yeast Hortaea werneckii - a model for intraspecific hybridization in clonal fungi

The polymorphic black yeast Hortaea werneckii (Capnodiales, Ascomycota) is extremely halotolerant (growth from 0 to 30% [w/v] NaCl) and has been extensively studied as a model for halotolerance in Eukaryotes for over two decades. Its most frequent sources are hypersaline environments and adjacent sea-water habitats in temperate, subtropical and tropical climates. Although typically saprobic, H. werneckii can also act as a commensal coloniser on human skin, causing tinea nigra on hands and soles. Here, we report that addition of NaCl to culture media expands the growth range of H. werneckii to 37 °C, which explains its colonisation of human skin, with its increased salinity. The morphological and physiological plasticity/ versatility of H. werneckii indicate that a species complex might be involved. This was investigated in this polyphasic taxonomic analysis based on the global diversity of H. werneckii strains collected from hypersaline environments, and from humans and animals. Analysis of D1/D2domains of 28S and internal transcribed spacer rDNA revealed 10 and 17 genotypes, respectively, that were not always compliant. The genotypes have global distributions. Human and environmental strains with the same genotypes are intermingled. Due to the limited number of phylogenetically informative characters in the ribosomal DNA dataset, the partial genes encoding for β-tubulin (BTB) and mini-chromosome maintenance protein (MCM7) were also sequenced. The use of these genes was hampered by ambiguous sequences obtained by Sanger sequencing, as a consequence of the diploid and highly heterozygous genome of many H. werneckii strains. Analysis of the BTB and MCM7 genes showed that in some cases two copies of the gene from the same genome are positioned in distant phylogenetic clusters of the intraspecific gene tree. Analysis of whole-genome sequences of selected H. werneckii strains generally confirmed the phylogenetic distances estimated on the basis of ribosomal genes, but also showed substantial reticulation within the phylogenetic history of the strains. This is in line with the hypothesis that the diploid genomes of H. werneckii were formed by hybridizations, which have sometimes occurred between relatively divergent strains.

Expansion of vomeronasal receptor genes (OlfC) in the evolution of fright reaction in Ostariophysan fishes

Ostariophysans are the most diverse group of freshwater fishes and feature a pheromone-elicited fright reaction. However, the genetic basis of fright reaction is unclear. Here, we compared vomeronasal type 2 receptor-like (OlfC) genes from fishes having and lacking fright reaction, to provide insight into evolution of pheromonal olfaction in fishes. We found OlfC genes expanded remarkably in ostariophysans having fright reaction compared with fishes lacking fright reaction. Phylogenetic analysis indicates OlfC subfamily 9 expanded specifically in ostariophysans having fright reaction. Principle component and phylogenetic logistic regression analysis partitioned fishes by ecotype (having or lacking fright reaction) and identified OlfC subfamily 9 as being an important factor for fright reaction. Expression levels of expanded OlfC subfamily genes after fright reaction in zebrafish changed more than did genes that had not expanded. Furthermore, evidence of positive selection was found in the expanded OlfC proteins in ostariophysan fishes having fright reaction. These results provide new insight into the genetic basis of fright reaction in ostariophysan fish and will enable future research into the mechanism of action of OlfC proteins.

Complex Gene Loss and Duplication Events Have Facilitated the Evolution of Multiple Loricrin Genes in Diverse Bird Species

The evolution of a mechanically resilient epidermis was a key adaptation in the transition of amniotes to a fully terrestrial lifestyle. Skin appendages usually form via a specialized type of programmed cell death known as cornification which is characterized by the formation of an insoluble cornified envelope (CE). Many of the substrates of cornification are encoded by linked genes located at a conserved genetic locus known as the epidermal differentiation complex (EDC). Loricrin is the main protein component of the mammalian CE and is encoded for by a gene located within the EDC. Recently, genes resembling mammalian loricrin, along with several other proteins most likely involved in CE formation, have been identified within the EDC of birds and several reptiles. To better understand the evolution and function of loricrin in birds, we screened the genomes of 50 avian species and 3 crocodilians to characterize their EDC regions. We found that loricrin is present within the EDC of all species investigated, and that three loricrin genes were present in birds. Phylogenetic and molecular evolution analyses found evidence that gene deletions and duplications as well as concerted evolution has shaped the evolution of avian loricrins. Our results suggest a complex evolutionary history of avian loricrins which has accompanied the evolution of bird species with diverse morphologies and lifestyles.

The Birth and Death of Toxins with Distinct Functions: A Case Study in the Sea Anemone Nematostella

The cnidarian Nematostella vectensis has become an established lab model, providing unique opportunities for venom evolution research. The Nematostella venom system is multimodal: involving both nematocytes and ectodermal gland cells, which produce a toxin mixture whose composition changes throughout the life cycle. Additionally, their modes of interaction with predators and prey vary between eggs, larvae, and adults, which is likely shaped by the dynamics of the venom system.

Nv1 is a major component of adult venom, with activity against arthropods (through specific inhibition of sodium channel inactivation) and fish. Nv1 is encoded by a cluster of at least 12 nearly identical genes that were proposed to be undergoing concerted evolution. Surprisingly, we found that Nematostella venom includes several Nv1 paralogs escaping a pattern of general concerted evolution, despite belonging to the Nv1-like family. Here, we show two of these new toxins, Nv4 and Nv5, are lethal for zebrafish larvae but harmless to arthropods, unlike Nv1. Furthermore, unlike Nv1, the newly identified toxins are expressed in early life stages. Using transgenesis and immunostaining, we demonstrate that Nv4 and Nv5 are localized to ectodermal gland cells in larvae.

The evolution of Nv4 and Nv5 can be described either as neofunctionalization or as subfunctionalization. Additionally, the Nv1-like family includes several pseudogenes being an example of nonfunctionalization and venom evolution through birth-and-death mechanism. Our findings reveal the evolutionary history for a toxin radiation and point toward the ecological function of the novel toxins constituting a complex cnidarian venom.

PCR amplification of a long rDNA segment with one primer pair in agriculturally important nematodes

Ribosomal DNA has been a reliable source of taxonomic and phylogenetic markers due to its high copy number in the genome and stable variation with few polymorphisms due to the homogenizing effect of concerted evolution. Typically specific regions are amplified through polymerase chain reaction (PCR) with multiple primer pairs that generate often incomplete and overlapping regions between adjacent segments of 18S, ITS1, 5.8S, ITS2, and 28S rDNA nucleotide sequences when combined in tandem. To improve the efficiency of this effort, a strategy for generating all these molecular sequences at once through PCR amplification of a large ribosomal 3.3 to 4.2 kb DNA target was developed using primer 18S-CL-F3 paired with D3B or a new alternative 28S PCR primer (28S-CL-R) and other well-positioned and ribosomal-specific sequencing primers (including novel primers 18S-CL-F7, 18S-CL-R6, 18S-CL-R7, 18S-CL-F8, 5.8S-CL-F1, 5.8S-CL-R1, 28S-CL-F1, 28S-CL-R3, 28S-CL-F3, 28S-CL-R1, and 28S-CL-F2). The D1 region between ITS2 and 28S boundaries and the flanking sequence between 18S and ITS1 boundaries were fully revealed in this large nucleotide segment. To demonstrate the value of this strategy, the long rDNA segment was amplified and directly sequenced in 17 agriculturally important nematodes from the Tylenchida, Aphelenchida, and Dorylaimida. The primers and their positions may be employed with traditional Sanger sequencing and with next-generation sequencing reagents and protocols.

Accurate mapping of tRNA reads

Motivation

Many repetitive DNA elements are transcribed at appreciable expression levels. Mapping the corresponding RNA sequencing reads back to a reference genome is notoriously difficult and error-prone task, however. This is in particular true if chemical modifications introduce systematic mismatches, while at the same time the genomic loci are only approximately identical, as in the case of tRNAs.

Results

We therefore developed a dedicated mapping strategy to handle RNA-seq reads that map to tRNAs relying on a modified target genome in which known tRNA loci are masked and instead intronless tRNA precursor sequences are appended as artificial 'chromosomes'. In a first pass, reads that overlap the boundaries of mature tRNAs are extracted. In the second pass, the remaining reads are mapped to a tRNA-masked target that is augmented by representative mature tRNA sequences. Using both simulated and real life data we show that our best-practice workflow removes most of the mapping artefacts introduced by simpler mapping schemes and makes it possible to reliably identify many of chemical tRNA modifications in generic small RNA-seq data. Using simulated data the FDR is only 2%. We find compelling evidence for tissue specific differences of tRNA modification patterns.

Availability and implementation

The workflow is available both as a bash script and as a Galaxy workflow from https://github.com/AnneHoffmann/tRNA-read-mapping.

Contact

fabian@tbi.univie.ac.at.

Supplementary information

Supplementary data are available at Bioinformatics online.

Contrasting patterns of coding and flanking region evolution in mammalian keratin associated protein-1 genes

Mammalian genomes contain a number of duplicated genes, and sequence identity between these duplicates can be maintained by purifying selection. However, between-duplicate recombination can also maintain sequence identity between copies, resulting in a pattern known as concerted evolution where within-genome repeats are more similar to each other than to orthologous repeats in related species. Here we investigated the tandemly-repeated keratin-associated protein 1 (KAP1) gene family, KRTAP1, which encodes proteins that are important components of hair and wool in mammals. Comparison of eutherian mammal KRTAP1 gene repeats within and between species shows a strong pattern of concerted evolution. However, in striking contrast to the coding regions of these genes, we find that the flanking regions have a divergent pattern of evolution. This contrast in evolutionary pattern transitions abruptly near the start and stop codons of the KRTAP1 genes. We reveal that this difference in evolutionary patterns is not explained by conventional purifying selection, nor is it likely a consequence of codon adaptation or reverse transcription of KRTAP1-n mRNA. Instead, the evidence suggests that these contrasting patterns result from short-tract gene conversion events that are biased to the KRTAP1 coding region by selection and/or differential sequence divergence. This work demonstrates the power that gene conversion has to finely shape the evolution of repetitive genes, and provides another distinctive pattern of contrasting evolutionary outcomes that results from gene conversion. A greater emphasis on exploring the evolution of multi-gene eukaryotic families will reveal how common different contrasting evolutionary patterns are in gene duplicates.

Rapid Divergence of Genome Architectures Following the Origin of an Ectomycorrhizal Symbiosis in the Genus Amanita

Fungi are evolutionary shape shifters and adapt quickly to new environments. Ectomycorrhizal (EM) symbioses are mutualistic associations between fungi and plants and have evolved repeatedly and independently across the fungal tree of life, suggesting lineages frequently reconfigure genome content to take advantage of open ecological niches. To date analyses of genomic mechanisms facilitating EM symbioses have involved comparisons of distantly related species, but here, we use the genomes of three EM and two asymbiotic (AS) fungi from the genus Amanita as well as an AS outgroup to study genome evolution following a single origin of symbiosis. Our aim was to identify the defining features of EM genomes, but our analyses suggest no clear differentiation of genome size, gene repertoire size, or transposable element content between EM and AS species. Phylogenetic inference of gene gains and losses suggests the transition to symbiosis was dominated by the loss of plant cell wall decomposition genes, a confirmation of previous findings. However, the same dynamic defines the AS species A. inopinata, suggesting loss is not strictly associated with origin of symbiosis. Gene expansions in the common ancestor of EM Amanita were modest, but lineage specific and large gene family expansions are found in two of the three EM extant species. Even closely related EM genomes appear to share few common features. The genetic toolkit required for symbiosis appears already encoded in the genomes of saprotrophic species, and this dynamic may explain the pervasive, recurrent evolution of ectomycorrhizal associations.

Pervasive Correlated Evolution in Gene Expression Shapes Cell and Tissue Type Transcriptomes

The evolution and diversification of cell types is a key means by which animal complexity evolves. Recently, hierarchical clustering and phylogenetic methods have been applied to RNA-seq data to infer cell type evolutionary history and homology. A major challenge for interpreting this data is that cell type transcriptomes may not evolve independently due to correlated changes in gene expression. This nonindependence can arise for several reasons, such as common regulatory sequences for genes expressed in multiple tissues, that is, pleiotropic effects of mutations. We develop a model to estimate the level of correlated transcriptome evolution (LCE) and apply it to different data sets. The results reveal pervasive correlated transcriptome evolution among different cell and tissue types. In general, tissues related by morphology or developmental lineage exhibit higher LCE than more distantly related tissues. Analyzing new data collected from bird skin appendages suggests that LCE decreases with the phylogenetic age of tissues compared, with recently evolved tissues exhibiting the highest LCE. Furthermore, we show correlated evolution can alter patterns of hierarchical clustering, causing different tissue types from the same species to cluster together. To identify genes that most strongly contribute to the correlated evolution signal, we performed a gene-wise estimation of LCE on a data set with ten species. Removing genes with high LCE allows for accurate reconstruction of evolutionary relationships among tissue types. Our study provides a statistical method to measure and account for correlated gene expression evolution when interpreting comparative transcriptome data.

Comparative Genomics of Apomictic Root-Knot Nematodes: Hybridization, Ploidy, and Dynamic Genome Change

The root-knot nematodes (genus Meloidogyne) are important plant parasites causing substantial agricultural losses. The Meloidogyne incognita group (MIG) of species, most of which are obligatory apomicts (mitotic parthenogens), are extremely polyphagous and important problems for global agriculture. While understanding the genomic basis for their variable success on different crops could benefit future agriculture, analyses of their genomes are challenging due to complex evolutionary histories that may incorporate hybridization, ploidy changes, and chromosomal fragmentation. Here, we sequence 19 genomes, representing five species of key root-knot nematodes collected from different geographic origins. We show that a hybrid origin that predated speciation within the MIG has resulted in each species possessing two divergent genomic copies. Additionally, the apomictic MIG species are hypotriploids, with a proportion of one genome present in a second copy. The hypotriploid proportion varies among species. The evolutionary history of the MIG genomes is revealed to be very dynamic, with noncrossover recombination both homogenizing the genomic copies, and acting as a mechanism for generating divergence between species. Interestingly, the automictic MIG species M. floridensis differs from the apomict species in that it has become homozygous throughout much of its genome.

Long read sequencing reveals poxvirus evolution through rapid homogenization of gene arrays

Poxvirus adaptation can involve combinations of recombination-driven gene copy number variation and beneficial single nucleotide variants (SNVs) at the same loci. How these distinct mechanisms of genetic diversification might simultaneously facilitate adaptation to host immune defenses is unknown. We performed experimental evolution with vaccinia virus populations harboring a SNV in a gene actively undergoing copy number amplification. Using long sequencing reads from the Oxford Nanopore Technologies platform, we phased SNVs within large gene copy arrays for the first time. Our analysis uncovered a mechanism of adaptive SNV homogenization reminiscent of gene conversion, which is actively driven by selection. This study reveals a new mechanism for the fluid gain of beneficial mutations in genetic regions undergoing active recombination in viruses and illustrates the value of long read sequencing technologies for investigating complex genome dynamics in diverse biological systems.

Formal description of sequence-based voucherless Fungi: promises and pitfalls, and how to resolve them

There is urgent need for a formal nomenclature of sequence-based, voucherless Fungi, given that environmental sequencing has accumulated more than one billion fungal ITS reads in the Sequence Read Archive, about 1,000 times as many as fungal ITS sequences in GenBank. These unnamed Fungi could help to bridge the gap between 115,000 to 140,000 currently accepted and 2.2 to 3.8 million predicted species, a gap that cannot realistically be filled using specimen or culture-based inventories. The Code never aimed at placing restrictions on the nature of characters chosen for taxonomy, and the requirement for physical types is now becoming a constraint on the advancement of science. We elaborate on the promises and pitfalls of sequence-based nomenclature and provide potential solutions to major concerns of the mycological community. Types of sequence-based taxa, which by default lack a physical specimen or culture, could be designated in four alternative ways: (1) the underlying sample ('bag' type), (2) the DNA extract, (3) fluorescent in situ hybridization (FISH), or (4) the type sequence itself. Only (4) would require changes to the Code and the latter would be the most straightforward approach, complying with three of the five principal functions of types better than physical specimens. A fifth way, representation of the sequence in an illustration, has been ruled as unacceptable in the Code. Potential flaws in sequence data are analogous to flaws in physical types, and artifacts are manageable if a stringent analytical approach is applied. Conceptual errors such as homoplasy, intragenomic variation, gene duplication, hybridization, and horizontal gene transfer, apply to all molecular approaches and cannot be used as a specific argument against sequence-based nomenclature. The potential impact of these phenomena is manageable, as phylogenetic species delimitation has worked satisfactorily in Fungi. The most serious shortcoming of sequence-based nomenclature is the likelihood of parallel classifications, either by describing taxa that already have names based on physical types, or by using different markers to delimit species within the same lineage. The probability of inadvertently establishing sequence-based species that have names available is between 20.4 % and 1.5 % depending on the number of globally predicted fungal species. This compares favourably to a historical error rate of about 30 % based on physical types, and this rate could be reduced to practically zero by adding specific provisions to this approach in the Code. To avoid parallel classifications based on different markers, sequence-based nomenclature should be limited to a single marker, preferably the fungal ITS barcoding marker; this is possible since sequence-based nomenclature does not aim at accurate species delimitation but at naming lineages to generate a reference database, independent of whether these lineages represent species, closely related species complexes, or infraspecies. We argue that clustering methods are inappropriate for sequence-based nomenclature; this approach must instead use phylogenetic methods based on multiple alignments, combined with quantitative species recognition methods. We outline strategies to obtain higher-level phylogenies for ITS-based, voucherless species, including phylogenetic binning, 'hijacking' species delimitation methods, and temporal banding. We conclude that voucherless, sequence-based nomenclature is not a threat to specimen and culture-based fungal taxonomy, but a complementary approach capable of substantially closing the gap between known and predicted fungal diversity, an approach that requires careful work and high skill levels.

NGS barcode sequencing in taxonomy and diagnostics, an application in "Candida" pathogenic yeasts with a metagenomic perspective

Species identification of yeasts and other Fungi is currently carried out with Sanger sequences of selected molecular markers, mainly from the ribosomal DNA operon, characterized by hundreds of tandem repeats of the 18S, ITS1, 5.8S, ITS2 and LSU loci. The ITS region has been recently proposed as a primary barcode marker making this region the most used one in taxonomy, phylogeny and diagnostics. The introduction of NGS is providing tools of high efficacy and relatively low cost to amplify two or more markers simultaneously with great sequencing depth. However, the presence of intra-genomic variability between the repeats requires specific analytical procedures and pipelines. In this study, 286 strains belonging to 11 pathogenic yeasts species were analysed with NGS of the region spanning from ITS1 to the D1/D2 domain of the LSU encoding ribosomal DNA. Results showed that relatively high heterogeneity can hamper the use of these sequences for the identification of single strains and even more of complex microbial mixtures. These observations point out that the metagenomics studies could be affected by species inflection at levels higher than currently expected.

Satellite DNA in Vicia faba is characterized by remarkable diversity in its sequence composition, association with centromeres, and replication timing

Satellite DNA, a class of repetitive sequences forming long arrays of tandemly repeated units, represents substantial portions of many plant genomes yet remains poorly characterized due to various methodological obstacles. Here we show that the genome of the field bean (Vicia faba, 2n = 12), a long-established model for cytogenetic studies in plants, contains a diverse set of satellite repeats, most of which remained concealed until their present investigation. Using next-generation sequencing combined with novel bioinformatics tools, we reconstructed consensus sequences of 23 novel satellite repeats representing 0.008–2.700% of the genome and mapped their distribution on chromosomes. We found that in addition to typical satellites with monomers hundreds of nucleotides long, V. faba contains a large number of satellite repeats with unusually long monomers (687–2033 bp), which are predominantly localized in pericentromeric regions. Using chromatin immunoprecipitation with CenH3 antibody, we revealed an extraordinary diversity of centromeric satellites, consisting of seven repeats with chromosome-specific distribution. We also found that in spite of their different nucleotide sequences, all centromeric repeats are replicated during mid-S phase, while most other satellites are replicated in the first part of late S phase, followed by a single family of FokI repeats representing the latest replicating chromatin.

Chromosomal inversions promote genomic islands of concerted evolution of Hsp70 genes in the Drosophila subobscura species subgroup

Heat-shock (HS) assays to understand the connection between standing inversion variation and evolutionary response to climate change in Drosophila subobscura found that "warm-climate" inversion O₃₊₄ exhibits non-HS levels of Hsp70 protein like those of "cold-climate" O_ST after HS induction. This was unexpected, as overexpression of Hsp70 can incur multiple fitness costs. To understand the genetic basis of this finding, we have determined the genomic sequence organization of the Hsp70 family in four different inversions, including O_ST , O₃₊₄ , O₃₊₄₊₈ and O₃₊₄₊₁₆ , using as outgroups the remainder of the subobscura species subgroup, namely Drosophila madeirensis and Drosophila guanche. We found (i) in all the assayed lines, the Hsp70 family resides in cytological locus 94A and consists of only two genes, each with four HS elements (HSEs) and three GAGA sites on its promoter. Yet, in O_{ST ,} the family is comparatively more compact; (ii) the two Hsp70 copies evolve in concert through gene conversion, except in D. guanche; (iii) within D. subobscura, the rate of concerted evolution is strongly structured by inversion, being higher in O_ST than in O₃₊₄ ; and (iv) in D. guanche, the two copies accumulated multiple differences, including a newly evolved "gap-type" HSE2. The absence of concerted evolution in this species may be related to a long-gone-unnoticed observation that it lacks Hsp70 HS response, perhaps because it has evolved within a narrow thermal range in an oceanic island. Our results point to a previously unrealized link between inversions and concerted evolution, with potentially major implications for understanding genome evolution.

Diversity and evolution of the repetitive genomic content in Cannabis sativa

Background

The repetitive content of the genome, once considered to be “junk DNA”, is in fact an essential component of genomic architecture and evolution. In this study, we used the genomes of three varieties of Cannabis sativa, three varieties of Humulus lupulus and one genotype of Morus notabilis to explore their repetitive content using a graph-based clustering method, designed to explore and compare repeat content in genomes that have not been fully assembled.

Results

The repetitive content in the C. sativa genome is mainly composed of the retrotransposons LTR/Copia and LTR/Gypsy (14% and 14.8%, respectively), ribosomal DNA (2%), and low-complexity sequences (29%). We observed a recent copy number expansion in some transposable element families. Simple repeats and low complexity regions of the genome show higher intra and inter species variation.

Conclusions

As with other sequenced genomes, the repetitive content of C. sativa’s genome exhibits a wide range of evolutionary patterns. Some repeat types have patterns of diversity consistent with expansions followed by losses in copy number, while others may have expanded more slowly and reached a steady state. Still, other repetitive sequences, particularly ribosomal DNA (rDNA), show signs of concerted evolution playing a major role in homogenizing sequence variation.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4494-3) contains supplementary material, which is available to authorized users.