Composition and epigenetic markers of heterochromatin in the aphid Aphis nerii (Hemiptera: Aphididae)

A detailed karyotype analysis of the oleander aphid Aphis nerii focusing on the distribution, molecular composition and epigenetic modifications of heterochromatin was done in order to better understand the structure and evolution of holocentric/holokinetic chromosomes in aphids. The female karyotype (2n = 8) consisted of 3 pairs of autosomes and a pair of X chromosomes that were the longest elements in the karyotype and carried a single, terminally located nucleolar organizer region. Males showed 2n = 7 chromosomes due to the presence of a single X chromosome. Heterochromatin was located in the X chromosomes only and consisted of 4 satellite DNAs that have been identified. A. nerii constitutive heterochromatin was enriched in mono-, di- and tri-methylated H3 histones and HP1 proteins but, interestingly, it lacked DNA methylation that was widespread in euchromatic chromosomal regions. These results suggest that aphid heterochromatin is assembled and condensed without any involvement of DNA methylation.     

The cytogenetic architecture of the aphid genome

In recent years aphids, with their well‐defined polyphenism, have become favoured as model organisms for the study of epigenetic processes. The availability of the pea aphid (Acyrthosiphon pisum) genome sequence has engendered much research aimed at elucidating the mechanisms by which the phenotypic plasticity of aphids is inherited and controlled. Yet so far this research effort has paid little attention to the cytogenetic processes that play a vital part in the organisation, expression and inheritance of the aphid genome. Aphids have holocentric chromosomes, which have very different properties from the chromosomes with localised centromeres that are found in most other organisms. Here we review the diverse forms of aphid chromosome behaviour that occur during sex determination and male and female meiosis, often in response to environmental changes and mediated by endocrine factors. Remarkable differences occur, even between related species, that could have significant effects on the inheritance of all or parts of the genome. In relation to this, we review the particular features of the distribution of heterochromatin, rDNA genes and other repetitive DNA in aphid chromosomes, and discuss the part that these may play in the epigenetic modification of chromatin structure and function.     

HIM-10 is required for kinetochore structure and function on Caenorhabditis elegans holocentric chromosomes

Macromolecular structures called kinetochores attach and move chromosomes within the spindle during chromosome segregation. Using electron microscopy, we identified a structure on the holocentric mitotic and meiotic chromosomes of Caenorhabditis elegans that resembles the mammalian kinetochore. This structure faces the poles on mitotic chromosomes but encircles meiotic chromosomes. Worm kinetochores require the evolutionarily conserved HIM-10 protein for their structure and function. HIM-10 localizes to the kinetochores and mediates attachment of chromosomes to the spindle. Depletion of HIM-10 disrupts kinetochore structure, causes a failure of bipolar spindle attachment, and results in chromosome nondisjunction. HIM-10 is related to the Nuf2 kinetochore proteins conserved from yeast to humans. Thus, the extended kinetochores characteristic of C. elegans holocentric chromosomes provide a guide to the structure, molecular architecture, and function of conventional kinetochores.     

Holocentric chromosomes of Luzula elegans are characterized by a longitudinal centromere groove, chromosome bending, and a terminal nucleolus organizer region

The structure of holocentric chromosomes was analyzed in mitotic cells of Luzula elegans. Light and scanning electron microscopy observations provided evidence for the existence of a longitudinal groove along each sister chromatid. The centromere-specific histone H3 variant, CENH3, colocalized with this groove and with microtubule attachment sites. The terminal chromosomal regions were CENH3-negative. During metaphase to anaphase transition, L. elegans chromosomes typically curved to a sickle-like shape, a process that is likely to be influenced by the pulling forces of microtubules along the holocentric axis towards the corresponding microtubule organizing regions. A single pair of 45S rDNA sites, situated distal to Arabidopsis-telomere repeats, was observed at the terminal region of one chromosome pair. We suggest that the 45S rDNA position in distal centromere-free regions could be required to ensure chromosome stability.     

A Satellite Explosion in the Genome of Holocentric Nematodes

Centromere sequences in the genome are associated with the formation of kinetochores, where spindle microtubules grow in mitosis. Centromere sequences usually have long tandem repeats (satellites). In holocentric nematodes it is not clear how kinetochores are formed during mitosis; they are distributed throughout the chromosomes. For this reason it appeared of interest to study the satellites in nematodes in order to determine if they offer any clue on how kinetochores are assembled in these species. We have studied the satellites in the genome of six nematode species. We found that the presence of satellites depends on whether the nematode chromosomes are holocentric or monocentric. It turns out that holocentric nematodes are unique because they have a large number of satellites scattered throughout their genome. Their number, length and composition are different in each species: they apparently have very little evolutionary conservation. In contrast, no scattered satellites are found in the monocentric nematode Trichinella spiralis. It appears that the absence/presence of scattered satellites in the genome distinguishes monocentric from holocentric nematodes. We conclude that the presence of satellites is related to the holocentric nature of the chromosomes of most nematodes. Satellites may stabilize a higher order structure of chromatin and facilitate the formation of kinetochores. We also present a new program, SATFIND, which is suited to find satellite sequences.     

Visualization of diffuse centromeres with centromere-specific histone H3 in the holocentric plant Luzula nivea

Although holocentric species are scattered throughout the plant and animal kingdoms, only holocentric chromosomes of the nematode worm Caenorhabditis elegans have been analyzed with centromeric protein markers. In an effort to determine the holocentric structure in plants, we investigated the snowy woodrush Luzula nivea. From the young roots, a cDNA encoding a putative centromere-specific histone H3 (LnCENH3) was successfully isolated based on sequence similarity among plant CENH3s. The deduced amino acid sequence was then used to raise an anti-LnCENH3 antibody. Immunostaining clearly revealed the diffuse centromere-like structure that appears in the linear shape at prophase to telophase. Furthermore, it was shown that the amount of LnCENH3 decreased significantly at interphase. The polar side positioning on each chromatid at metaphase to anaphase also confirmed that LnCENH3 represents one of the centromere-specific proteins in L. nivea. These data from L. nivea are compared with those from C. elegans, and common features of holocentric chromosomes are discussed.     

利用原子力显微镜对A型流感病毒的形态学研究

利用透射电子显微镜(TEM)和原子力显微镜(AFM)观察流感病毒(H1N1),探讨AFM在病毒形态研究中的应用,为病毒形态学研究提供一种新型、简便、快捷的工具.TEM采用磷钨酸负染方法,AFM采用轻敲模式在大气常温下扫描成像,并对主要指标长度(直径)、Ra、Rq等进行测量.两种方法最终得到相似的形态学结果,流感病毒呈球状、丝状,并有一些形状介于两者之间.TEM提供了流感病毒二维图像,可见钉状突起,AFM则呈现了流感病毒三维图像,且可见病毒表面有凹凸不平的特征和边缘有齿轮状的突起,同时获得表面粗糙度等可以量化指标.与TEM观察相比,原子力显微镜是一种制样简单、观察直观的新型病毒形态学研究工具,其表征参数可以作为病毒形态学研究的量化指标.     

Sublethal effects of insecticides on the intrinsic rate of increase of cotton aphid

Insecticides are often implicated in causing outbreaks of the cotton aphid, Aphis gossypii (Glover) (Homoptera: Aphididae), through stimulation of reproduction. In this study we report the sublethal effects of dosages of bifenthrin, acephate, carbofuran or pyriproxifen on cotton aphid reproduction. We could not detect any increase or decrease in the intrinsic rate of increase of cotton aphids exposed to bifenthrin, acephate or carbofuran. However, we did detect some increases in the net reproductive rate of aphids treated with bifenthrin justifying further investigation of the effect on reproduction by this insecticide. Trends based on simple linear regression models suggest that sublethal dosages of bifenthrin or carbofuran have a negative impact on aphid population growth as dosages increase. These data suggest that stimulation of reproduction by these insecticides probably does not play a major role in cotton aphid outbreaks or resurgence. Pyriproxyfen is a juvenoid currently used for control of whiteflies in cotton. It demonstrated significant activity towards cotton aphid reared on treated cotton in our bioassays. Pyriproxyfen caused sterility in most aphids exposed to dosages exceeding 1 ppm, and reduced aphid longevity by approximately 50%. However, it did not appear to greatly influence the reproductive potential or longevity of reproductively mature aphids. A field study indicates that pyriproxyfen affects cotton aphid population structure and may have potential in managing cotton aphid outbreaks. Modifying aphid population structure and growth through the use of juvenoids such as pyriproxifen may prove to be an effective proactive approach to pest control without adversely impacting beneficial organisms or causing pest resurgence.     

A simple and effective sample preparation method for atomic force microscopy visualization of individual DNA molecules in situ

A simple, controllable and effective sample preparation method was established for atomic force microscopy (AFM) imaging of individual DNA molecules in aqueous solution. Firstly, magnesium ion (Mg²⁺) at a concentration of 5.0–10.0 mM as a positively charged bridge was transferred onto mica to immobilize DNA molecules. Then Mg²⁺-modified mica was used to investigate DNA molecules in any buffer without magnesium ion by AFM. AFM images demonstrated that DNA molecules can be successfully observed in solution with good resolution, reproducibility, and stability. Further, this DNA sample preparation method makes AFM successful to investigate DNA molecular interaction in situ and DNA/chitosan complex in gene delivery.     

Chromosome Analysis and Molecular Characterization of Highly Repeated DNAs in the Aphid Acyrthosiphon Pisum (Aphididae, Hemiptera)

Despite the interest in aphid biology, information on chromatin organization of their holocentric chromosomes is still limited to few species. In order to fill this gap, we have performed an extensive survey on pea aphid mitotic chromosomes using both classical and molecular cytogenetic techniques. Our results after silver, CMA₃ and DAPI-staining, C-banding and fluorescent in situ hybridization (FISH) using 28S rDNA and 5S rDNA as probes evidenced a tendency of repetitive DNAs to be concentrated on the X chromosomes. FISH experiments with the telomeric probe (TTAGG) _n revealed bright hybridization signals on each telomere of all Acyrthosiphon pisum chromosomes. No interstitial signals were seen. This revised version was published online in August 2006 with corrections to the Cover Date.     

The “acrostyle”: A newly described anatomical structure in aphid stylets

The recent demonstration that a plant virus could be retained on protein receptors located exclusively in a small area inside the common duct at the tip of aphid maxillary stylets indicated the possible existence of a distinct anatomical structure at this level. Since no distinct feature within the common duct of any aphid species has ever been reported in the literature, we first carefully re-examined the distal extremity of the maxillary stylets of Acyrthosiphon pisum using transmission- and scanning-electron microscopy. Here, we describe an area of the cuticle surface displaying a different structure that is limited to a “band” paving the bottom of the common duct in each opposing maxillary stylet. This band starts at the very distal extremity, adopts a “comma-like” shape as it continues up towards the salivary canal, reducing in width and disappearing before actually reaching it. Investigations on several aphid species led to the conclusion that this anatomical feature—which we have tentatively named the “acrostyle”—is highly conserved among aphids. We then produced an antibody recognizing a consensus peptide located in the middle of the RR-2 motif of cuticular proteins from A. pisum and showed that this motif is accessible specifically within the acrostyle, indicating a higher concentration of cuticular proteins. While it is clear that at least some viruses can use the acrostyle to interact with their aphid vectors to ensure plant-to-plant transmission, the role of this new “organ” in aphid biology is unknown and calls for further investigation in the near future.     

The holocentric species Luzula elegans shows interplay between centromere and large‐scale genome organization

In higher plants, the large‐scale structure of monocentric chromosomes consists of distinguishable eu‐ and heterochromatic regions, the proportions and organization of which depend on a species' genome size. To determine whether the same interplay is maintained for holocentric chromosomes, we investigated the distribution of repetitive sequences and epigenetic marks in the woodrush Luzula elegans (3.81 Gbp/1C). Sixty‐one per cent of the L. elegans genome is characterized by highly repetitive DNA, with over 30 distinct sequence families encoding an exceptionally high diversity of satellite repeats. Over 33% of the genome is composed of the Angela clade of Ty1/copia LTR retrotransposons, which are uniformly dispersed along the chromosomes, while the satellite repeats occur as bands whose distribution appears to be biased towards the chromosome termini. No satellite showed an almost chromosome‐wide distribution pattern as expected for a holocentric chromosome and no typical centromere‐associated LTR retrotransposons were found either. No distinguishable large‐scale patterns of eu‐ and heterochromatin‐typical epigenetic marks or early/late DNA replicating domains were found along mitotic chromosomes, although super‐high‐resolution light microscopy revealed distinguishable interspersed units of various chromatin types. Our data suggest a correlation between the centromere and overall genome organization in species with holocentric chromosomes.     

Visual representation by atomic force microscopy (AFM) of tomato spotted wilt virus ribonucleoproteins.

Atomic force microscopy (AFM) allows the observation of biological material without fixation procedures. Here we present AFM images of ribonucleoproteins (nucleocapsids) derived from a plant infecting RNA virus (tomato spotted wilt virus, TSWV), which have been recorded in contact mode. The nucleocapsids, prepared from systemically infected leaves of tobacco, were spreaded on a glass surface and dried in air, and appeared as regularly formed rings, resembling the proposed pseudocircular and panhandle structure of encapsidated genomic RNA. Average values between 1300 and 2200 nm of nucleocapsid lengths could be related to dimensions estimated by electron microscopy, thereby validating a filamentous configuration of the TSWV ribonucleoproteins. However, to our knowledge regular, ring-like forms of ribonucleoproteins have not been obtained by electron microscopy, which rather showed an amorphous structure of the virus particles. Hence, the AFM approach provides a starting point for further detailed studies on TSWV ribonucleoprotein complexes.     

Surface Characteristics of Gram-Negative and Gram-Positive Bacteria in an Atomic Force Microscope Image

Bacterial images can be obtained rather easily with an atomic-force microscope (AFM) in the magnification range of 5,000 to 30,000 times without any pretreatment of the specimens for such observations as chemical fixation, dehydration or staining. The bacterial shapes or the presence of flagella can be clearly recognized in these magnification ranges. In addition, we were also able to distinguish between Gramnegative and Gram-positive bacteria based on the specific wavy surface appearance of the former. AFM could thus be a useful tool for the identification of bacteria in the resolution range between electron and light microscopy.     

Continuous occurrence of intra-individual chromosome rearrangements in the peach potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae)

Analysis of the holocentric mitotic chromosomes of the peach-potato aphid, Myzus persicae (Sulzer), from clones labelled 50, 51 and 70 revealed different chromosome numbers, ranging from 12 to 14, even within each embryo, in contrast to the standard karyotype of this species (2n?=?12). Chromosome length measurements, combined with fluorescent in situ hybridization experiments, showed that the observed chromosomal mosaicisms are due to recurrent fragmentations of chromosomes X, 1 and 3. Contrary to what has generally been reported in the literature, X chromosomes were frequently involved in recurrent fragmentations, in particular at their telomeric ends opposite to the nucleolar organizer region. Supernumerary B chromosomes have been also observed in clones 50 and 51. The three aphid clones showed recurrent fissions of the same chromosomes in the same regions, thereby suggesting that the M. persicae genome has fragile sites that are at the basis of the observed changes in chromosome number. Experiments to induce males also revealed that M. persicae clones 50, 51 and 70 are obligately parthenogenetic, arguing that the reproduction by apomictic parthenogenesis favoured the stabilization and inheritance of the observed chromosomal fragments.     

Tobacco mosaic virus as an AFM tip calibrator

The study of high-resolution topographic surfaces of isolated single molecules is one of the applications of atomic force microscopy (AFM). Since tip-induced distortions are significant in topographic images the exact AFM tip shape must be known in order to correct dilated AFM height images using mathematical morphology operators. In this work, we present a protocol to estimate the AFM tip apex radius using tobacco mosaic virus (TMV) particles. Among the many advantages of TMV, are its non-abrasivity, thermal stability, bio-compatibility with other isolated single molecules and stability when deposited on divalent ion pretreated mica. Compared to previous calibration systems, the advantage of using TMV resides in our detailed knowledge of the atomic structure of the entire rod-shaped particle. This property makes it possible to interpret AFM height images in term of the three-dimensional structure of TMV. Results obtained in this study show that when a low imaging force is used, the tip is sensing viral protein loops whereas at higher imaging force the tip is sensing the TMV particle core. The known size of the TMV particle allowed us to develop a tip-size estimation protocol which permits the successful erosion of tip-convoluted AFM height images. Our data shows that the TMV particle is a well-adapted calibrator for AFM tips for imaging single isolated biomolecules. The procedure developed in this study is easily applicable to any other spherical viral particles.     

