Formation and expression of pseudogenes on the B chromosome of rye

Does chromoanagenesis play a role in the origin of B chromosomes?

B chromosomes (Bs) exist in addition to the standard (A) chromosomes in a wide range of species. The process underlying their origin is still unclear. We propose pathways of intra- and interspecific origin of B chromosomes based on known mechanisms of chromosome evolution and available knowledge of their sequence composition in different species. We speculate that a mitotic or meiotic segregation error of one or more A chromosomes initiates, via chromoanagenesis, the formation of a proto-B chromosome. In the second step, proto-B chromosomes accumulate A chromosome- and organelle-derived sequences over time, most likely via DNA double-strand break (DSB) mis-repair. Consequently, the original structure of the early stage proto-B chromosomes becomes masked by continuous sequence incorporation. The similarity between A chromosome sequences integrated into B chromosomes and the original sequences on the donor chromosomes decreases over time if there is no selection pressure on these sequences on B chromosomes. However, besides chromoanagenesis, also other mechanisms leading to the formation of B chromosomes might exist.

The genetic mechanism of B chromosome drive in rye illuminated by chromosome-scale assembly

The genomes of many plants, animals, and fungi frequently comprise dispensable B chromosomes that rely upon various chromosomal drive mechanisms to counteract the tendency of non-essential genetic elements to be purged over time. The B chromosome of rye - a model system for nearly a century - undergoes targeted nondisjunction during first pollen mitosis, favouring segregation into the generative nucleus, thus increasing their numbers over generations. However, the genetic mechanisms underlying this process are poorly understood. Here, using a newly-assembled, ~430 Mb-long rye B chromosome pseudomolecule, we identify five candidate genes whose role as trans-acting moderators of the chromosomal drive is supported by karyotyping, chromosome drive analysis and comparative RNA-seq. Among them, we identify DCR28, coding a microtubule-associated protein related to cell division, and detect this gene also in the B chromosome of Aegilops speltoides. The DCR28 gene family is neo-functionalised and serially-duplicated with 15 B chromosome-located copies that are uniquely highly expressed in the first pollen mitosis of rye.

Chromosome-level genome assemblies reveal genome evolution of an invasive plant Phragmites australis

Biological invasions pose a significant threat to ecosystems, disrupting local biodiversity and ecosystem functions. The genomic underpinnings of invasiveness, however, are still largely unknown, making it difficult to predict and manage invasive species effectively. The common reed (Phragmites australis) is a dominant grass species in wetland ecosystems and has become particularly invasive when transferred from Europe to North America. Here, we present a high-quality gap-free, telomere-to-telomere genome assembly of Phragmites australis consisting of 24 pseudochromosomes and a B chromosome. Fully phased subgenomes demonstrated considerable subgenome dominance and revealed the divergence of diploid progenitors approximately 30.9 million years ago. Comparative genomics using chromosome-level scaffolds for three other lineages and a previously published draft genome assembly of an invasive lineage revealed that gene family expansions in the form of tandem duplications may have contributed to the invasiveness of the lineage. This study sheds light on the genome evolution of Arundinoideae grasses and suggests that genetic drivers, such as gene family expansions and tandem duplications, may underly the processes of biological invasion in plants. These findings provide a crucial step toward understanding and managing the genetic basis of invasiveness in plant species.

Transcribing the enigma: the B chromosome as a territory of uncharted RNAs

B chromosomes are supernumerary elements found in several groups of eukaryotes, including fungi, plants, and animals. Typically, these chromosomes either originate from their hosts through errors in meiosis or interspecifically through horizontal transfer. While many B chromosomes are primarily heterochromatic and possess a low number of coding genes, these additional elements are still capable of transcribing sequences and exerting influence on the expression of host genes. How B chromosomes escape elimination and which impacts can be promoted in the cell always intrigued the cytogeneticists. In pursuit of understanding the behavior and functional impacts of these extra elements, cytogenetic studies meet the advances of molecular biology, incorporating various techniques into investigating B chromosomes from a functional perspective. In this review, we present a timeline of studies investigating B chromosomes and RNAs, highlighting the advances and key findings throughout their history. Additionally, we identified which RNA classes are reported in the B chromosomes and emphasized the necessity for further investigation into new perspectives on the B chromosome functions. In this context, we present a phylogenetic tree that illustrates which branches either report B chromosome presence or have functional RNA studies related to B chromosomes. We propose investigating other unexplored RNA classes and conducting functional analysis in conjunction with cytogenetic studies to enhance our understanding of the B chromosome from an RNA perspective.

Unravelling the due importance of pseudogenes and their resurrection in plants

The complexities of a genome are underpinned to the vast expanses of the intergenic region, which constitutes ∼97-98% of the genome. This region is essentially composed of what is colloquially referred to as the "junk DNA" and is composed of various elements like transposons, repeats, pseudogenes, etc. The latter have long been considered as dead elements merely contributing to transcriptional noise in the genome. Many studies now describe the previously unknown regulatory functions of these genes. Recent advances in the Next-generation sequencing (NGS) technologies have allowed unprecedented access to these regions. With the availability of whole genome sequences of more than 788 different plant species in past 20 years, genome annotation has become feasible like never before. Different bioinformatic pipelines are available for the identification of pseudogenes. However, still little is known about their biological functions. The functional validation of these genes remains challenging and research in this area is still in infancy, particularly in plants. CRISPR/Cas-based genome editing could provide solutions to understand the biological roles of these genes by allowing creation of precise edits within these genes. The possibility of pseudogene reactivation or resurrection as has been demonstrated in a few studies might open new avenues of genetic manipulation to yield a desirable phenotype. This review aims at comprehensively summarizing the progress made with regards to the identification of pseudogenes and understanding their biological functions in plants.

Comprehending the dynamism of B chromosomes in their journey towards becoming unselfish

Investigated for more than a century now, B chromosomes (Bs) research has come a long way from Bs being considered parasitic or neutral to becoming unselfish and bringing benefits to their hosts. B chromosomes exist as accessory chromosomes along with the standard A chromosomes (As) across eukaryotic taxa. Represented singly or in multiple copies, B chromosomes are largely heterochromatic but also contain euchromatic and organellar segments. Although B chromosomes are derived entities, they follow their species-specific evolutionary pattern. B chromosomes fail to pair with the standard chromosomes during meiosis and vary in their number, size, composition and structure across taxa and ensure their successful transmission through non-mendelian mechanisms like mitotic, pre-meiotic, meiotic or post-meiotic drives, unique non-disjunction, self-pairing or even imparting benefits to the host when they lack drive. B chromosomes have been associated with cellular processes like sex determination, pathogenicity, resistance to pathogens, phenotypic effects, and differential gene expression. With the advancements in B-omics research, novel insights have been gleaned on their functions, some of which have been associated with the regulation of gene expression of A chromosomes through increased expression of miRNAs or differential expression of transposable elements located on them. The next-generation sequencing and emerging technologies will further likely unravel the cellular, molecular and functional behaviour of these enigmatic entities. Amidst the extensive fluidity shown by B chromosomes in their structural and functional attributes, we perceive that the existence and survival of B chromosomes in the populations most likely seem to be a trade-off between the drive efficiency and adaptive significance versus their adverse effects on reproduction.

First characterization of PIWI-interacting RNA clusters in a cichlid fish with a B chromosome

BACKGROUND

B chromosomes are extra elements found in several eukaryote species. Usually, they do not express a phenotype in the host. However, advances in bioinformatics over the last decades have allowed us to describe several genes and molecular functions related to B chromosomes. These advances enable investigations of the relationship between the B chromosome and the host to understand how this element has been preserved in genomes. However, considering that transposable elements (TEs) are highly abundant in this supernumerary chromosome, there is a lack of knowledge concerning the dynamics of TE control in B-carrying cells. Thus, the present study characterized PIWI-interacting RNA (piRNA) clusters and pathways responsible for silencing the mobilization of TEs in gonads of the cichlid fish Astatotilapia latifasciata carrying the B chromosome.

RESULTS

Through small RNA-seq and genome assembly, we predicted and annotated piRNA clusters in the A. latifasciata genome for the first time. We observed that these clusters had biased expression related to sex and the presence of the B chromosome. Furthermore, three piRNA clusters, named curupira, were identified in the B chromosome. Two of them were expressed exclusively in gonads of samples with the B chromosome. The composition of these curupira sequences was derived from LTR, LINE, and DNA elements, representing old and recent transposition events in the A. latifasciata genome and the B chromosome. The presence of the B chromosome also affected the expression of piRNA pathway genes. The mitochondrial cardiolipin hydrolase-like (pld6) gene is present in the B chromosome, as previously reported, and an increase in its expression was detected in gonads with the B chromosome.

CONCLUSIONS

Due to the high abundance of TEs in the B chromosome, it was possible to investigate the origin of piRNA from these jumping genes. We hypothesize that the B chromosome has evolved its own genomic guardians to prevent uncontrolled TE mobilization. Furthermore, we also detected an expression bias in the presence of the B chromosome over A. latifasciata piRNA clusters and pathway genes.

B Chromosomes in Psalidodon scabripinnis (Characiformes, Characidae) Species Complex

B chromosomes are extra-genomic components of cells found in individuals and in populations of some eukaryotic organisms. They have been described since the first observations of chromosomes, but several aspects of their biology remain enigmatic. Despite being present in hundreds of fungi, plants, and animal species, only a small number of B chromosomes have been investigated through high-throughput analyses, revealing the remarkable mechanisms employed by these elements to ensure their maintenance. Populations of the Psalidodon scabripinnis species complex exhibit great B chromosome diversity, making them a useful material for various analyses. In recent years, important aspects of their biology have been revealed. Here, we review these studies presenting a comprehensive view of the B chromosomes in the P. scabripinnis complex and a new hypothesis regarding the role of the B chromosome in the speciation process.

Rye B chromosomes differently influence the expression of A chromosome-encoded genes depending on the host species

The B chromosome (B) is a dispensable component of the genome in many species. To evaluate the impact of Bs on the transcriptome of the standard A chromosomes (A), comparative RNA-seq analyses of rye and wheat anthers with and without additional rye Bs were conducted. In both species, 5-6% of the A-derived transcripts across the entire genomes were differentially expressed in the presence of  2Bs. The GO term enrichment analysis revealed that Bs influence A chromosome encoded processes like "gene silencing"; "DNA methylation or demethylation"; "chromatin silencing"; "negative regulation of gene expression, epigenetic"; "post-embryonic development"; and "chromosome organization." 244 B chromosome responsive A-located genes in + 2B rye and + B wheat shared the same biological function. Positively correlated with the number of Bs, 939 and 1391 B-specific transcripts were identified in + 2B and + 4B wheat samples, respectively. 85% of B-transcripts in + 2B were also found in + 4B transcriptomes. 297 B-specific transcripts were identified in + 2B rye, and 27% were common to the B-derived transcripts identified in + B wheat. Bs encode mobile elements and housekeeping genes, but most B-transcripts were without detectable similarity to known genes. Some of these genes are involved in cell division-related functions like Nuf2 and might indicate their importance in maintaining Bs. The transcriptome analysis provides new insights into the complex interrelationship between standard A chromosomes and supernumerary B chromosomes.

B Chromosomes’ Sequences in Yellow-Necked Mice Apodemus flavicollis—Exploring the Transcription

B chromosomes (Bs) are highly polymorphic additional chromosomes in the genomes of many species. Due to the dispensability of Bs and the lack of noticeable phenotypic effects in their carriers, they were considered genetically inert for a long time. Recent studies on Bs in Apodemus flavicollis revealed their genetic composition, potential origin, and spatial organization in the interphase nucleus. Surprisingly, the genetic content of Bs in this species is preserved in all studied samples, even in geographically distinct populations, indicating its biological importance. Using RT-PCR we studied the transcription activity of three genes (Rraga, Haus6, and Cenpe) previously identified on Bs in A. flavicollis. We analysed mRNA isolated from spleen tissues of 34 animals harboring different numbers of Bs (0–3).The products of transcriptional activity of the analysed sequences differ in individuals with and without Bs. We recorded B-genes and/or genes from the standard genome in the presence of Bs, showing sex-dependent higher levels of transcriptional activity. Furthermore, the transcriptional activity of Cenpe varied with the age of the animals differently in the group with and without Bs. With aging, the amount of product was only found to significantly decrease in B carriers. The potential biological significance of all these differences is discussed in the paper.

Evolution of B Chromosomes: From Dispensable Parasitic Chromosomes to Essential Genomic Players

B chromosomes represent additional chromosomes found in many eukaryotic organisms. Their origin is not completely understood but recent genomic studies suggest that they mostly arise through rearrangements and duplications from standard chromosomes. They can occur in single or multiple copies in a cell and are usually present only in a subset of individuals in the population. Because B chromosomes frequently show unstable inheritance, their maintenance in a population is often associated with meiotic drive or other mechanisms that increase the probability of their transmission to the next generation. For all these reasons, B chromosomes have been commonly considered to be nonessential, selfish, parasitic elements. Although it was originally believed that B chromosomes had little or no effect on an organism's biology and fitness, a growing number of studies have shown that B chromosomes can play a significant role in processes such as sex determination, pathogenicity and resistance to pathogens. In some cases, B chromosomes became an essential part of the genome, turning into new sex chromosomes or germline-restricted chromosomes with important roles in the organism's fertility. Here, we review such cases of "cellular domestication" of B chromosomes and show that B chromosomes can be important genomic players with significant evolutionary impact.

Identification of rye B chromosome-associated peptides by mass spectrometry

B chromosomes (Bs) are supernumerary dispensable components of the standard genome (A chromosomes, As) that have been found in many eukaryotes. So far, it is unkown whether the B-derived transcripts translate to proteins or if the host proteome is changed due to the presence of Bs. Comparative mass spectrometry was performed using the protein samples isolated from shoots of rye plants with and without Bs. We aimed to identify B-associated peptides and analyzed the effects of Bs on the total proteome. Our comparative proteome analysis demonstrates that the presence of rye Bs affects the total proteome including different biological function processes. We found 319 of 16 776 quantified features in at least three out of five +B plants but not in 0B plants; 31 of 319 features were identified as B-associated peptide features. According to our data mining, one B-specific protein fragment showed similarity to a glycine-rich RNA binding protein which differed from its A-paralogue by two amino acid insertions. Our result represents a milestone in B chromosome research, because this is the first report to demonstrate the existence of Bs changing the proteome of the host.

