Reduction In Chromosome Number Research Articles

Repeated Whole-Genome Duplication, Karyotype Reshuffling, and Biased Retention of Stress-Responding Genes in Buckler Mustard.

Whole-genome duplication (WGD) is usually followed by gene loss and karyotype repatterning. Despite evidence of new adaptive traits associated with WGD, the underpinnings and evolutionary significance of such genome fractionation remain elusive. Here, we use Buckler mustard (Biscutella laevigata) to infer processes that have driven the retention of duplicated genes after recurrent WGDs. In addition to the β- and α-WGD events shared by all Brassicaceae, cytogenetic and transcriptome analyses revealed two younger WGD events that occurred at times of environmental changes in the clade of Buckler mustard (Biscutelleae): a mesopolyploidy event from the late Miocene that was followed by considerable karyotype reshuffling and chromosome number reduction and a neopolyploidy event during the Pleistocene. Although a considerable number of the older duplicates presented signatures of retention under positive selection, the majority of retained duplicates arising from the younger mesopolyploidy WGD event matched predictions of the gene balance hypothesis and showed evidence of strong purifying selection as well as enrichment in gene categories responding to abiotic stressors. Retention of large stretches of chromosomes for both genomic copies supported the hypothesis that cycles of WGD and biased fractionation shaped the genome of this stress-tolerant polypolyloid, promoting the adaptive recruitment of stress-responding genes in the face of environmental challenges.

Cytogenetics and molecular differentiation in the African armoured ground bushcrickets (Orthoptera: Tettigoniidae: Hetrodinae)

Spalacomimus, Enyaliopsis, Eugaster, and Eugasteroides are tettigoniid genera belonging to the African subfamily Hetrodinae. In order to understand its diversification and unravel relationships between the species, we performed molecular analysis based on a combined set of nuclear genes (18S, ITS1, ITS2). The monophyly of Hetrodinae was supported by Bayesian inference and maximum likelihood analysis. Tree topology and cytogenetic data revealed two main lineages: (1) Eugaster and Eugasteroides and (2) Enyaliopsis and Spalacomimus. The molecular results were compared with previous and new cytogenetic and morphological data. Species of the North African genus Eugaster and the East African species Eugasteroides loricatus showed the same chromosome number (29), but differed in the number of rDNA signals, whereas the East African Enyaliopsis ephippiatus exhibited a reduced chromosome number (27) and only one rDNA-bearing chromosome pair. Both genera have the same X0 (plesiomorphic) sex chromosome system. The greatest chromosome reduction (to 24) was observed in two species of the East African genus Spalacomimus, both having neo-XY sex chromosome systems. However, the morphology of the sex chromosomes differed between these species, indicating different origins of their neo-XY systems. More molecular and cytotaxonomic analyses are necessary to further elucidate relationships within Hetrodinae. The present work sets the stage for future experimental investigations of the phylogeny and systematics of this group.

The wondrous cycles of polyploidy in plants.

The wondrous cycles of polyploidy in plants.

PATRONUS1 is expressed in meiotic prophase I to regulate centromeric cohesion in Arabidopsis and shows synthetic lethality with OSD1.

BackgroundRetention of sister centromere cohesion during meiosis I and its dissolution at meiosis II is necessary for balanced chromosome segregation and reduction of chromosome number. PATRONUS1 (PANS1) has recently been proposed to regulate centromere cohesion in Arabidopsis after meiosis I, during interkinesis. pans1 mutants lose centromere cohesion prematurely during interkinesis and segregate randomly at meiosis II. PANS1 protein interacts with components of the Anaphase Promoting Complex/Cyclosome (APC/C).ResultsWe show here that PANS1 protein is found mainly in prophase I of meiosis, with its level declining late in prophase I during diplotene. PANS1 also shows expression in dividing tissues. We demonstrate that, in addition to the previously reported premature loss of centromere cohesion during interkinesis, pans1 mutants show partially penetrant defects in centromere cohesion during meiosis I. We also determine that pans1 shows synthetic lethality at the level of the sporophyte, with Omission of Second Division 1 (osd1), which encodes a known inhibitor of the APC/C that is required for cell cycle progression during mitosis, as well as meiosis I and II.ConclusionsOur results show that PANS1 is expressed mainly in meiosis I where it has an important function and together with previous studies indicate that PANS1 and OSD1 are part of a network linking centromere cohesion and cell cycle progression through control of APC/C activity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0558-6) contains supplementary material, which is available to authorized users.

Review of the Plangia graminea (Serville) complex and the description of new Plangia species from East Africa (Orthoptera: Phaneropteridae, Phaneropterinae) with data on habitat, bioacoustics, and chromosomes

The Plangia graminea complex is partly reviewed and two new species of Plangia are described, Plangia multimaculata n. sp. from savanna habitats and Plangia satiscaerulea n. sp. from the submontane zones in northern Tanzania. Plangia compressa (Walker 1869) is synonymized with P. graminea (Serville, 1838). Data on habitat, bioacoustics, and chromosomes are provided. Both analyzed Plangia species showed the same chromosomal number. Compared to other investigated African Phaneropterinae, Plangia had a reduced chromosome number and thus shows a derived condition in its genome. The sex chromosomes differed morphologically in both investigated species of Plangia suggesting different mechanisms leading to these differences. Niche specialization and the male calling song are discussed as drivers for speciation in fully alate and thus mobile taxa.

Sequence-tagged high-density genetic maps of Zoysia japonica provide insights into genome evolution in Chloridoideae.

Zoysiagrass (Zoysia spp.), belonging to the genus Zoysia in the subfamily Chloridoideae, is widely used in domestic lawns, sports fields and as forage. We constructed high-density genetic maps of Zoysia japonica using a restriction site-associated DNA sequencing (RAD-Seq) approach and an F1 mapping population derived from a cross between 'Carrizo' and 'El Toro'. Two linkage maps were constructed, one for each of the parents. A map consisting of 2408 RAD markers distributed on 21 linkage groups was constructed for 'Carrizo'. Another map with 1230 RAD markers mapped on 20 linkage groups was constructed for 'El Toro'. The average distance between adjacent markers of the two maps was at 0.56 and 1.4 cM, respectively. Comparative genomics analysis was carried out among zoysiagrass, rice and sorghum genomes and a highly conserved collinearity in the gene order was observed among the three genomes. Chromosome collinearity was disrupted at centromeric regions for each chromosome pair between zoysiagrass and sorghum genomes. However, no obvious synteny gaps were observed across the centromeric regions between zoysiagrass and rice genomes. Two homologous chromosomes for each of the 10 sorghum chromosomes were found in the zoysiagrass genome, indicating an allotetraploid origin for zoysiagrass. The reduction of the basic chromosome number from 12 to 10 in chloridoids and panicoids took place via independent single-step nested chromosome fusion events after the two subfamilies diverged from a common ancestor. The genetic maps will assist in genome sequence assembly, targeted gene isolation and comparative genomic analyses among grasses.

Ecology, acoustics and chromosomes of the East African genus Afroanthracites Hemp & Ingrisch (Orthoptera, Tettigoniidae, Conocephalinae, Agraeciini) with the description of new species

The flightless Agraeciini genus Afroanthracites, a genus restricted to East Africa, is reviewed and two new species are described. Exemplary for the genus Afroanthracites, the ecological niche of Afroanthracites montium from Mt Kilimanjaro is defined. A. montium occupies habitats in humid and perhumid conditions and thus shows a broad altitudinal range (1250–2700 m). Concerning the acoustic communication, it is remarkable that within the micropterous genus a trend to low carrier frequencies is observed combined with the evolution of larger stridulatory organs (mirror; resonating part of tegmen). This trend starts with species using the same ultrasonic frequencies as a brachypterous out-group and ends with species presenting clearly audible songs. Cytogenetic data are given for five Afroanthracites and one Afroagraecia species. Differences in chromosome numbers Afroanthracites 2n = 29 and Afroagraecia 2n = 27 as well as a number of major rDNA clusters (one and two, respectively) are probably useful markers to separate both genera. It is discussed whether the African species evolved from a species with a presumably derived karyotype (e.g. 27 or 29 chromosomes) or if the proposed reduction of chromosome number occurred independently in Africa, Asia and Australia. The data set suggests that the African Agraeciini is of monophyletic origin, with a common ancestor of Afroagraecia and Afroanthracites in Africa. The Afroanthracites species can be divided into three groups on base of their morphology and colour pattern. Species of adjacent areas are morphologically sister groups. The most derived forms as seen in their morphology and acoustics are found in the West Usambara Mountains, part of the geologically old Eastern Arc Mountains.

Abstract 5521: TTFields reduce cancer cell clonogenic potential through abnormal chromosome segregation during mitosis

Abstract TTFields cause mitotic arrest and cell death by the disruption of spindle microtubule arrangement and interference with cytokinesis. Recently it was reported that TTFields application results in reduced clonogenicity of cells surviving treatment. We hypothesized that TTFields can impair chromosome segregation during mitosis thus leading to a long term effect on surviving cancer cells. A2780 human ovarian cancer cells were treated with TTFields for 24, 48 and 72 hours and the cytotoxic effect and clonogenic potential were determined. Abnormal mitotic figures were analyzed using fluorescence microscopy and cell karyotyping was performed using Giemsa staining and SKY analysis. TTFields application led to 36, 52 and 43% cell growth inhibition after 24, 48 and 72 hours, respectively. Clonogenic assays revealed that only 12.9% of cells treated with TTFields for 72 hours maintained their ability to form colonies after 2 weeks. Immunofluorescence images revealed a significant increase in the number of abnormal mitotic figures amongst treated cells. Karyotyping analysis following 24 and 48 hours of TTFields application revealed 38.5% and 53.8% abnormal segregation events, respectively. After 24 hours of treatment, the proportion of cells with reduced and increased chromosomes number was equal, however, following 48 hours of treatment the population with a reduced chromosome number disappeared. These results demonstrate that in addition to their cytotoxic effect leading to mitotic death, TTFields also lead to abnormal chromosome segregation. The surviving aneuploid cells are impaired, providing a possible explanation for the reduced clonogenic potential reported in cancer cells treated with TTFields. Citation Format: Rosa S. Schneiderman, Moshe Giladi, Yaara Porat, Mijal Munster, Eilon D. Kirson, Uri Weinberg, Yoram Palti. TTFields reduce cancer cell clonogenic potential through abnormal chromosome segregation during mitosis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5521. doi:10.1158/1538-7445.AM2014-5521

Telomere-centric genome repatterning determines recurring chromosome number reductions during the evolution of eukaryotes.

Whole-genome duplication (WGD) is central to the evolution of many eukaryotic genomes, in particular rendering angiosperm (flowering plant) genomes much less stable than those of animals. Following repeated duplication/triplication(s), angiosperm chromosome numbers have usually been restored to a narrow range, as one element in a 'diploidization' process that re-establishes diploid heredity. In several angiosperms affected by WGD, we show that chromosome number reduction (CNR) is best explained by intra- and/or inter-chromosomal crossovers to form new chromosomes that utilize the existing telomeres of 'invaded' and centromeres of 'invading' chromosomes, the alternative centromeres and telomeres being lost. Comparison with the banana (Musa acuminata) genome supports a 'fusion model' for the evolution of rice (Oryza sativa) chromosomes 2 and 3, implying that the grass common ancestor had seven chromosomes rather than the five implied by a 'fission model.' The 'invading' and 'invaded' chromosomes are frequently homoeologs, originating from duplication of a common ancestral chromosome and with greater-than-average DNA-level correspondence to one another. Telomere-centric CNR following recursive WGD in plants is also important in mammals and yeast, and may be a general mechanism of restoring small linear chromosome numbers in higher eukaryotes.

Comparative Relationships and Chromosome Evolution in Switchgrass (Panicum virgatum) and Its Genomic Model, Foxtail Millet (Setaria italica)

A switchgrass (Panicum virgatum L.) consensus map was developed that combined data from two mapping populations and integrated recombination data from both parents of this largely obligate outcrossing species. The consensus map consisted of 1,321 loci and spanned 2,122 cM. An analysis of the distribution of genic and genomic markers across the length of a linkage group showed that genic markers were relatively overrepresented in distal regions, while genomic markers were relatively overrepresented in pericentromeric regions. Furthermore, genic markers located in distal and pericentromeric regions identified orthologs in the genome of the closely related species foxtail millet (Setaria italica (L.) P. Beauv.) at a similar rate, but fewer orthologs were found for genomic markers that mapped to pericentromeric regions compared to distal regions. Because cross-species sequence conservation is largely limited to genes, our data suggest that genomic markers that map to distal regions are more likely to be derived from genic regions than those that map to pericentromeric regions. A comparative analysis between loci on the switchgrass consensus map and their orthologs in the foxtail millet genome showed that the two species varied by at least nine inversions and one reciprocal translocation. Extending the comparative analysis to sorghum demonstrated that the majority of the rearrangements occurred in the foxtail millet genome in the past 13 million years. The reduction in chromosome number from 10 in the Andropogoneae to 9 in the Paniceae was achieved through a complex set of rearrangements involving three ancestral chromosomes orthologous to sorghum chromosomes 6, 8, and 9 and chromosomes III and VII in the lineage leading to switchgrass and foxtail millet. Upon insertion of ancestral chromosome 9 into ancestral chromosome 8, the centromere of the recipient chromosome was lost, and this was accompanied by a loss of repetitive DNA. The switchgrass–foxtail millet comparative map provides a guide of the rearrangements that need to be taken into account when using foxtail millet as a surrogate for switchgrass in genetic analyses.

Karyotype structure and chromosome fragility in the grass Phleum echinatum Host.

Phleum echinatum Host (2n = 2x = 10) is an annual Mediterranean species which differs from other representatives of the genus Phleum by reduced chromosome number, asymmetric karyotype and unusually high amount of DNA in the genome. Chromosomes of this plant were studied using conventional acetic-orcein staining and fluorescence in situ hybridization (FISH). FISH showed the major 35S ribosomal DNA (rDNA) site at the secondary constriction of satellite chromosome (3) and the minor 35S rDNA site near 5S rDNA cluster in the monobrachial chromosome 5. Telomeric repeats were detected at all chromosome ends within secondary constriction in satellited chromosome 3 and at the centromeric regions of chromosomes 1 and 2. Intrachromosomally located telomeric repeats are probably traces of chromosomal rearrangements that have shaped P.echinatum genome; they were prone to breakage which was manifested in chromosome fragmentation. The most distinct telomeric signals, suggesting massive amplification of interstitial telomeric sequences (ITRs), were observed at the nucleolar organizer region (NOR) of the third chromosome pair. Double FISH confirmed co-localization of telomeric and 35S rDNA repeats in this locus characterized by the biggest fragility in the karyotype. Fragile sites of P.echinatum, composed of amplified telomeric repeats, may bear a resemblance to metazoan rare fragile sites enriched in microsatellite repeats.Electronic supplementary materialThe online version of this article (doi:10.1007/s00709-014-0681-5) contains supplementary material, which is available to authorized users.

The Robertsonian phenomenon in the house mouse: mutation, meiosis and speciation.

Many different chromosomal races with reduced chromosome number due to the presence of Robertsonian fusion metacentrics have been described in western Europe and northern Africa, within the distribution area of the western house mouse Mus musculus domesticus. This subspecies of house mouse has become the ideal model for studies to elucidate the processes of chromosome mutation and fixation that lead to the formation of chromosomal races and for studies on the impact of chromosome heterozygosities on reproductive isolation and speciation. In this review, we briefly describe the history of the discovery of the first and subsequent metacentric races in house mice; then, we focus on the molecular composition of the centromeric regions involved in chromosome fusion to examine the molecular characteristics that may explain the great variability of the karyotype that house mice show. The influence that metacentrics exert on the nuclear architecture of the male meiocytes and the consequences on meiotic progression are described to illustrate the impact that chromosomal heterozygosities exert on fertility of house mice-of relevance to reproductive isolation and speciation. The evolutionary significance of the Robertsonian phenomenon in the house mouse is discussed in the final section of this review.

Live and let die: centromere loss during evolution of plant chromosomes

SummaryFunctional centromeres, ensuring regular chromosome segregation in mitosis and meiosis, are a prerequisite for the evolutionary success of pre‐existing and new chromosome variants. The rapid progress in plant comparative genomics and cytogenetics brings new insights into the evolutionary fate of centromeres and mechanisms of chromosome number reduction (descending dysploidy). Centromere loss and relocation in chromosome regions with otherwise conserved collinearity can be explained by conventional mechanisms of chromosome rearrangements or, as newly available phylogenomic and cytogenomic data suggest, by centromere inactivation through epigenetic chromatin modifications and/or intra‐ and inter‐chromosomal recombination.

A RAD-tag genetic map for the platyfish (Xiphophorus maculatus) reveals mechanisms of karyotype evolution among teleost fish.

Mammalian genomes can vary substantially in haploid chromosome number even within a small taxon (e.g., 3-40 among deer alone); in contrast, teleost fish genomes are stable (24-25 in 58% of teleosts), but we do not yet understand the mechanisms that account for differences in karyotype stability. Among perciform teleosts, platyfish (Xiphophorus maculatus) and medaka (Oryzias latipes) both have 24 chromosome pairs, but threespine stickleback (Gasterosteus aculeatus) and green pufferfish (Tetraodon nigroviridis) have just 21 pairs. To understand the evolution of teleost genomes, we made a platyfish meiotic map containing 16,114 mapped markers scored on 267 backcross fish. We tiled genomic contigs along the map to create chromosome-length genome assemblies. Genome-wide comparisons of conserved synteny showed that platyfish and medaka karyotypes remained remarkably similar with few interchromosomal translocations but with numerous intrachromosomal rearrangements (transpositions and inversions) since their lineages diverged ∼120 million years ago. Comparative genomics with platyfish shows how reduced chromosome numbers in stickleback and green pufferfish arose by fusion of pairs of ancestral chromosomes after their lineages diverged from platyfish ∼195 million years ago. Zebrafish and human genomes provide outgroups to root observed changes. These studies identify likely genome assembly errors, characterize chromosome fusion events, distinguish lineage-independent chromosome fusions, show that the teleost genome duplication does not appear to have accelerated the rate of translocations, and reveal the stability of syntenies and gene orders in teleost chromosomes over hundreds of millions of years.

Mouse CCDC79 (TERB1) is a meiosis-specific telomere associated protein.

BackgroundTelomeres have crucial meiosis-specific roles in the orderly reduction of chromosome numbers and in ensuring the integrity of the genome during meiosis. One such role is the attachment of telomeres to trans-nuclear envelope protein complexes that connect telomeres to motor proteins in the cytoplasm. These trans-nuclear envelope connections between telomeres and cytoplasmic motor proteins permit the active movement of telomeres and chromosomes during the first meiotic prophase. Movements of chromosomes/telomeres facilitate the meiotic recombination process, and allow high fidelity pairing of homologous chromosomes. Pairing of homologous chromosomes is a prerequisite for their correct segregation during the first meiotic division. Although inner-nuclear envelope proteins, such as SUN1 and potentially SUN2, are known to bind and recruit meiotic telomeres, these proteins are not meiosis-specific, therefore cannot solely account for telomere-nuclear envelope attachment and/or for other meiosis-specific characteristics of telomeres in mammals.ResultsWe identify CCDC79, alternatively named TERB1, as a meiosis-specific protein that localizes to telomeres from leptotene to diplotene stages of the first meiotic prophase. CCDC79 and SUN1 associate with telomeres almost concurrently at the onset of prophase, indicating a possible role for CCDC79 in telomere-nuclear envelope interactions and/or telomere movements. Consistent with this scenario, CCDC79 is missing from most telomeres that fail to connect to SUN1 protein in spermatocytes lacking the meiosis-specific cohesin SMC1B. SMC1B-deficient spermatocytes display both reduced efficiency in telomere-nuclear envelope attachment and reduced stability of telomeres specifically during meiotic prophase. Importantly, CCDC79 associates with telomeres in SUN1-deficient spermatocytes, which strongly indicates that localization of CCDC79 to telomeres does not require telomere-nuclear envelope attachment.ConclusionCCDC79 is a meiosis-specific telomere associated protein. Based on our findings we propose that CCDC79 plays a role in meiosis-specific telomere functions. In particular, we favour the possibility that CCDC79 is involved in telomere-nuclear envelope attachment and/or the stabilization of meiotic telomeres. These conclusions are consistent with the findings of an independently initiated study that analysed CCDC79/TERB1 functions.

Chromosome number reduction in Eremothecium coryli by two telomere-to-telomere fusions.

The genus Eremothecium belongs to the Saccharomyces complex of pre-whole-genome duplication (WGD) yeasts and contains both dimorphic and filamentous species. We established the 9.1-Mb draft genome of Eremothecium coryli, which encodes 4,682 genes, 186 tRNA genes, and harbors several Ty3 transposons as well as more than 60 remnants of transposition events (LTRs). The initial de novo assembly resulted in 19 scaffolds, which were assembled based on synteny to other Eremothecium genomes into six chromosomes. Interestingly, we identified eight E. coryli loci that bear centromeres in the closely related species E. cymbalariae. Two of these E. coryli loci, CEN1 and CEN8, however, lack conserved DNA elements and did not convey centromere function in a plasmid stability assay. Correspondingly, using a comparative genomics approach we identified two telomere-to-telomere fusion events in E. coryli as the cause of chromosome number reduction from eight to six chromosomes. Finally, with the genome sequences of E. coryli, E. cymbalariae, and Ashbya gossypii a reconstruction of three complete chromosomes of an Eremothecium ancestor revealed that E. coryli is more syntenic to this ancestor than the other Eremothecium species.

The Role of Fusion in Ant Chromosome Evolution: Insights from Cytogenetic Analysis Using a Molecular Phylogenetic Approach in the Genus Mycetophylax

Among insect taxa, ants exhibit one of the most variable chromosome numbers ranging from n = 1 to n = 60. This high karyotype diversity is suggested to be correlated to ants diversification. The karyotype evolution of ants is usually understood in terms of Robertsonian rearrangements towards an increase in chromosome numbers. The ant genus Mycetophylax is a small monogynous basal Attini ant (Formicidae: Myrmicinae), endemic to sand dunes along the Brazilian coastlines. A recent taxonomic revision validates three species, Mycetophylax morschi, M. conformis and M. simplex. In this paper, we cytogenetically characterized all species that belongs to the genus and analyzed the karyotypic evolution of Mycetophylax in the context of a molecular phylogeny and ancestral character state reconstruction. M. morschi showed a polymorphic number of chromosomes, with colonies showing 2n = 26 and 2n = 30 chromosomes. M. conformis presented a diploid chromosome number of 30 chromosomes, while M. simplex showed 36 chromosomes. The probabilistic models suggest that the ancestral haploid chromosome number of Mycetophylax was 17 (Likelihood framework) or 18 (Bayesian framework). The analysis also suggested that fusions were responsible for the evolutionary reduction in chromosome numbers of M. conformis and M. morschi karyotypes whereas fission may determines the M. simplex karyotype. These results obtained show the importance of fusions in chromosome changes towards a chromosome number reduction in Formicidae and how a phylogenetic background can be used to reconstruct hypotheses about chromosomes evolution.

Comparative linkage maps suggest that fission, not polyploidy, underlies near-doubling of chromosome number within monkeyflowers (Mimulus; Phrymaceae)

Changes in chromosome number and structure are important contributors to adaptation, speciation and macroevolution. In flowering plants, polyploidy and subsequent reductions in chromosome number by fusion are major sources of chromosomal evolution, but chromosome number increase by fission has been relatively unexplored. Here, we use comparative linkage mapping with gene-based markers to reconstruct chromosomal synteny within the model flowering plant genus Mimulus (monkeyflowers). Two sections of the genus with haploid numbers ≥ 14 have been inferred to be relatively recent polyploids because they are phylogenetically nested within numerous taxa with low base numbers (n=8-10). We combined multiple data sets to build integrated genetic maps of the M. guttatus species complex (section Simiolus, n=14) and the M. lewisii group (section Erythranthe; n=8), and then aligned the two integrated maps using >100 shared markers. We observed strong segmental synteny between M. lewisii and M. guttatus maps, with essentially 1-to-1 correspondence across each of 16 chromosomal blocks. Assuming that the M. lewisii (and widespread) base number of 8 is ancestral, reconstruction of 14 M. guttatus chromosomes requires at least eight fission events (likely shared by Simiolus and sister section Paradanthus (n=16)), plus two fusion events. This apparent burst of fission in the yellow monkeyflower lineages raises new questions about mechanisms and consequences of chromosomal fission in plants. Our comparative maps also provide insight into the origins of a chromosome exhibiting centromere-associated female meiotic drive and create a framework for transferring M. guttatus genome resources across the entire genus.

A new diploid chromosome number in Gymnopleurus Illiger, 1803 (Coleoptera: Scarabaeidae, Scarabaeinae) from Haryana, India

The chromosomes obtained from scarab beetle Gymnopleurus mundus Wied., 1819 of subfamily Scarabaeinae were studied using standard staining and C-banding. The karyotype is comprised of 14 chromosomes with meioformula, 6AA+Xyp. The analysis of constitutive heterochromatin (CH) revealed small blocks located in the centromeric region of all chromosomes. Some of the anomalies like polypoidy, decondensation, stickyness and fragmentation of the chromatids were also encountered. The reduction of chromosome number and with conserved sex chromosome mechanism in G. mundus as compared to other Gymnopleurus spp. is suggestive of the Robertsonian fusion of autosomes having played some role in the evolution of karyotype in this genus.

Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga

Loss of sexual reproduction is considered an evolutionary dead end for metazoans, but bdelloid rotifers challenge this view as they appear to have persisted asexually for millions of years. Neither male sex organs nor meiosis have ever been observed in these microscopic animals: oocytes are formed through mitotic divisions, with no reduction of chromosome number and no indication of chromosome pairing. However, current evidence does not exclude that they may engage in sex on rare, cryptic occasions. Here we report the genome of a bdelloid rotifer, Adineta vaga (Davis, 1873), and show that its structure is incompatible with conventional meiosis. At gene scale, the genome of A. vaga is tetraploid and comprises both anciently duplicated segments and less divergent allelic regions. However, in contrast to sexual species, the allelic regions are rearranged and sometimes even found on the same chromosome. Such structure does not allow meiotic pairing; instead, we find abundant evidence of gene conversion, which may limit the accumulation of deleterious mutations in the absence of meiosis. Gene families involved in resistance to oxidation, carbohydrate metabolism and defence against transposons are significantly expanded, which may explain why transposable elements cover only 3% of the assembled sequence. Furthermore, 8% of the genes are likely to be of non-metazoan origin and were probably acquired horizontally. This apparent convergence between bdelloids and prokaryotes sheds new light on the evolutionary significance of sex.