Nonreductive Iron Uptake Mechanism in the Marine Alveolate Chromera velia

Chromera velia is a newly cultured photosynthetic marine alveolate. This microalga has a high iron requirement for respiration and photosynthesis, although its natural environment contains less than 1 nm of this metal. We found that this organism uses a novel mechanism of iron uptake, differing from the classic reductive and siderophore-mediated iron uptake systems characterized in the model yeast Saccharomyces cerevisiae and present in most yeasts and terrestrial plants. C. velia has no trans-plasma membrane electron transfer system, and thus cannot reduce extracellular ferric chelates. It is also unable to use hydroxamate siderophores as iron sources. Iron uptake from ferric citrate by C. velia is not inhibited by a ferrous chelator, but the rate of uptake is strongly decreased by increasing the ferric ligand (citrate) concentration. The cell wall contains a large number of iron binding sites, allowing the cells to concentrate iron in the vicinity of the transport sites. We describe a model of iron uptake in which aqueous ferric ions are first concentrated in the cell wall before being taken up by the cells without prior reduction. We discuss our results in relation to the strategies used by the phytoplankton to take up iron in the oceans.Chromera velia is a newly cultured marine alveolate containing a photosynthetic plastid phylogenetically related to vestigial plastids in apicomplexan (Moore et al., 2008). It represents the closest free-living photosynthetic relative to apicomplexan parasites, thus providing a powerful model to study the evolution of eukaryotic adaptability (Moore et al., 2008). To gain further insight into the biology of this organism, the genome of which remains unsequenced, we investigated its iron metabolism and its mechanisms of iron uptake. We compared the data obtained with other phytoplanktonic organisms sharing the same ecological niche, and with a terrestrial unicellular eukaryote, the yeast Saccharomyces cerevisiae. S. cerevisiae is phylogenetically distant from C. velia, but its mechanisms of iron uptake are well characterized, and thus constitutes a useful model in these studies.Iron uptake by terrestrial microorganisms and plants is mostly based on the use of two main strategies, both of which have been previously characterized in S. cerevisiae. The first strategy is the reductive mechanism of uptake. Extracellular ferric complexes are first dissociated by reduction, via trans-plasma membrane electron transfer catalyzed by specialized flavohemoproteins (Fre). Free iron is then imported by a high-affinity permease system (Ftr1) coupled to a copper-dependent oxidase (Fet3), allowing iron to be channeled through the plasma membrane. In the second strategy, the siderophore-mediated mechanism, siderophores excreted by the cells or produced by other bacterial or fungal species are taken up without prior dissociation, via specific, copper-independent high-affinity receptors. Iron is then dissociated from the siderophores inside the cells, probably by reduction (for review, see Kosman, 2003; Philpott, 2006). Chlamydomonas reinhardtii is a photosynthetic eukaryotic model organism for the study of iron homeostasis, which shares with yeast the strategy 1 of iron uptake (copper-dependent reductive iron uptake; Merchant et al., 2006).Much less is known about the strategies used by marine phytoplankton to acquire iron. Some data suggest that these two strategies are used by some marine microalgae (Soria-Dengg and Horstmann, 1995; Allen et al., 2008). However, for most marine unicellular eukaryotes the mechanisms of iron assimilation are completely unknown. The strategies used by these organisms to acquire iron must have evolved to adapt to the very particular conditions that prevail in their surrounding natural environment: The transition metal composition of the ocean differs greatly from that of terrestrial environments (Butler, 1998). In particular, iron levels in surface seawater are extremely low (0.02–1 nm; Turner et al., 2001). Therefore a strategy of iron uptake operating efficiently in a terrestrial environment that contains iron at a micromolar level may be inefficient in a marine environment. No classic iron uptake system with an affinity constant in the nanomolar range has ever been found. Additionally, the marine environment imposes physical limits on the classic strategies of uptake, including the high diffusion rate of the species of interest (siderophores or reduced iron; Völker and Wolf-Gladrow, 1999). It is well known that the low levels of iron limits primary production of phytoplankton and carbon fluxes across vast regions of the world’s oceans (Coale et al., 2004; Pollard et al., 2009). It is thus of particular interest to elucidate the molecular mechanisms underlying acquisition of iron by marine phytoplankton and to determine which iron sources are preferentially assimilated with regards to the yield of carbon fixation.In this study, we investigated the mechanisms of iron uptake by C. velia, and found that this organism uses a nonreductive uptake system of ferric ions, which are first concentrated in the cell wall. Our findings provide a better understanding of the biology of this organism, and highlights the need for further study on the mechanisms of iron acquisition in marine phytoplankton.     

Phylogenetic analysis of the light-harvesting system in Chromera velia

Chromera velia is a newly discovered photosynthetic eukaryotic alga that has functional chloroplasts closely related to the apicoplast of apicomplexan parasites. Recently, the chloroplast in C. velia was shown to be derived from the red algal lineage. Light-harvesting protein complexes (LHC), which are a group of proteins involved in photon capture and energy transfer in photosynthesis, are important for photosynthesis efficiency, photo-adaptation/accumulation and photo-protection. Although these proteins are encoded by genes located in the nucleus, LHC peptides migrate and function in the chloroplast, hence the LHC may have a different evolutionary history compared to chloroplast evolution. Here, we compare the phylogenetic relationship of the C. velia LHCs to LHCs from other photosynthetic organisms. Twenty-three LHC homologues retrieved from C. velia EST sequences were aligned according to their conserved regions. The C.?velia LHCs are positioned in four separate groups on trees constructed by neighbour-joining, maximum likelihood and Bayesian methods. A major group of seventeen LHCs from C. velia formed a separate cluster that was closest to dinoflagellate LHC, and to LHC and fucoxanthin chlorophyll-binding proteins from diatoms. One C. velia LHC sequence grouped with LI1818/LI818-like proteins, which were recently identified as environmental stress-induced protein complexes. Only three LHC homologues from C. velia grouped with the LHCs from red algae.     

Identification of plant-like galactolipids in Chromera velia, a photosynthetic relative of malaria parasites

Apicomplexa are protist parasites that include Plasmodium spp., the causative agents of malaria, and Toxoplasma gondii, responsible for toxoplasmosis. Most Apicomplexa possess a relict plastid, the apicoplast, which was acquired by secondary endosymbiosis of a red alga. Despite being nonphotosynthetic, the apicoplast is otherwise metabolically similar to algal and plant plastids and is essential for parasite survival. Previous studies of Toxoplasma gondii identified membrane lipids with some structural features of plastid galactolipids, the major plastid lipid class. However, direct evidence for the plant-like enzymes responsible for galactolipid synthesis in Apicomplexan parasites has not been obtained. Chromera velia is an Apicomplexan relative recently discovered in Australian corals. C. velia retains a photosynthetic plastid, providing a unique model to study the evolution of the apicoplast. Here, we report the unambiguous presence of plant-like monogalactosyldiacylglycerol and digalactosyldiacylglycerol in C. velia and localize digalactosyldiacylglycerol to the plastid. We also provide evidence for a plant-like biosynthesis pathway and identify candidate galactosyltranferases responsible for galactolipid synthesis. Our study provides new insights in the evolution of these important enzymes in plastid-containing eukaryotes and will help reconstruct the evolution of glycerolipid metabolism in important parasites such as Plasmodium and Toxoplasma.     

Effect of Nutrient Concentration and Salinity on Immotile–Motile Transformation of Chromera velia

ABSTRACT. Chromera velia (Chromerida: Alveolata) is a photosynthetic, unicellular organism closely related to parasitic apicomplexa. Diurnal rhythmicity of an immotile–motile transformation has been observed but its role in the life cycle remains largely unknown. Using a multiwell system, we show that salinity and f‐medium concentration significantly affect the percentage of motile C. velia cells. An inverse relationship between salinity and motility in C. velia occurred, and flagellation was also suppressed at high nutrient levels. These results suggest a low salinity environment with relatively low nutrient levels enables flagellate transformation during the diurnal cycle of C. velia.     

Life at elevated CO2 modifies the cell composition of Chromera velia (Chromerida)

We investigated the response to high CO₂ of Chromera velia, a photosynthetic relative of apicomplexan parasites that is possibly involved in symbiotic associations with scleractinian corals. The inorganic C content in the proximity of the symbiotic algal cells within the tissues of scleractinians is disputed. According to some authors, it is very high. A higher C content in the endodermal tissues of scleractinians than in the external environment may have favoured the constitution of symbiosis with organisms such as Symbiodinium and Chromera that have a type II Rubisco, which is intrinsically ill suited to low CO₂ environments. We thus cultured C. velia at the very high inorganic C estimated by some authors and assessed its growth and photosynthetic performance. We also evaluated whether these conditions affected C allocation and elemental stoichiometry in C. velia cells by state-of-the-art Fourier transform infrared spectroscopy and total reflection X-ray fluorescence spectrometry in combination with more traditional biochemical and physiological techniques. Our results demonstrated that C. velia was capable of coping with very high CO₂, which even stimulated biomass production and increased N, P, Mn, