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A Virus Infection in the Marine Brown Alga Ectocarpus siliculosus (Phaeophyceae)
Abstract
Laboratory cultures of Ectocarpus siliculosus originating from New Zealand showed a defect in gametangium formation. Nuclear divisions in gametangium initials are not followed by cell wall formation. In the resulting multinucleate cells nuclear DNA increases dramatically, and nuclear membranes disintegrate. Eventually, the entire structure is filled with hexagonal particles of approximately 130 nm diameter. These were isolated and shown by EM to consist of a dense core surrounded by a 3-layered shell. They are released into the culture medium when the host cells burst. Ectocarpus gametes from healthy cultures could be infected by these particles. The resulting partheno-sporophytes developed pathological symptoms, suggesting that the particles are viruses.The expression of the defect is temperature dependent. At 10°C all gametangia are abnormal, while between 15 and 20 °C defective and normal gametangia and gametes are formed on the same plant. Partheno-sporophytes developing from such gametes carry the viral particles expressed in deformed unilocular and plurilocular sporangia.
... As the host develops into a mature macroalga, every cell inherits a copy of the phaeoviral genome via mitosis [57,58]. The genome remains latent except in the host reproductive organs (sporangia and gametangia), which become filled with virus particles when the virus is induced [59,60]. In addition to infection by virus particles, phaeoviruses are vertically transmitted by inheritance of the latent phaeoviral genome. ...
... Whether viruses play similar roles in the range shifts of kelp remains to be explored. Since Ectocarpales phaeoviruses are temperature sensitive [60,102] and can reduce rates of host photosynthesis and respiration [102], it should be determined whether phaeoviral infection impacts the biogeographical distributions of kelp. ...
Two sister orders of the brown macroalgae (class Phaeophyceae), the morphologically complex Laminariales (commonly referred to as kelp) and the morphologically simple Ectocarpales are natural hosts for the dsDNA phaeoviruses (family Phycodnaviridae) that persist as proviruses in the genomes of their hosts. We have previously shown that the major capsid protein (MCP) and DNA polymerase concatenated gene phylogeny splits phaeoviruses into two subgroups, A and B (both infecting Ectocarpales), while MCP-based phylogeny suggests that the kelp phaeoviruses form a distinct third subgroup C. Here we used MCP to better understand the host range of phaeoviruses by screening a further 96 and 909 samples representing 11 and 3 species of kelp and Ectocarpales, respectively. Sporophyte kelp samples were collected from their various natural coastal habitats spanning five continents: Africa, Asia, Australia, Europe, and South America. Our phylogenetic analyses showed that while most of the kelp phaeoviruses, including one from Macrocystis pyrifera, belonged to the previously designated subgroup C, new lineages of Phaeovirus in 3 kelp species, Ecklonia maxima, Ecklonia radiata, Undaria pinnatifida, grouped instead with subgroup A. In addition, we observed a prevalence of 26% and 63% in kelp and Ectocarpales, respectively. Although not common, multiple phaeoviral infections per individual were observed, with the Ectocarpales having both intra-and inter-subgroup phaeoviral infections. Only intra-subgroup phaeoviral infections were observed in kelp. Furthermore, prevalence of phaeoviral infections within the Ectocarpales is also linked to their exposure to waves. We conclude that phaeoviral infection is a widely occurring phenomenon in both lineages, and that phaeoviruses have diversified with their hosts at least since the divergence of the Laminariales and Ectocarpales.
... Heavily DAPI stained cells were associated with many opaque and not translucent cells (Figures 2b-d). It has been previously reported that similar cells in Ectocarpales were a result of viral infection and that the phaeovirus DNA genomes could be detected through DAPI staining (Müller et al., 1990). Transmission electron microscopy (TEM) of the L. digitata strain LdigPH10-30 m suggests that LdigV-1, similar to phaeovirus infections in Ectocarpales, targets the nucleus resulting in the eventual degeneration (Figures 2f and g) as the cytoplasm fills with long tubular structures (arrows; Figures 2h, i and k), followed by the development of virus-like particles (VLPs) (Figures 2f-l). ...
... Mature VLPs were observed in ultrafiltered gametophyte culture medium (Figures 2m and n) showing a structure similar to intracellular VLPs. Our observations in kelp compare well with the characteristics of EsV-1 in Ectocarpus as described by Müller et al. (1990). However, unlike the ectocarpoid phaeoviruses, the infection in kelp appears to be common in vegetative cells (Figures 2d and e) and we do not know yet how the virions are released. ...
Phaeoviruses are latent double-stranded DNA viruses that insert their genomes into those of their brown algal (Phaeophyceae) hosts. So far these viruses are known only from members of the Ectocarpales, which are small and short-lived macroalgae. Here we report molecular and morphological evidence for a new Phaeovirus cluster, referred to as sub-group C, infecting kelps (Laminariales) of the genera Laminaria and Saccharina, which are ecologically and commercially important seaweeds. Epifluorescence and TEM observations indicate that the Laminaria digitata Virus (LdigV), the type species of sub-group C, targets the host nucleus for its genome replication, followed by gradual degradation of the chloroplast and assembly of virions in the cytoplasm of both vegetative and reproductive cells. This study is the first to describe phaeoviruses in kelp. In the field, these viruses infected two thirds of their host populations; however, their biological impact remains unknown.
... Muller et al. (103) found viruses in Ectocarpus siliculosus, a related ectocarpoid species also isolated near the coast of New Zealand, which contained particles about 130 nm in diameter. These particles were located exclusively in the gametangia. ...
Until recently there was little interest or information on viruses and viruslike particles of eukaryotic algae. However, this situation is changing. In the past decade many large double-stranded DNA-containing viruses that infect two culturable, unicellular, eukaryotic green algae have been discovered. These viruses can be produced in large quantities, assayed by plaque formation, and analyzed by standard bacteriophage techniques. The viruses are structurally similar to animal iridoviruses, their genomes are similar to but larger (greater than 300 kbp) than that of poxviruses, and their infection process resembles that of bacteriophages. Some of the viruses have DNAs with low levels of methylated bases, whereas others have DNAs with high concentrations of 5-methylcytosine and N6-methyladenine. Virus-encoded DNA methyltransferases are associated with the methylation and are accompanied by virus-encoded DNA site-specific (restriction) endonucleases. Some of these enzymes have sequence specificities identical to those of known bacterial enzymes, and others have previously unrecognized specificities. A separate rod-shaped RNA-containing algal virus has structural and nucleotide sequence affinities to higher plant viruses. Quite recently, viruses have been associated with rapid changes in marine algal populations. In the next decade we envision the discovery of new algal viruses, clarification of their role in various ecosystems, discovery of commercially useful genes in these viruses, and exploitation of algal virus genetic elements in plant and algal biotechnology.
... Features of its inorganic biochemistry include a remarkable, non-ferritin mode of iron storage [15]. Ectocarpus is also well studied with regards to its pathologies which include viruses [16,17], fungi [18], oomycetes [18] and plasmodiophoraleans [19]. ...
This study explores key features of bromine and iodine metabolism in the filamentous brown alga and genomics model Ectocarpus siliculosus. Both elements are accumulated in Ectocarpus, albeit at much lower concentration factors (2-3 orders of magnitude for iodine, and < 1 order of magnitude for bromine) than e.g. in the kelp Laminaria digitata. Iodide competitively reduces the accumulation of bromide. Both iodide and bromide are accumulated in the cell wall (apoplast) of Ectocarpus, with minor amounts of bromine also detectable in the cytosol. Ectocarpus emits a range of volatile halogenated compounds, the most prominent of which by far is methyl iodide. Interestingly, biosynthesis of this compound cannot be accounted for by vanadium haloperoxidase since the latter have not been found to catalyze direct halogenation of an unactivated methyl group or hydrocarbon so a methyl halide transferase-type production mechanism is proposed.
... In retrospect, there have historically been many other large viruses observed by scientists. Several algal viruses such as Ectocaprus siliculosus virus (EsV-1) [5] and Micromonas pusilla virus (MpV) [6] were observed decades ago and have been studied sporadically over the years. Others, highlighted by Emiliania huxleyi virus (EhV) [7] and perhaps the best-studied of the large virus systems, the Chlorella-infecting virus group [8], have been characterized in studies ranging from biochemical to emerging ecological studies in recent years. ...