Gauging the trends of pseudogenes in plants

Pseudogenes, the debilitated parts of ancient genes, were previously scrapped off as junk or discarded genes with no functional significance. Pseudogenes have come under scrutiny for their functionality, since recent studies have unveiled their importance in the regulation of their corresponding parent genes and various biological mechanisms. Despite the enormous occurrence of pseudogenes in plants, the lack of experimental validation has contributed toward their unresolved roles in gene regulation. Contrarily, most of the studies associated with gene regulation have been mainly reported for humans, mice, and other mammalian genomes. Consequently, in order to present a cumulative report on plant-based pseudogenes research, an attempt has been made to assemble multiple studies presenting the pseudogene classification, the prediction and the determination of comparative accuracies of various computational pipelines, and recent trends in analyzing their biological functions, and regulatory mechanisms. This review represents the classical, as well as the recent advances on pseudogene identification and their potential roles in transcriptional regulation, which could possibly invigorate the quality of genome annotation, evolutionary analysis, and complexity surrounding the regulatory pathways in plants. Thus, when the ambiguous boundary girdling the pseudogenes eventually recedes on account of their explicit orchestration role, research in flora would no longer saunter compared to that on fauna.

Differential expression of miRNAs in the presence of B chromosome in the cichlid fish Astatotilapia latifasciata

BACKGROUND

B chromosomes (Bs) are extra elements observed in diverse eukaryotes, including animals, plants and fungi. Although Bs were first identified a century ago and have been studied in hundreds of species, their biology is still enigmatic. Recent advances in omics and big data technologies are revolutionizing the B biology field. These advances allow analyses of DNA, RNA, proteins and the construction of interactive networks for understanding the B composition and behavior in the cell. Several genes have been detected on the B chromosomes, although the interaction of B sequences and the normal genome remains poorly understood.

RESULTS

We identified 727 miRNA precursors in the A. latifasciata genome, 66% which were novel predicted sequences that had not been identified before. We were able to report the A. latifasciata-specific miRNAs and common miRNAs identified in other fish species. For the samples carrying the B chromosome (B⁺), we identified 104 differentially expressed (DE) miRNAs that are down or upregulated compared to samples without B chromosome (B^-) (p < 0.05). These miRNAs share common targets in the brain, muscle and gonads. These targets were used to construct a protein-protein-miRNA network showing the high interaction between the targets of differentially expressed miRNAs in the B⁺ chromosome samples. Among the DE-miRNA targets there are protein-coding genes reported for the B chromosome that are present in the protein-protein-miRNA network. Additionally, Gene Ontology (GO) terms related to nuclear matrix organization and response to stimulus are exclusive to DE miRNA targets of B⁺ samples.

CONCLUSIONS

This study is the first to report the connection of B chromosomes and miRNAs in a vertebrate species. We observed that the B chromosome impacts the miRNAs expression in several tissues and these miRNAs target several mRNAs involved with important biological processes.

Jekyll or Hyde? The genome (and more) of Nesidiocoris tenuis, a zoophytophagous predatory bug that is both a biological control agent and a pest

Nesidiocoris tenuis (Reuter) is an efficient predatory biological control agent used throughout the Mediterranean Basin in tomato crops but regarded as a pest in northern European countries. From the family Miridae, it is an economically important insect yet very little is known in terms of genetic information and no genomic or transcriptomic studies have been published. Here, we use a linked-read sequencing strategy on a single female N. tenuis. From this, we assembled the 355 Mbp genome and delivered an ab initio, homology-based and evidence-based annotation. Along the way, the bacterial "contamination" was removed from the assembly. In addition, bacterial lateral gene transfer (LGT) candidates were detected in the N. tenuis genome. The complete gene set is composed of 24 688 genes; the associated proteins were compared to other hemipterans (Cimex lectularis, Halyomorpha halys and Acyrthosiphon pisum). We visualized the genome using various cytogenetic techniques, such as karyotyping, CGH and GISH, indicating a karyotype of 2n = 32. Additional analyses include the localization of 18S rDNA and unique satellite probes as well as pooled sequencing to assess nucleotide diversity and neutrality of the commercial population. This is one of the first mirid genomes to be released and the first of a mirid biological control agent.

B Chromosomes in Genus Sorghum (Poaceae)

B chromosomes (Bs) are supernumerary dispensable genomic elements that have been reported in several thousand eukaryotic species. Since their discovery, Bs have been subjected to countless studies aiming at the clarification of their origin, composition, and influence on the carriers. Despite these efforts, we still have very limited knowledge of the processes that led to the emergence of Bs, the mechanisms of their transmission, and the effects of Bs on the hosts. In the last decade, sophisticated molecular methods, including next-generation sequencing, have provided powerful tool to help answer some of these questions, but not many species have received much attention yet. In this review, we summarize the currently available information about Bs in the genus Sorghum, which has so far been on the periphery of scientific interest. We present an overview of the occurrence and characteristics of Bs in various Sorghum species, discuss the possible mechanisms involved in their maintenance and elimination, and outline hypotheses of the origin of Bs in this genus.

A New Variant B Chromosome in Auchenipteridae: The Role of (GATA)n and (TTAGGG)n Sequences in Understanding the Evolution of Supernumeraries in Trachelyopterus

Basic and molecular cytogenetic techniques were carried out in 3 Neotropical region populations of catfishes, two of Trachelyopterus galeatus (one from the marshlands of Paraguay River basin and another from Lago Catalão, Amazon River basin) and one of Trachelyopterus porosus, a sympatric population to T. galeatus from the Amazon River basin. This study aimed to describe and understand the structure and evolution of Trachelyopterus B chromosomes, mainly through physical mapping of repetitive elements. A diploid number of 58 chromosomes was found for all individuals, as well as the presence of B chromosomes. For T. porosus this is the first report of a supernumerary. The sympatric species of T. galeatus and T. porosus from Amazon River had 1-3 B chromosomes and T. galeatus from Paraguay River had 1-2 B chromosomes, all of them showed intra- and interindividual numerical variation. Two females of T. porosus exhibited a new variant B chromosome (B2), previously not seen in Auchenipteridae, which might have originated from B1 chromosomes. All B chromosomes were entirely heterochromatic. In contrast to all complement A and B2 chromosomes, in which the telomeric sequences were found in the telomeric regions, B1 chromosomes of all populations were totally marked by (TTAGGG)n probes. (GATA)n sequence sites were found through all complement A chromosomes, but B1 and B2 chromosomes exhibited only a clustered block in one of the chromosome arms. The most frequent B chromosomes (B1) in all populations/species, including those previously studied in Auchenipteridae catfishes, share the following characteristics: totally heterochromatic, small, metacentric, with accumulation of repetitive (TTAGGG)n sequences, and a low number of (GATA)n copies, which might suggest a common ancient origin in Trachelyopterus species/populations.

Satellite DNA Is an Inseparable Fellow Traveler of B Chromosomes

Next-Generation Sequencing (NGS) has revealed that B chromosomes in several species are enriched in repetitive DNA, mostly satellite DNA (satDNA). This raises the question of whether satDNA is important to B chromosomes for functional reasons or else its abundance on Bs is simply a consequence of properties of B chromosomes such as their dispensability and late replication. Here we review current knowledge in this respect and contextualize it within the frame of practical difficulties to perform this kind of research, the most important being the absence of good full genome sequencing for B-carrying species, which is an essential requisite to ascertain the intragenomic origin of B chromosomes. Our review analysis on 16 species revealed that 38% of them showed B-specific satDNAs whereas only one of them (6%) carried an inter-specifically originated B chromosome. This shows that B-specific satDNA families can eventually evolve in intraspecifically arisen B chromosomes. Finally, the possibility of satDNA accumulation on B chromosomes for functional reasons is exemplified by B chromosomes in rye, as they contain B-specific satDNAs which are transcribed and occupy chromosome locations where they might facilitate the kind of drive shown by this B chromosome during pollen grain mitosis.

B chromosomes of multiple species have intense evolutionary dynamics and accumulated genes related to important biological processes

BACKGROUND

One of the biggest challenges in chromosome biology is to understand the occurrence and complex genetics of the extra, non-essential karyotype elements, commonly known as supernumerary or B chromosomes (Bs). The non-Mendelian inheritance and non-pairing abilities of B chromosomes make them an interesting model for genomics studies, thus bringing to bear different questions about their genetic composition, evolutionary survival, maintenance and functional role inside the cell. This study uncovers these phenomena in multiple species that we considered as representative organisms of both vertebrate and invertebrate models for B chromosome analysis.

RESULTS

We sequenced the genomes of three animal species including two fishes Astyanax mexicanus and Astyanax correntinus, and a grasshopper Abracris flavolineata, each with and without Bs, and identified their B-localized genes and repeat contents. We detected unique sequences occurring exclusively on Bs and discovered various evolutionary patterns of genomic rearrangements associated to Bs. In situ hybridization and quantitative polymerase chain reactions further validated our genomic approach confirming detection of sequences on Bs. The functional annotation of B sequences showed that the B chromosome comprises regions of gene fragments, novel genes, and intact genes, which encode a diverse set of functions related to important biological processes such as metabolism, morphogenesis, reproduction, transposition, recombination, cell cycle and chromosomes functions which might be important for their evolutionary success.

CONCLUSIONS

This study reveals the genomic structure, composition and function of Bs, which provide new insights for theories of B chromosome evolution. The selfish behavior of Bs seems to be favored by gained genes/sequences.

Wide dispersion of B chromosomes in Rhammatocerus brasiliensis (Orthoptera, Acrididae)

The grasshopper Rhammatocerus brasiliensis shows polymorphism of B chromosomes, but the magnitude of B-chromosome occurrence and the factors that may contribute to their dispersion in the species remain unknown thus far. The present study analyzed the occurrence and dispersion of B chromosomes in R. brasiliensis individuals from 21 populations widely distributed in the Brazilian Northeast. The genetic connectivity between 10 populations was verified through analysis of ISSR markers from 200 individuals. Of the 21 populations, 19 presented individuals with one B chromosome, three with two, and one with three B chromosomes. The B chromosome is of medium size and constitutive heterochromatin (CH) located in the pericentromeric region. A variant B chromosome was observed in three populations, similar in size to that of chromosome X, gap and CH, and located in the terminal region. B chromosome frequencies in different populations varied from 0% to 18,8%, mean 8,5%. The wide distribution of the B chromosome is likely a consequence of the positive gene flow among the analyzed populations. B-chromosome occurrence in populations of R. brasiliensis possibly follows the population genetic structure of the species and, owing to the existence of a variant, its origin may not be recent.

Genome elimination mediated by gene expression from a selfish chromosome

Numerous plants and animals harbor selfish B chromosomes that "drive" or transmit themselves at super-Mendelian frequencies, despite long-term fitness costs to the organism. Currently, it is unknown how B chromosome drive is mediated, and whether B-gene expression plays a role. We used modern sequencing technologies to analyze the fine-scale sequence composition and expression of paternal sex ratio (PSR), a B chromosome in the jewel wasp Nasonia vitripennis. PSR causes female-to-male conversion by destroying the sperm's hereditary material in young embryos to drive. Using RNA interference, we demonstrate that testis-specific expression of a PSR-linked gene, named haploidizer, facilitates this genome elimination-and-sex conversion effect. haploidizer encodes a putative protein with a DNA binding domain, suggesting a functional link with the sperm-derived chromatin.

Novel B-chromosome-specific transcriptionally active sequences are present throughout the maize B chromosome

Supernumerary B chromosomes are dispensable parts of the nuclear genome and occur in all eukaryotic groups. They differ from the normal A chromosomes in morphology, genetic behavior, and inheritance. Because they are nonessential for individual development, B chromosomes are considered to be genetically inert and to lack functional genes. However, the maize B chromosome carries control elements that direct its behavior and affects A chromosomes during cell division. Therefore, the maize B chromosome might contain genic regions that differ from the genic regions of A chromosomes. Yet, only a few B-specific transcript sequences have been isolated. To identify more B-specific transcriptionally active sequences, we constructed de novo transcriptome assemblies for maize B73 inbred lines with 0B (+0B) and 2B (+2B). Comparative analysis of the B73 + 0B and B73 + 2B assemblies revealed that unigenes annotated to 201 gene ontology terms were differentially expressed. Using RT-PCR analysis of novel transcript sequences specific to B73 + 2B, we identified 32 novel B-related transcript sequences, and most sequences showed consistent B-specific transcription in different inbred lines. Moreover, 20 of those novel B-related transcript sequences were further confirmed to be located only on the B chromosome by genomic PCR analysis. A total of 19 novel B-specific transcript sequences were mapped to various positions along the B chromosome using B-10L translocations. Taken together, our results suggest that the maize B chromosome indeed affects the expression of A-located genes and that a substantial amount of novel B-specific transcriptionally active sequences are present throughout the maize B chromosome. Therefore, the maize B chromosome seems not to be genetically inert.

Satellite DNA content of B chromosomes in the characid fish Characidium gomesi supports their origin from sex chromosomes

The origin of supernumerary (B) chromosomes is clearly conditioned by their ancestry from the standard (A) chromosomes. Sequence similarity between A and B chromosomes is thus crucial to determine B chromosome origin. For this purpose, we compare here the DNA sequences from A and B chromosomes in the characid fish Characidium gomesi using two main approaches. First, we found 59 satellite DNA (satDNA) families constituting the satellitome of this species and performed FISH analysis for 18 of them. This showed the presence of six satDNAs on the B chromosome: one shared with sex chromosomes and autosomes, two shared with sex chromosomes, one shared with autosomes and two being B-specific. This indicated that B chromosomes most likely arose from the sex chromosomes. Our second approach consisted of the analysis of five repetitive DNA families: 18S and 5S ribosomal DNA (rDNA), the H3 histone gene, U2 snDNA and the most abundant satDNA (CgoSat01-184) on DNA obtained from microdissected B chromosomes and from B-lacking genomes. PCR and sequence analysis of these repetitive sequences was successful for three of them (5S rDNA, H3 histone gene and CgoSat01-184), and sequence comparison revealed that DNA sequences obtained from the B chromosomes displayed higher identity with C. gomesi genomic DNA than with those obtained from other Characidium species. Taken together, our results support the intraspecific origin of B chromosomes in C. gomesi and point to sex chromosomes as B chromosome ancestors, which raises interesting prospects for future joint research on the genetic content of sex and B chromosomes in this species.

De novo genome assembly of the cichlid fish Astatotilapia latifasciata reveals a higher level of genomic polymorphism and genes related to B chromosomes

Supernumerary B chromosomes (Bs) are accessory elements to the regular chromosome set (As) and have been observed in a huge diversity of eukaryotic species. Although extensively investigated, the biological significance of Bs remains enigmatic. Here, we present de novo genome assemblies for the cichlid fish Astatotilapia latifasciata, a well-known model to study Bs. High coverage data with Illumina sequencing was obtained for males and females with 0B (B-), 1B, and 2B (B+) chromosomes to provide information regarding the diversity among these genomes. The draft assemblies comprised 771 Mb for the B- genome and 781 Mb for the B+ genome. Comparative analysis of the B+ and B- assemblies reveals syntenic discontinuity, duplicated blocks and several insertions, deletions, and inversions indicative of rearrangements in the B+ genome. Hundreds of transposable elements and 1546 protein coding sequences were annotated in the duplicated B+ regions. Our work contributes a list of thousands of genes harbored on the B chromosome, with functions in several biological processes, including the cell cycle.

Origin, Behaviour, and Transmission of B Chromosome with Special Reference to Plantago lagopus

B chromosomes have been reported in many eukaryotic organisms. These chromosomes occur in addition to the standard complement of a species. Bs do not pair with any of the A chromosomes and they have generally been considered to be non-essential and genetically inert. However, due to tremendous advancements in the technologies, the molecular composition of B chromosomes has been determined. The sequencing data has revealed that B chromosomes have originated from A chromosomes and they are rich in repetitive elements. In our laboratory, a novel B chromosome was discovered in Plantago lagopus. Using molecular cytogenetic techniques, the B chromosome was found to be composed of ribosomal DNA sequences. However, further characterization of the chromosome using next generation sequencing (NGS) etc. revealed that the B chromosome is a mosaic of sequences derived from A chromosomes, 5S ribosomal DNA (rDNA), 45S rDNA, and various types of repetitive elements. The transmission of B chromosome through the female sex track did not follow the Mendelian principles. The chromosome was found to have drive due to which it was perpetuating in populations. The present paper attempts to summarize the information on nature, transmission, and origin of B chromosomes, particularly the current status of our knowledge in P. lagopus.

The Modern View of B Chromosomes Under the Impact of High Scale Omics Analyses

Supernumerary B chromosomes (Bs) are extra karyotype units in addition to A chromosomes, and are found in some fungi and thousands of animals and plant species. Bs are uniquely characterized due to their non-Mendelian inheritance, and represent one of the best examples of genomic conflict. Over the last decades, their genetic composition, function and evolution have remained an unresolved query, although a few successful attempts have been made to address these phenomena. A classical concept based on cytogenetics and genetics is that Bs are selfish and abundant with DNA repeats and transposons, and in most cases, they do not carry any function. However, recently, the modern quantum development of high scale multi-omics techniques has shifted B research towards a new-born field that we call "B-omics". We review the recent literature and add novel perspectives to the B research, discussing the role of new technologies to understand the mechanistic perspectives of the molecular evolution and function of Bs. The modern view states that B chromosomes are enriched with genes for many significant biological functions, including but not limited to the interesting set of genes related to cell cycle and chromosome structure. Furthermore, the presence of B chromosomes could favor genomic rearrangements and influence the nuclear environment affecting the function of other chromatin regions. We hypothesize that B chromosomes might play a key function in driving their transmission and maintenance inside the cell, as well as offer an extra genomic compartment for evolution.

Sequence Expression of Supernumerary B Chromosomes: Function or Fluff?

B chromosomes are enigmatic heritable elements found in the genomes of numerous plant and animal species. Contrary to their broad distribution, most B chromosomes are non-essential. For this reason, they are regarded as genome parasites. In order to be stably transmitted through generations, many B chromosomes exhibit the ability to "drive", i.e., they transmit themselves at super-Mendelian frequencies to progeny through directed interactions with the cell division apparatus. To date, very little is understood mechanistically about how B chromosomes drive, although a likely scenario is that expression of B chromosome sequences plays a role. Here, we highlight a handful of previously identified B chromosome sequences, many of which are repetitive and non-coding in nature, that have been shown to be expressed at the transcriptional level. We speculate on how each type of expressed sequence could participate in B chromosome drive based on known functions of RNA in general chromatin- and chromosome-related processes. We also raise some challenges to functionally testing these possible roles, a goal that will be required to more fully understand whether and how B chromosomes interact with components of the cell for drive and transmission.

Gene expression changes elicited by a parasitic B chromosome in the grasshopper Eyprepocnemis plorans are consistent with its phenotypic effects

Parasitism evokes adaptive physiological changes in the host, many of which take place through gene expression changes. This response can be more or less local, depending on the organ or tissue affected by the parasite, or else systemic when the parasite affects the entire host body. The most extreme of the latter cases is intragenomic parasitism, where the parasite is present in all host nuclei as any other genomic element. Here, we show the molecular crosstalk between a parasitic chromosome (also named B chromosome) and the host genome, manifested through gene expression changes. The transcriptome analysis of 0B and 1B females of the grasshopper Eyprepocnemis plorans, validated by a microarray experiment performed on four B-lacking and five B-carrying females, revealed changes in gene expression for 188 unigenes being consistent in both experiments. Once discarded B-derived transcripts, there were 46 differentially expressed genes (30 up- and 16 downregulated) related with the adaptation of the host genome to the presence of the parasitic chromosome. Interestingly, the functions of these genes could explain some of the most important effects of B chromosomes, such as nucleotypic effects derived from the additional DNA they represent, chemical defense and detoxification, protein modification and response to stress, ovary function, and regulation of gene expression. Collectively, these changes uncover an intimate host-parasite interaction between A and B chromosomes during crucial steps of gene expression and protein function.

Genomic Characterization of a B Chromosome in Lake Malawi Cichlid Fishes

B chromosomes (Bs) were discovered a century ago, and since then, most studies have focused on describing their distribution and abundance using traditional cytogenetics. Only recently have attempts been made to understand their structure and evolution at the level of DNA sequence. Many questions regarding the origin, structure, function, and evolution of B chromosomes remain unanswered. Here, we identify B chromosome sequences from several species of cichlid fish from Lake Malawi by examining the ratios of DNA sequence coverage in individuals with or without B chromosomes. We examined the efficiency of this method, and compared results using both Illumina and PacBio sequence data. The B chromosome sequences detected in 13 individuals from 7 species were compared to assess the rates of sequence replacement. B-specific sequence common to at least 12 of the 13 datasets were identified as the "Core" B chromosome. The location of B sequence homologs throughout the genome provides further support for theories of B chromosome evolution. Finally, we identified genes and gene fragments located on the B chromosome, some of which may regulate the segregation and maintenance of the B chromosome.

Sequence Composition and Evolution of Mammalian B Chromosomes

B chromosomes (Bs) revealed more than a hundred years ago remain to be some of the most mysterious elements of the eukaryotic genome. Their origin and evolution, DNA composition, transcriptional activity, impact on adaptiveness, behavior in meiosis, and transfer to the next generation require intensive investigations using modern methods. Over the past years, new experimental techniques have been applied and helped us gain a deeper insight into the nature of Bs. Here, we consider mammalian Bs, taking into account data on their DNA sequencing, transcriptional activity, positions in nuclei of somatic and meiotic cells, and impact on genome functioning. Comparative cytogenetics of Bs suggests the existence of different mechanisms of their formation and evolution. Due to the long and complicated evolvement of Bs, the similarity of their morphology could be explained by the similar mechanisms involved in their development while the difference between Bs even of the same origin could appear due to their positioning at different stages of their evolution. A complex analysis of their DNA composition and other features is required to clarify the origin and evolutionary history of Bs in the species studied. The intraspecific diversity of Bs makes this analysis a very important element of B chromosome studies.

Sequencing of Supernumerary Chromosomes of Red Fox and Raccoon Dog Confirms a Non-Random Gene Acquisition by B Chromosomes

B chromosomes (Bs) represent a variable addition to the main karyotype in some lineages of animals and plants. Bs accumulate through non-Mendelian inheritance and become widespread in populations. Despite the presence of multiple genes, most Bs lack specific phenotypic effects, although their influence on host genome epigenetic status and gene expression are recorded. Previously, using sequencing of isolated Bs of ruminants and rodents, we demonstrated that Bs originate as segmental duplications of specific genomic regions, and subsequently experience pseudogenization and repeat accumulation. Here, we used a similar approach to characterize Bs of the red fox (Vulpes vulpes L.) and the Chinese raccoon dog (Nyctereutes procyonoides procyonoides Gray). We confirm the previous findings of the KIT gene on Bs of both species, but demostrate an independent origin of Bs in these species, with two reused regions. Comparison of gene ensembles in Bs of canids, ruminants, and rodents once again indicates enrichment with cell-cycle genes, development-related genes, and genes functioning in the neuron synapse. The presence of B-chromosomal copies of genes involved in cell-cycle regulation and tissue differentiation may indicate importance of these genes for B chromosome establishment.

Tackling Plant Meiosis: From Model Research to Crop Improvement

Genetic engineering and traditional plant breeding, which harnesses the natural genetic variation that arises during meiosis, will have key roles to improve crop varieties and thus deliver Food Security in the future. Meiosis, a specialized cell division producing haploid gametes to maintain somatic diploidy following their fusion, assures genetic variation by regulated genetic exchange through homologous recombination. However, meiotic recombination events are restricted in their total number and their distribution along chromosomes limiting allelic variations in breeding programs. Thus, modifying the number and distribution of meiotic recombination events has great potential to improve and accelerate plant breeding. In recent years much progress has been made in understanding meiotic progression and recombination in plants. Many genes and factors involved in these processes have been identified primarily in Arabidopsis thaliana but also more recently in crops such as Brassica, rice, barley, maize, or wheat. These advances put researchers in the position to translate acquired knowledge to various crops likely improving and accelerating breeding programs. However, although fundamental aspects of meiotic progression and recombination are conserved between species, differences in genome size and organization (due to repetitive DNA content and ploidy level) exist, particularly among plants, that likely account for differences in meiotic progression and recombination patterns found between species. Thus, tools and approaches are needed to better understand differences and similarities in meiotic progression and recombination among plants, to study fundamental aspects of meiosis in a variety of plants including crops and non-model species, and to transfer knowledge into crop species. In this article, we provide an overview of tools and approaches available to study plant meiosis, highlight new techniques, give examples of areas of future research and review distinct aspects of meiosis in non-model species.

Transcription of a B chromosome CAP-G pseudogene does not influence normal Condensin Complex genes in a grasshopper

Parasitic B chromosomes invade and persist in natural populations through several mechanisms for transmission advantage (drive). They may contain gene-derived sequences which, in some cases, are actively transcribed. A further interesting question is whether B-derived transcripts become functional products. In the grasshopper Eyprepocnemis plorans, one of the gene-derived sequences located on the B chromosome shows homology with the gene coding for the CAP-G subunit of condensin I. We show here, by means of fluorescent in situ hybridization coupled with tyramide signal amplification (FISH-TSA), that this gene is located in the distal region of the B24 chromosome variant. The DNA sequence located in the B chromosome is a pseudogenic version of the CAP-G gene (B-CAP-G). In two Spanish populations, we found active transcription of B-CAP-G, but it did not influence the expression of CAP-D2 and CAP-D3 genes coding for corresponding condensin I and II subunits, respectively. Our results indicate that the transcriptional regulation of the B-CAP-G pseudogene is uncoupled from the standard regulation of the genes that constitute the condensin complex, and suggest that some of the B chromosome known effects may be related with its gene content and transcriptional activity, thus opening new exciting avenues for research.

How Next-Generation Sequencing Has Aided Our Understanding of the Sequence Composition and Origin of B Chromosomes

Accessory, supernumerary, or-most simply-B chromosomes, are found in many eukaryotic karyotypes. These small chromosomes do not follow the usual pattern of segregation, but rather are transmitted in a higher than expected frequency. As increasingly being demonstrated by next-generation sequencing (NGS), their structure comprises fragments of standard (A) chromosomes, although in some plant species, their sequence also includes contributions from organellar genomes. Transcriptomic analyses of various animal and plant species have revealed that, contrary to what used to be the common belief, some of the B chromosome DNA is protein-encoding. This review summarizes the progress in understanding B chromosome biology enabled by the application of next-generation sequencing technology and state-of-the-art bioinformatics. In particular, a contrast is drawn between a direct sequencing approach and a strategy based on a comparative genomics as alternative routes that can be taken towards the identification of B chromosome sequences.

The satellite DNA AflaSAT-1 in the A and B chromosomes of the grasshopper Abracris flavolineata

BACKGROUND

Satellite DNAs (satDNAs) are organized in repetitions directly contiguous to one another, forming long arrays and composing a large portion of eukaryote genomes. These sequences evolve according to the concerted evolution model, and homogenization of repeats is observed at the intragenomic level. Satellite DNAs are the primary component of heterochromatin, located primarily in centromeres and telomeres. Moreover, satDNA enrichment in specific chromosomes has been observed, such as in B chromosomes, that can provide clues about composition, origin and evolution of this chromosome. In this study, we isolated and characterized a satDNA in A and B chromosomes of Abracris flavolineata by integrating cytogenetic, molecular and genomics approaches at intra- and inter-population levels, with the aim to understand the evolution of satDNA and composition of B chromosomes.

RESULTS

AflaSAT-1 satDNA was shared with other species and in A. flavolineata, was associated with another satDNA, AflaSAT-2. Chromosomal mapping revealed centromeric blocks variable in size in almost all chromosomes (except pair 11) of A complement for both satDNAs, whereas for B chromosome, only a small centromeric signal occurred. In distinct populations, variable number of AflaSAT-1 chromosomal sites correlated with variability in copy number. Instead of such variability, low sequence diversity was observed in A complement, but monomers from B chromosome were more variable, presenting also exclusive mutations. AflaSAT-1 was transcribed in five tissues of adults in distinct life cycle phases.

CONCLUSIONS

The sharing of AflaSAT-1 with other species is consistent with the library hypothesis and indicates common origin in a common ancestor; however, AflaSAT-1 was highly amplified in the genome of A. flavolineata. At the population level, homogenization of repeats in distinct populations was documented, but dynamic expansion or elimination of repeats was also observed. Concerning the B chromosome, our data provided new information on the composition in A. flavolineata. Together with previous results, the sequences of heterochromatic nature were not likely highly amplified in the entire B chromosome. Finally, the constitutive transcriptional activity suggests a possible unknown functional role, which should be further investigated.

The hnRNP Q-like gene is retroinserted into the B chromosomes of the cichlid fish Astatotilapia latifasciata

B chromosomes are dispensable elements observed in many eukaryotic species, including the African cichlid Astatotilapia latifasciata, which might have one or two B chromosomes. Although there have been many studies focused on the biology of these chromosomes, questions about the evolution, maintenance, and potential effects of these chromosomes remain. Here, we identified a variant form of the hnRNP Q-like gene inserted into the B chromosome of A. latifasciata that is characterized by a high copy number and intron-less structure. The absence of introns and presence of transposable elements with a reverse transcriptase domain flanking hnRNP Q-like sequences suggest that this gene was retroinserted into the B chromosome. RNA-Seq analysis did not show that the B variant retroinserted copies are transcriptionally active. However, RT-qPCR results showed variations in the canonical hnRNP Q-like copy expression levels among exons, tissues, sex, and B presence/absence. Although the patterns of transcription are not well understood, the exons of the B retrocopies were overexpressed, and a bias for female B+ expression was also observed. These results suggest that retroinsertion is an additional and important mechanism contributing to B chromosome formation. Furthermore, these findings indicate a bias towards female differential expression of B chromosome sequences, suggesting that B chromosomes and sex determination are somehow associated in cichlids.

Protein-coding genes in B chromosomes of the grasshopper Eyprepocnemis plorans

For many years, parasitic B chromosomes have been considered genetically inert elements. Here we show the presence of ten protein-coding genes in the B chromosome of the grasshopper Eyprepocnemis plorans. Four of these genes (CIP2A, GTPB6, KIF20A, and MTG1) were complete in the B chromosome whereas the six remaining (CKAP2, CAP-G, HYI, MYCB2, SLIT and TOP2A) were truncated. Five of these genes (CIP2A, CKAP2, CAP-G, KIF20A, and MYCB2) were significantly up-regulated in B-carrying individuals, as expected if they were actively transcribed from the B chromosome. This conclusion is supported by three truncated genes (CKAP2, CAP-G and MYCB2) which showed up-regulation only in the regions being present in the B chromosome. Our results indicate that B chromosomes are not so silenced as was hitherto believed. Interestingly, the five active genes in the B chromosome code for functions related with cell division, which is the main arena where B chromosome destiny is played. This suggests that B chromosome evolutionary success can lie on its gene content.

B Chromosomes - A Matter of Chromosome Drive

B chromosomes are supernumerary chromosomes which are often preferentially inherited, deviating from usual Mendelian segregation. The balance between the so-called chromosome drive and the negative effects that the presence of Bs applies on the fitness of their host determines the frequency of Bs in a particular population. Drive is the key for understanding most B chromosomes. Drive occurs in many ways at pre-meiotic, meiotic or post-meiotic divisions, but the molecular mechanism remains unclear. The cellular mechanism of drive is reviewed based on the findings obtained for the B chromosomes of rye, maize and other species. How novel analytical tools will expand our ability to uncover the biology of B chromosome drive is discussed.

Rye B chromosomes encode a functional Argonaute-like protein with in vitro slicer activities similar to its A chromosome paralog

B chromosomes (Bs) are supernumerary, dispensable parts of the nuclear genome, which appear in many different species of eukaryote. So far, Bs have been considered to be genetically inert elements without any functional genes. Our comparative transcriptome analysis and the detection of active RNA polymerase II (RNAPII) in the proximity of B chromatin demonstrate that the Bs of rye (Secale cereale) contribute to the transcriptome. In total, 1954 and 1218 B-derived transcripts with an open reading frame were expressed in generative and vegetative tissues, respectively. In addition to B-derived transposable element transcripts, a high percentage of short transcripts without detectable similarity to known proteins and gene fragments from A chromosomes (As) were found, suggesting an ongoing gene erosion process. In vitro analysis of the A- and B-encoded AGO4B protein variants demonstrated that both possess RNA slicer activity. These data demonstrate unambiguously the presence of a functional AGO4B gene on Bs and that these Bs carry both functional protein coding genes and pseudogene copies. Thus, B-encoded genes may provide an additional level of gene control and complexity in combination with their related A-located genes. Hence, physiological effects, associated with the presence of Bs, may partly be explained by the activity of B-located (pseudo)genes.

B chromosomes: from cytogenetics to systems biology

Though hundreds to thousands of reports have described the distribution of B chromosomes among diverse eukaryote groups, a comprehensive theory of their biological role has not yet clearly emerged. B chromosomes are classically understood as a sea of repetitive DNA sequences that are poor in genes and are maintained by a parasitic-drive mechanism during cell division. Recent developments in high-throughput DNA/RNA analyses have increased the resolution of B chromosome biology beyond those of classical and molecular cytogenetic methods; B chromosomes contain many transcriptionally active sequences, including genes, and can modulate the activity of autosomal genes. Furthermore, the most recent knowledge obtained from omics analyses, which is associated with a systemic view, has demonstrated that B chromosomes can influence cell biology in a complex way, possibly favoring their own maintenance and perpetuation.

Exploring Supernumeraries - A New Marker for Screening of B-Chromosomes Presence in the Yellow Necked Mouse Apodemus flavicollis

Since the density of simple sequence repeats (SSRs) may vary between different chromosomes of the same species in eukaryotic genomes, we screened SSRs of the whole genome of the yellow necked mouse, Apodemus flavicollis, in order to reveal SSR profiles specific for animals carrying B chromosomes. We found that the 2200 bp band was amplified by primer (CAG)₄AC to a highly increased level in samples with B chromosomes. This quantitative difference (B-marker) between animals with (+B) and without (0B) B chromosomes was used to screen 20 populations (387 animals). The presence/absence of Bs was confirmed in 96.5% of 342 non mosaic individuals, which recommends this method for noninvasive B-presence detection. A group of 45 animals with mosaic and micro B (μB) karyotypes was considered separately and showed 55.6% of overall congruence between karyotyping and molecular screening results. Relative quantification by qPCR of two different targeted sequences from B-marker indicated that these B-specific fragments are multiplied on B chromosomes. It also confirms our assumption that different types of Bs with variable molecular composition may exist in the same individual and between individuals of this species. Our results substantiate the origin of Bs from the standard chromosomal complement. The B-marker showed 98% sequence identity with the serine/threonine protein kinase VRK1 gene, similarly to findings reported for Bs from phylogenetically highly distant mammalian species. Evolutionarily conserved protein-coding genes found in Bs, including this one in A. flavicollis, could suggest a common evolutionary pathway.

Development of chromosomal markers based on next-generation sequencing: the B chromosome of the cichlid fish Astatotilapia latifasciata as a model

BACKGROUND

B chromosomes (Bs) are additional chromosomal elements found in a wide range of eukaryotes including fungi, plants and animals. B chromosomes are still enigmatic despite being the subject of hundreds, even thousands of reports. As yet there is no comprehensive theory for the biological role of B chromsomes thus, new studies are needed. Next-generation sequencing (NGS) holds promise for investigating classical issues in chromosome biology. NGS uses a large-scale approach that is required for advancing classical cytogenetic studies. Based on 454 sequencing data of a microdissected B chromosome and Illumina whole-genome sequencing data generated for 0B, 1B and 2B animals, we developed PCR- and qPCR-based markers for the B chromosomes of the cichlid fish Astatotilapia latifasciata (that possess 0, 1 or 2 B chromosomes).

RESULTS

Specific PCR primers were designed to produce two amplified fragments for B-positive samples and the control fragment for B-negative samples. Thus, PCR markers detected the presence/absence of Bs but did not provide information about the number of Bs. However, quantitative PCR (qPCR) markers clearly discriminated between 1B and 2B samples. The high copy number of the marker identified in the B chromosomes was confirmed by chromosome mapping.

CONCLUSIONS

The analysis of chromosome polymorphisms based on a NGS approach is a powerful strategy to obtain markers that detect the presence/absence of extra chromosomes or the gain or loss of genomic blocks. Further, qPCR can also provide information regarding the relative copy number of specific DNA fragments. These methods are useful to investigate various chromosome polymorphisms, including B and sex chromosomes, as well as chromosomal duplications and deletions. NGS data provide a detailed analysis of the composition of genomic regions that are thought to be present in B chromosomes.

Restructuring of Holocentric Centromeres During Meiosis in the Plant Rhynchospora pubera

Centromeres are responsible for the correct segregation of chromosomes during mitosis and meiosis. Holocentric chromosomes, characterized by multiple centromere units along each chromatid, have particular adaptations to ensure regular disjunction during meiosis. Here we show by detecting CENH3, CENP-C, tubulin, and centromeric repeats that holocentromeres may be organized differently in mitosis and meiosis of Rhynchospora pubera Contrasting to the mitotic linear holocentromere organization, meiotic centromeres show several clusters of centromere units (cluster-holocentromeres) during meiosis I. They accumulate along the poleward surface of bivalents where spindle fibers perpendicularly attach. During meiosis II, the cluster-holocentromeres are mostly present in the midregion of each chromatid. A linear holocentromere organization is restored after meiosis during pollen mitosis. Thus, a not yet described case of a cluster-holocentromere organization, showing a clear centromere restructuration between mitosis and meiosis, was identified in a holocentric organism.

B-chromosome effects on Hsp70 gene expression does not occur at transcriptional level in the grasshopper Eyprepocnemis plorans

As intragenomic parasites, B chromosomes can elicit stress in the host genome, thus inducing a response for host adaptation to this kind of continuous parasitism. In the grasshopper Eyprepocnemis plorans, B-chromosome presence has been previously associated with a decrease in the amount of the heat-shock protein 70 (HSP70). To investigate whether this effect is already apparent at transcriptional level, we analyze the expression levels of the Hsp70 gene in gonads and somatic tissues of males and females with and without B chromosomes from two populations, where the predominant B chromosome variants (B2 and B24) exhibit different levels of parasitism, by means of quantitative real-time PCR (qPCR) on complementary DNA (cDNA). The results revealed the absence of significant differences for Hsp70 transcripts associated with B-chromosome presence in virtually all samples. This indicates that the decrease in HSP70 protein levels, formerly reported in this species, may not be a consequence of transcriptional down-regulation of Hsp70 genes, but the result of post-transcriptional regulation. These results will help to design future studies oriented to identifying factors modulating Hsp70 expression, and will also contribute to uncover the biological role of B chromosomes in eukaryotic genomes.

Meiotic behavior and H3K4m distribution in B chromosomes of Characidium gomesi (Characiformes, Crenuchidae)

Characidium gomesi Travasso, 1956 specimens from the Pardo River have up to four heterochromatic supernumerary chromosomes, derived from the sex chromosomes. To access the meiotic behavior and distribution of an active chromatin marker, males and females of Characidium gomesi with two or three B chromosomes were analyzed. Mitotic chromosomes were characterized using C-banding and FISH with B chromosome probes. Meiocytes were subjected to immunofluorescence-FISH assay using anti-SYCP3, anti-H3K4m, and B chromosomes probes. Molecular homology of supernumeraries was confirmed by FISH and by its bivalent conformation in individuals with two of these chromosomes. In individuals with three Bs, these elements formed a bivalent and a univalent. Supernumerary and sex chromosomes exhibited H3K4m signals during pachytene contrasting with their heterochromatic and asynaptic nature, which suggest a more structural role than functional of this histone modification. The implications of this result are discussed in light of the homology, meiotic nuclear organization, and meiotic silencing of unsynapsed chomatin.

The repetitive DNA element BncDNA, enriched in the B chromosome of the cichlid fish Astatotilapia latifasciata, transcribes a potentially noncoding RNA

Supernumerary chromosomes have been studied in many species of eukaryotes, including the cichlid fish, Astatotilapia latifasciata. However, there are many unanswered questions about the maintenance, inheritance, and functional aspects of supernumerary chromosomes. The cichlid family has been highlighted as a model for evolutionary studies, including those that focus on mechanisms of chromosome evolution. Individuals of A. latifasciata are known to carry up to two B heterochromatic isochromosomes that are enriched in repetitive DNA and contain few intact gene sequences. We isolated and characterized a transcriptionally active repeated DNA, called B chromosome noncoding DNA (BncDNA), highly represented across all B chromosomes of A. latifasciata. BncDNA transcripts are differentially processed among six different tissues, including the production of smaller transcripts, indicating transcriptional variation may be linked to B chromosome presence and sexual phenotype. The transcript lengths and lack of similarity with known protein/gene sequences indicate BncRNA might represent a novel long noncoding RNA family (lncRNA). The potential for interaction between BncRNA and known miRNAs were computationally predicted, resulting in the identification of possible binding of this sequence in upregulated miRNAs related to the presence of B chromosomes. In conclusion, Bnc is a transcriptionally active repetitive DNA enriched in B chromosomes with potential action over B chromosome maintenance in somatic cells and meiotic drive in gametic cells.

B chromosome contains active genes and impacts the transcription of A chromosomes in maize (Zea mays L.)

BACKGROUND

The dispensable maize (Zea mays L.) B chromosome is highly heterochromatic and widely believed to be devoid of functional genes. Although low-copy B chromosome causes no obvious phenotype variation, its existence might influence A genome gene expression. Previous studies suggested that B chromosomes are evolved from standard chromosomes; therefore, they might contain genic regions showing homology with A chromosome sequences.

RESULTS

Our data suggested that maize B chromosome influences the A-genome transcription with stronger effect associated with an increase in copy number of B chromosome. In total 130 differently expressed genes were detected in comparison between with and without B chromosome lines. These differentially expressed genes are mainly involved in cell metabolism and nucleotide binding. Using Starter + B, we amplified ten B chromosome loci with high sequence similarity to A-genome genes. Fluorescence in situ hybridization (FISH) confirmed that at least four ~5 kb-sized genes are located on the B chromosome. In addition, through de novo assembly of the reads not unmapped to maize B73 reference genome together with PCR validation, we found three B-located LTR; in particular, one of them, the 3.2 kb comp75688, is expressed in a B-dosage dependent manner.

CONCLUSION

We found that in the presence of maize B chromosome, the transcription of A genome genes was altered, with more impact by the increase of the B chromosome number. The B-located transcriptionally active genes showed high similarity to their A-genome homologues, and retrotransposons on B chromosome also have partial homologous to A genome sequences. Our data shed more lights on the genome structure and evolution of the maize B chromosome.

Non-random expression of ribosomal DNA units in a grasshopper showing high intragenomic variation for the ITS2 region

We analyse intragenomic variation of the ITS2 internal transcribed spacer of ribosomal DNA (rDNA) in the grasshopper Eyprepocnemis plorans, by means of tagged PCR 454 amplicon sequencing performed on both genomic DNA (gDNA) and RNA-derived complementary DNA (cDNA), using part of the ITS2 flanking coding regions (5.8S and 28S rDNA) as an internal control for sequencing errors. Six different ITS2 haplotypes (i.e. variants for at least one nucleotide in the complete ITS2 sequence) were found in a single population, one of them (Hap4) being specific to a supernumerary (B) chromosome. The analysis of both gDNA and cDNA from the same individuals provided an estimate of the expression efficiency of the different haplotypes. We found random expression (i.e. about similar recovery in gDNA and cDNA) for three haplotypes (Hap1, Hap2 and Hap5), but significant underexpression for three others (Hap3, Hap4 and Hap6). Hap4 was the most extremely underexpressed and, remarkably, it showed the lowest sequence conservation for the flanking 5.8-28S coding regions in the gDNA reads but the highest conservation (100%) in the cDNA ones, suggesting the preferential expression of mutation-free rDNA units carrying this ITS2 haplotype. These results indicate that the ITS2 region of rDNA is far from complete homogenization in this species, and that the different rDNA units are not expressed at random, with some of them being severely downregulated.