Direct observation of defect structure in protein crystals by atomic force and transmission electron microscopy.

We have examined the structure of S-layers isolated from Sulfolobus acidocaldarius using atomic force microscopy (AFM) and transmission electron microscopy (TEM). From the AFM images, we were able to directly observe individual dimers of the crystal, defects in the crystal structure, and twin boundaries. We have identified two types of boundaries, one defined by a mirror plane and the other by a glide plane. This work shows that twin boundaries are highly structured regions that are directly related to the organization of units within each crystal domain. Projection maps from TEM images have shown that there are significant differences in the final average maps has allowed us to relate high magnification views obtained by AFM to the relatively high resolution information obtained by electron microscopy and image processing.     

A novel in situ atomic force microscopy imaging technique to probe surface morphological features of starch granules

The application of atomic force microscopy (AFM) for observing iodine complexes in starch has been limited due to limitations including granular sample fixation techniques and possible unintended reactions with embedding materials such as epoxy resins or adhesives. In this paper, a new method is described that employs an optical microscopic technique to ensure that the tip of the AFM is scanning a specified granule without any probe-induced particle movement by the AFM probe motion. The direct sprinkling of samples on a two-sided adhesive tape allows investigations in an in situ environment of the un-embedded starch granule surface and thus provides high-resolution images of granule morphology and phase changes of starches in the presence of humidity and with iodine vapor. These observations demonstrate that this novel in situ AFM imaging technique allows us to visualize the hair-like structures on the surface of granular starches when starches are exposed to iodine vapor under humid environments. This study reveals that the hair-like extensions on the starch granule surfaces are strongly dependent on the organization of the glucan polymers within corn or potato starch.     

Holocentric chromosomes may be an apomorphy of Droseraceae

Holocentric chromosomes have evolved in various plant and animal taxa, which suggests they may confer a selective advantage in certain conditions, yet their adaptive potential has scarcely been studied. One of the reasons may reside in our insufficient knowledge of the phylogenetic distribution of holocentric chromosomes across eukaryotic phylogeny. In the present study, we focused on Droseraceae, a carnivorous plant family with an unknown chromosomal structure in monotypic genera Dionaea and Aldrovanda, and a closely related monotypic family Drosophyllaceae. We used flow cytometry to detect holocentric chromosomes by measuring changes in the ratio of the number of G2 nuclei to the number of G1 nuclei in response to gamma irradiation and determined chromosomal structures in Aldrovanda vesiculosa, Dionaea muscipula, Drosera tokaiensis, and Drosera ultramafica from Droseraceae and Drosophyllum lusitanicum from Drosophyllaceae. We confirmed monocentric chromosomes in D. lusitanicum and detected holocentric chromosomes in all four Droseraceae. Our novel finding of holocentric chromosomes in monotypic genera Aldrovanda and Dionaea suggests that all Droseraceae may be holocentric, but to confirm that further research is needed due to previously reported conflicting results in Drosera rotundifolia.     

Fast chromosomal evolution and karyotype instability: recurrent chromosomal rearrangements in the peach potato aphid Myzus persicae (Hemiptera: Aphididae)

The occurrence of karyotype variations with respect to both chromosome number and structure has been frequently reported in aphids. Here, we review recent data attesting to the presence of recurrent chromosomal changes in the karyotype of the peach potato aphid Myzus persicae, where clones presenting metaphases with different chromosome number (from 12 to 17) have been observed, also comparing plates obtained within the same embryo. According to the available data, M. persicae autosomes 3 and 1 are the chromosomes mostly involved in changes compared to other autosomes, suggesting that they could have sites more susceptible to fragmentation. Chromosomal fissions involving the X chromosomes have also been observed, suggesting that they may have fragile sites located at the termini opposite to the nucleolar organizer regions‐bearing telomere. The presence of holocentric chromosomes and reproduction by apomictic parthenogenesis, together with a constitutive expression of telomerase, could explain the inheritance of the observed chromosomal instability in aphids. Considering that chromosomal changes may affect the host choice and could also favour speciation, it would be intriguing to confirm whether the observed karyotype variants have effects over short temporal and spatial scales.