Viruses are generally considered to be amongst the smallest bioactive particles; dating back to the original observations, including those of luminaries such as Ivanosky and Beijerinck, size has always been at issue within the definition, a tradition that continued for many years [1]. It was thus a surprise to the scientific community in the early 2000s when French scientists demonstrated that a particle, previously thought to be a bacterium, was indeed a virus [2]. The discovery of the Mimivirus and the other “giants” that have followed, including Mamavirus, Pandoravirus, Faustovirus, and Mollivirus, has blurred the definition of what constitutes a virus and, indeed, the boundaries between viral particles and cellular life [3].
... Other characteristics of Ectocarpus studied include a detailed description of its life cycle (Müller 1967), the sexual pheromone (Maier 1995), ultrastructure (Maier 1997a, b), physiology (Schmidt and Dring 1993, Busch and Schmid 2001, ecophysiological variation (Bolton 1983), and phylogeography (Stache-Crain et al. 1997). The virus EsV-1, which infects Ectocarpus siliculosus, has been studied in considerable detail, and the complete sequence of its 336 kbp genome has been determined (Müller et al. 1990, Müller 1991, Kuhlenkamp and Müller 1994, Bräutigam et al. 1995, Sengco et al. 1996, Del Campo et al. 1997, Delaroque et al. 1999, 2001. The EsV-1 genome is potentially an interesting source of tools such as promoter regions for the development of molecular approaches in Ectocarpus. ...
Seaweeds is the common collective name for multicellular or macro-algae that play very important ecological roles in many aquatic communities. Major concerns have been raised about the health and sustainability of ecosystems reliant on key seaweed species in response to increasing anthropogenic influences, changing environmental conditions, and pathogen-related effects. To date, viral infection has been reported in at least 54 species of seaweeds. These data are comprised primarily of transmission electron microscopy observations of virus-like particles (VLPs) and viral sequences isolated from seaweed material (either from seaweed-associated virus particles or integrated in seaweed DNA as endogenous viral elements). Most VLPs in red seaweed are small (<80 nm) and icosahedral, and, therefore, not distinctive enough to be identified to a particular family or species. In contrast, the large (typically >150 nm) icosahedral VLPs reported in several brown and one green seaweed likely belong to the distinctive giant or megaviruses known as nucleo-cytoplasmic large DNA viruses (NCLDVs). The available evidence suggests that: (i) brown seaweeds and other stramenopile groups are infected by NCLDVs, (ii) red seaweeds are infected by dsRNA viruses related to fungal viruses, and (iii) Archaeplastida (plants, red, and green seaweed) was infected by NCLDVs in the past. The best studied seaweed virus genus is the NCLDV Phaeovirus. Currently, members of the genus Phaeovirus are known to infect seven species from four families of the order Ectocarpales and putatively infect eight species from three families of the order Laminariales, commonly known as kelp. This chapter seeks to provide an overview of the current state of seaweed virology research with a focus on the expansion in Phaeovirus diversity.
Chondrus crispus Stackhouse (Gigartinales) is a red seaweed found on North Atlantic rocky shores. Electrophoresis of RNA extracts showed a prominent band with a size of around 6,000 bp. Sequencing of the band revealed several sequences with similarity to totiviruses, double-stranded RNA viruses that normally infect fungi. This virus-like entity was named CcV. It should probably be regarded as an extreme viral quasispecies or a mutant swarm since low identity (<65%) was found between sequences. Totiviruses typically code for two genes: one capsid gene (gag) and one RNA-dependent RNA polymerase gene (pol) with a pseudoknot structure between the genes. Both the genes and the intergenic structures were found in the CcV sequences. A non-identical gag gene was also found in the nuclear genome of C. crispus, with associated EST and upstream regulatory features. The gene was presumably horizontally transferred from the virus to the alga. Similar dsRNA bands were seen in extracts from different life cycle stages of C. crispus and from all geographical locations tested. In addition, similar bands were also observed in RNA extractions from other red algae; however, the significance of this apparently widespread phenomenon is unknown. No phenotype caused by the infection nor any virus particles, or capsid proteins were identified; thus, the presence of viral particles has not been validated. These findings increase the known host range of totiviruses to include marine red algae. This article is protected by copyright. All rights reserved.
Ectocarpus-like marine brown algae are frequently parasitized by polyhedric DNA viruses. Infected hosts have been studied in unialgal and axenic cultures, and the present state of knowledge is summarized in regard to stage-specific virus expression, discharge and survival time of virus particles, infection mechanism, association with host's nuclear genome, passage of the virus genome through mitosis and meiosis of the host, suppression of symptoms and spontaneous recovery of infected plants, host specificity and intergeneric transmission, vitality of infected plants, pandemic occurrence of virus infections, molecular data on Ectocarpus and Feldmannia viruses, and algal DNA-viruses as potential vectors for gene transfer. A scheme for the nomenclature of brown algal viruses is proposed.
Sexual compatibility in the marine brown alga Ectocarpus siliculosus was studied using clonal laboratory cultures from various geographic areas. Complete interfertility was found in European material ranging from the Mediterranean to Norway. Plasmogamy was also found between strains from Florida, North Carolina and all European clones. Male and female plants from Massachusetts were almost fully, female plants from Texas partially sterile with strains from other localities. In addition, South Australian Ectocarpus siliculosus was interfertile with almost all other strains examined. These findings are interpreted to show that local populations of the species are related closely enough to permit plasmogamy on a world-wide basis. Some local populations (Massachusetts and Texas), however, are in the process of isolating themselves from the original gene pool.
The life cycle of Ectocarpus siliculosus from Naples (Italy) was investigated, using well defined cultured material. Gametophytes and sporophytes differ morphologically and functionally. The gametophytes are dioecious, with genotypic determination of their sex. Sporophytes exist in the haploid, diploid and tetraploid phase. All sporophytes can form unilocular sporangia. In tetraploid and diploid sporophytes the formation of unilocular sporangia is connected with meiosis. Certain motile cells of haploid plants may spontaneously give rise to diploid or tetraploid sporophytes which are homozygous. Gametophytes can only be formed together with sporophytes form the swarmers of unilocular sporangia (heteroblasty). Meiosis in tetraploid sporophytes resulted in diploid gametophytes, the gametes of which fused with haploid female gametes. All observed nuclear phases and growth forms are connected with each other by meiosis, heteroblasty, fusion of gametes and spontaneous increase in chromosome number.
Clonal isolates of the brown algaEctocarpus siliculosus (Ectocarpales) originating from Naples (Mediterranean Sea) and southern Chile were compared in laboratory culture studies.
The two isolates showed distinctly different morphological characters, but very similar details of life history and sexual
reproduction. Gametes are sexually compatible; hybrid zygotes are formed and sporophytes develop, which are fertile on the
basis of mitotic spores. However, unilocular sporangia were abortive, indicating segregational sterility caused by chromosomal
mismatch during meiosis. Although the biological species concept in a strict sense does not apply, and appreciable morphological
variability exists in this cosmopolitan taxon, local populations are considered as representatives of the same species.
Summary Ultrastructural examination ofStreblonema sp. revealed icosahedral virus-like particles (135–150 nm) throughout the cytoplasm of vegetative cells. The densely packed particles consist of an osmiophilic coat around a fibrillar core. Most cytoplasmic organelles are excluded from the regions where the particles are extremely abundant, but no degeneration of plastids, mitochondria or dictyosomes is evident. The “virogenic stroma” contains many ribosomes and fibers possibly representing DNA strands remaining from the lysed nucleus. No decrease in vigor seemed to be associated with the presence of the particles.
Virus-like particles were obsemed in zoospores and less frequently in vegetative cells of the filaments of mature plants of the brown alga Sorocarpus uvaeformls . The particles, measuring approximately 170 nm in diameter, are isometric in profile and show three distinct zones. An electron dense rim (coat), 10 nm in thickness, is separated from a dense core, 110 nm in diameter, by an electron light space 20 nm in width. When closely packed the particles are usually separated from each other by a regular halo-like space. Besides the isometric particles long flexuous structures of variable length and measuring 75 nm in width were also found. The infection could be induced experimentally in healthy cells by using either medium prevenient from infected cultures or crude extracts obtained from infected plants.