Mouse centric and pericentric satellite repeats form distinct functional heterochromatin

Heterochromatin is thought to play a critical role for centromeric function. However, the respective contributions of the distinct repetitive sequences found in these regions, such as minor and major satellites in the mouse, have remained largely unsolved. We show that these centric and pericentric repeats on the chromosomes have distinct heterochromatic characteristics in the nucleus. Major satellites from different chromosomes form clusters associated with heterochromatin protein 1alpha, whereas minor satellites are individual entities associated with centromeric proteins. Both regions contain methylated histone H3 (Me-K9 H3) but show different micrococcal nuclease sensitivities. A dinucleosome repeating unit is found specifically associated with major satellites. These domains replicate asynchronously, and chromatid cohesion is sustained for a longer time in major satellites compared with minor satellites. Such prolonged cohesion in major satellites is lost in the absence of Suv39h histone methyltransferases. Thus, we define functionally independent centromeric subdomains, which spatio-temporal isolation is proposed to be important for centromeric cohesion and dissociation during chromosome segregation.     

Satellite RNAs of plant viruses: structures and biological effects.

Plant viruses often contain parasites of their own, referred to as satellites. Satellite RNAs are dependent on their associated (helper) virus for both replication and encapsidation. Satellite RNAs vary from 194 to approximately 1,500 nucleotides (nt). The larger satellites (900 to 1,500 nt) contain open reading frames and express proteins in vitro and in vivo, whereas the smaller satellites (194 to 700 nt) do not appear to produce functional proteins. The smaller satellites contain a high degree of secondary structure involving 49 to 73% of their sequences, with the circular satellites containing more base pairing than the linear satellites. Many of the smaller satellites produce multimeric forms during replication. There are various models to account for their formation and role in satellite replication. Some of these smaller satellites encode ribozymes and are able to undergo autocatalytic cleavage. The enzymology of satellite replication is poorly understood, as is the replication of their helper viruses. In many cases the coreplication of satellites suppresses the replication of the helper virus genome. This is usually paralleled by a reduction in the disease induced by the helper virus; however, there are notable exceptions in which the satellite exacerbates the pathogenicity of the helper virus, albeit on only a limited number of hosts. The ameliorative satellites are being assessed as biocontrol agents of virus-induced disease. In greenhouse studies, satellites have been known to "spontaneously" appear in virus cultures. The possible origin of satellites will be briefly considered.     

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.     

The Relationship Between the Presence of Satellite Males and Nest-Holders' Mating Success in the Azorean Rock-Pool Blenny Parablennius sanguinolentus parvicornis

In the Azorean rock-pool blenny, sexually active males may adopt alternative reproductive tactics. In the present paper the relationship between the presence of satellite males and the reproductive success of nest-holders was investigated by comparing nests with and without an associated satellite male. Males with an associated satellite male suffered more conspecific intrusions but they did not display a higher frequency of attacks towards conspecifics. Nest-holder males were more aggressive towards other conspecifics than towards satellites and the tolerance of nest-holders towards satellites was inversely correlated with the time spent by the satellites in the breeding territory, which suggests control by the nest-holder male of the satellite investment in shared territorial defence. Nest-holders with an associated satellite male had higher condition factors and received more female visits and more spawnings. These results bear two possible interpretations. (1) Nest-holders benefit from the presence of a satellite male by increased attractiveness of their nests to females; satellite males are mutualists helping to defend the nest-owner's territory and to attract females, which is why they are tolerated. (2) Satellite males associate preferentially with more successful nest-holder males which have higher condition factors, and by doing so have more opportunities to achieve parasitic fertilizations. Only experiments will allow these two hypotheses to be distinguished.     

The centriolar satellite proteins Cep72 and Cep290 interact and are required for recruitment of BBS proteins to the cilium

Defects in centrosome and cilium function are associated with phenotypically related syndromes called ciliopathies. Centriolar satellites are centrosome-associated structures, defined by the protein PCM1, that are implicated in centrosomal protein trafficking. We identify Cep72 as a PCM1-interacting protein required for recruitment of the ciliopathy-associated protein Cep290 to centriolar satellites. Loss of centriolar satellites by depletion of PCM1 causes relocalization of Cep72 and Cep290 from satellites to the centrosome, suggesting that their association with centriolar satellites normally restricts their centrosomal localization. We identify interactions between PCM1, Cep72, and Cep290 and find that disruption of centriolar satellites by overexpression of Cep72 results in specific aggregation of these proteins and the BBSome component BBS4. During ciliogenesis, BBS4 relocalizes from centriolar satellites to the primary cilium. This relocalization occurs normally in the absence of centriolar satellites (PCM1 depletion) but is impaired by depletion of Cep290 or Cep72, resulting in defective ciliary recruitment of the BBSome subunit BBS8. We propose that Cep290 and Cep72 in centriolar satellites regulate the ciliary localization of BBS4, which in turn affects assembly and recruitment of the BBSome. Finally, we show that loss of centriolar satellites in zebrafish leads to phenotypes consistent with cilium dysfunction and analogous to those observed in human ciliopathies.     

Location and Underreplication of Satellite DNA in DROSOPHILA MELANOGASTER

The two light nuclear satellites (PCsC1 = 1.672 and PCsC1 = 1.687) have been quantified in DNA isolated from the larvel imaginal discs and brains of Drosophila melanogaster with the genotypes X/O, X/X and X/Y. By comparing the results from these different genotypes, the amounts of the two satellites in the X and Y chromosomes and in the autosomes have been determined. The lightest satellite is not located to any appreciable extent in the X chromosome. The heterochromatic regions are not completely filled by these satellites. --Satellite DNA has also been quantified in DNA isolated from adults containing different genotypes. The two satellites are underreplicated to different extents. The apparent amount of underreplication for one of the satellites is different in different parts of the genome.     

NUCLEATING SITES FOR THE ASSEMBLY OF CYTOPLASMIC MICROTUBULES IN THE ECTODERMAL CELLS OF BLASTULAE OF ARBACIA PUNCTULATA

In the ectodermal cells of sea urchin blastulae, the microtubules converge and appear to make contact with three distinct cytoplasmic foci or satellites associated with the basal body of the cilium. Beneath the nucleus, which lies in the apical end of the cell, the microtubules are aligned predominantly parallel to the cell's long axis and could thus make contact with the satellites as is directly suggested by observations on sections at or near the planes of the satellites. After the embryos are treated with low temperature (0°C), the microtubules disassemble; however, the satellites can still be recognized. Upon rewarming, the microtubules reappear. In early stages of reformation, when the tubules in the cell consist of short segments, tubules have only been found in the apical part of the cell. One end of each microtubule appears to make contact with its respective satellite, or is aligned so that it could contact one of the satellites, provided serial sections were cut and collected in order. After longer periods of recovery, the microtubules elongate; as before, one end of each makes contact with a satellite or is aligned so that it could attach to a satellite. Segments of microtubules seen in the basal region of the cell are aligned parallel to the long axis of the cell as in the untreated ectodermal cells and are therefore interpreted as extensions of those tubules making contact with one of the satellites. On the basis of these observations, we suggest that assembly of microtubules is initiated at the satellites. These sites, perhaps best referred to as "nucleating sites," thereby could exert considerable control over the distribution of microtubules in cells. It is hoped that this preliminary report will be followed up by a more detailed study using serial sections.     

Geminivirus disease complexes: an emerging threat

Small circular single-stranded DNA satellites have recently been isolated from plants infected with whitefly-transmitted monopartite begomoviruses. The satellites, named DNA beta, depend on the helper viruses for their proliferation and, in turn, are required for helper virus accumulation and symptom expression. They are highly diverse yet retain an overall conserved structure with respect to potential coding regions and regulatory elements. The begomovirus-satellite disease complexes are associated with economically important diseases, and have been isolated from vegetable and fibre crops, ornamental plants and weeds throughout Africa and Asia. Their widespread distribution and diversity, coupled to the global movement of plant material and the dissemination of the whitefly vector, suggests that these disease complexes pose a serious threat to tropical and sub-tropical agro-ecosystems worldwide.     

Morphometric analysis of human chromosome satellites and NOR asymmetries by transmission electron microscopy

Human chromosome satellites appear as roughly spherical telomeric structures of 0.215 +/- 0.013 microns mean diameter by electron microscopy. Morphometric evaluations showed that in the short arms of D and G chromosomes lacking secondary constrictions, the chromatin which constituted the satellites appeared virtually integrated within the short arms. Asymmetry was detected in sister satellites from almost identical dimensions to the near absence of one of them. When Ag-NOR staining was employed to locate active nucleolar organizer regions (NORs) these appeared associated with satellite short arms and active NORs were never found in non-satellite chromosomes. Asymmetry was also evident between sister NORs.     

Chromatin opening of DNA satellites by targeted sequence-specific drugs

There are few tools available for dissecting and elucidating the functions of DNA satellites and other nongenic DNA. To address this, we have explored the experimental potential of DNA sequence-specific drugs containing pyrrole and imidazole amino acids (polyamides). Compounds were synthesized that target different Drosophila melanogaster satellites. Dimeric oligopyrroles were shown to target the AT-rich satellites I, III, and SARs (scaffold associated regions). One polyamide (P31) specifically binds the GAGAA satellite V. Specificity of targeting was established by footprinting, epifluorescence of nuclei, and polytene chromosomes stained with fluorescent derivatives. These polyamides were shown to mediate satellite-specific chromatin opening of the chromatin fiber. Remarkably, certain polyamides induced defined gain or loss-of-function phenotypes when fed to Drosophila melanogaster.     

Molecular variation of satellite DNA beta molecules associated with Malvastrum yellow vein virus and their role in pathogenicity

Previous studies have found that the diversity of begomovirus-associated DNA beta satellites is related to host and geographical origin. In this study, we have cloned and sequenced 20 different isolates of DNA beta molecules associated with Malvastrum yellow vein virus (MYVV) isolated from Malvastrum coromandelianum plants in different geographical locations of Yunnan Province, China. Analyses of their molecular variation indicate that the satellites are clustered together according to their geographical location but that they have only limited sequence diversity. Infectivity tests using infectious clones of MYVV and its associated DNA beta molecule indicate that MYVV DNA beta is indispensable for symptom induction in Nicotiana benthamiana, N. glutinosa, Petunia hybrida, and M. coromandelianum plants. Furthermore, we showed that MYVV interacts functionally with heterologous DNA beta molecules in N. benthamiana plants.     

Comparative study of satellite sequences and phylogeny of five species from the genus Palorus (Insecta, Coleoptera).

Major satellite sequences are analysed in the three tenebrionid beetles Palorus cerylonoides, P. genalis, and P. ficicola, and compared with the ones from P. ratzeburgii and P. subdepressus reported elsewhere. All of them are A+T rich, pericentromerically located, and with lengths of about 150 bp, either in the form of monomers or formed by more complex repeating units. A preliminary phylogenetic analysis of Palorus species using the 3' end of the mitochondrial Cytochrome Oxidase I gene shows that the five Palorus species have been diverging for a considerable amount of evolutionary time, with the pair P. ratzeburgii and P. genalis being the most closely related. Only these two taxa showed some similarity between their respective high-copy-number satellite sequences, while other satellites are mutually unrelated and might have originated independently. However, all the satellites have in common tertiary structure induced by intrinsic DNA curvature, a characteristic which is conserved within the genus. Palorus major satellites were previously detected in the genomes of congeneric species as low-copy-number clusters (Mestrovi? et al., Mol. Biol. Evol. 15: 1062-1068. 1998). Given the divergences between the analysed species, the substitution rate deduced from high- and low-copy-number repeats is unexpectedly low. The presence of sequence-induced DNA curvature in all Palorus satellites and similar satellite DNAs in the species pair P. ratzeburgii and P. genalis suggest (i) that constraints are at the tertiary structure; and (ii) that the satellite DNA evolutionary turnover can be dependent on the history of the taxa under study, resulting in retention of similar satellites in related taxa.     

Three-player social parasitism games: implications for resource defense and group formation

Abstract: Individuals that produce resources are often exploited by several individuals; such exploitation may or may not be tolerated. We modeled the decision of a resource owner to accept one scrounger (the "satellite") and of both of these to accept being joined by another (the "floater"). In general, satellites tolerated floaters when competition between them was low, while owners tolerated satellites when facilitation between satellite and floater was high. When floaters were likely to find resources without joining, owners were more likely to resist satellites. In some cases, Nash equilibria were also mutually beneficial for two of the three individuals. Our model makes the counterintuitive prediction that mutually beneficial coalitions between satellites and floaters can only arise when the net benefits arising from the other's presence are low. When facilitation between satellites and floaters is high, satellites and owners may form mutually beneficial groups and groups with division of labor, or alternatively, owners may benefit from encouraging floaters to join. Finally, our model suggests there must be differences in competitive ability or some benefit of familiarity for owners to tolerate satellites but not floaters. We discuss empirical evidence for these and other predictions of the model.     

Association of the developing acrosome with multiple small Golgi units, the Golgi satellites, in spermatids of the musk shrew, Suncus murinus

The spermatozoa of the musk shrew, Suncus murinus, have a fan-like giant acrosome with a diameter of approximately 20 mm. The aim of this study was to investigate how this giant acrosome is constructed in the musk shrew spermatid and, in particular, how the Golgi apparatus involved in acrosome formation behaves. The behaviour of the Golgi apparatus was monitored by confocal laser scanning microscopy with antibody against a Golgi-associated Rab6 small GTPase. In the early Golgi phase, small Golgi units, the Golgi satellites, localized as a large aggregate in the juxtanuclear cytoplasm. As acrosome formation progressed, the Golgi satellites gradually dispersed, associated with proacrosomal vesicles and an acrosomal vesicle, and finally became distributed as multiple small units over the whole surface of an acrosomal cap in the round spermatid. The mode of acrosome formation in musk shrews was distinctly different from that in rats and mice, in which the Golgi apparatus remains as a single unit throughout acrosome formation. In musk shrews, the proacrosomal vesicles formed successively by the Golgi satellites coalesced, one after another, into a potential acrosomal vesicle. This process may result in further enlargement of the acrosome. The results of the present study indicate that Golgi satellites are necessary for the biogenesis and development of the giant acrosome in musk shrew spermatozoa.     

Satellite DNA of Anopheles stephensi liston (Diptera: Culicidae)

Four satellite DNAs in the Anopheles stephensi genome have been defined on the basis of their banding properties in Hoechst 33258-CsCl density gradients. Two of these satellites, satellites I and II, are visible on neutral CsCl density gradients as a light density peak forming approximately 15% of total cellular DNA. Hoechst-CsCl density gradient profiles of DNA extracted from polytene tissues indicates that these satellites are underreplicated in larval salivary gland cells and adult female Malpighian tubules and possibly also in ovarian nurse cells. The chromosomal location of satellite I on mitotic and polytene chromosomes has been determined by in situ hybridisation. Sequences complementary to satellite I are present in approximately equal amounts on a heterochromatic arm of the X and Y chromosomes and are also present, in smaller amounts, at the centromere of chromosome 3. A quantitative analysis of the in situ hybridisation experiments indicates that sequences complementary to satellite I at these two sites differ in their replicative behaviour during polytenisation: heterosomal satellite I sequences are under-replicated relative to chromosome 3 sequences in polytene larval salivary gland and ovarian nurse cell nuclei.     

Sex chromosome associated satellite DNA: Evolution and conservation

Satellites visible in female but not in male DNA were isolated from the snakesElaphe radiata (satellite IV, p = 1.708 g · cm^–3) andBungarus fasciatus (BK¹ minor, p=1.709 g · cm^–3). The satellites cross hybridize. Hybridization of³H labelled nick translated BK minor satellite DNA with the total male and female DNA and/or chromosomes in situ of different species of snakes revealed that its sequences are conserved throughout the snake group and are mainly concentrated on the W chromosome. Snakes lacking sex chromosomes do possess related sequences but there is no sex difference and visible related satellites are absent. The following conclusions have been reached on the basis of these results. 1. The W chromosome associated satellite DNA is related to similar sequences scattered in the genome. 2. The origin and increment in the number of the W satellite DNA sequence on the W chromosome is associated with the heterochromatinization of the W. 3. Satellite sequences have become distributed along the length of the W and resulted in morphological differentiation of sex chromosomes. 4. Evolutionary conservation of W satellite DNA strongly suggests that functional constraints may have limited sequence divergence.     

An experimental approach to altering mating tactics in male horseshoe crabs (Limulus polyphemus)

Alternative reproductive tactics are often correlated with phenotype, density, environment, or social context. Male horseshoe crabs(Limulus polyphemus) have two mating tactics that are associatedwith phenotype. Males in good condition arrive at the nestingbeach and spawn while attached to females, whereas those inpoorer condition come ashore unattached and crowd around thenesting couples as satellites, fertilizing eggs through sperm competition. The correlation between mating tactic and phenotypemay be due to males choosing tactics based on condition, orit may be that males that have not found a female choose tocome ashore as satellites. To distinguish between these twopossibilities, I conducted an experiment on male horseshoe crabsin the field at Seahorse Key on the northern Gulf coast ofFlorida. I prevented males from attaching to females by placingsmall plastic bags over the claws they use to attach. The resultsshowed that males in poor condition came ashore as satellites,whereas males in good condition remained at sea. This meansthat mating tactics are cued by information about the male'scondition and not about whether he found a female. The evolutionof phenotype-correlated mating tactics can be represented bya model in which the fitness of each tactic changes with conditionand fitness curves cross. I hypothesize that male horseshoecrabs in good condition have higher fitness when attached and that males in poorer condition to better when unattached.     

Comparative analysis of three guinea pig satellite DNA's by restriction nucleases.

The structures of guinea pig satellite DNAs I, II, and III have been analyzed by digestion with seven restriction nucleases. From the cleavage patterns it is obvious that the long-range periodicities in these three satellites differ rather characteristically Satellite I is fairly resistant to six nucleases and gives only a number of weak discrete bands which do not show a simple regularity. By the restriction nuclease from Arthrobacter luteus, however, it is cleaved extensively and yields very heterogeneous breakdown products. This is consistent with the high extent of divergence previously found for this satellite, e. g. by sequence analysis. Satellite II is almost completely resistant to all nucleases, indicative of a high degree of sequence homogeneity of this satellite. Satellite III is completely broken by the restriction nuclease from Bacillus subtilis into fragments which form a novel, highly regular series of bands in gel electrophoresis. The patterns show that the satellite is composed of tandem repeats ofapproximately 215 nucleotide pairs length, each repeat unit containing two cleavage sites for this nuclease. The data are consistent with the assumption that 30--40% of all cleavage sites have been eliminated by a random process. Satellite III DNA yields weak degradation patterns of the same periodicity with a number of other restriction nucleases. Cleavage sites for these nuclease are clustered on separatesmall segments of the satellite DNA. In this respect, the satellite is similar to others, notably the mouse satellite DNA. The three guinea pig satellites are examples of more general types of satellite structures also found in othe organisms. Similarities and differences to other satellites are discussed with special consideration to theories on the evolution of this class of DNA.     

Ecotype-specific and chromosome-specific expansion of variant centromeric satellites in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Despite the conserved roles and conserved protein machineries of centromeres, their nucleotide sequences can be highly diverse even among related species. The diversity reflects rapid evolution, but the underlying mechanism is largely unknown. One approach to monitor rapid evolution is examination of intra-specific variation. Here we report variant centromeric satellites of Arabidopsis thaliana found through survey of 103 natural accessions (ecotypes). Among them, a cluster of variant centromeric satellites was detected in one ecotype, Cape Verde Islands (Cvi). Recombinant inbred mapping revealed that the variant satellites are distributed in centromeric region of the chromosome 5 (CEN5) of this ecotype. This apparently recent variant accumulation is associated with large deletion of a pericentromeric region and the expansion of satellite region. The variant satellite was bound to HTR12 (centromeric variant histone H3), although expansion of the satellite was not associated with comparable increase in the HTR12 binding. The results suggest that variant satellites with centromere function can rapidly accumulate in one centromere, supporting the model that the satellite repeats in the array are homogenized by occasional unequal crossing-over, which has a potential to generate an expansion of local sequence variants within a centromere cluster. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.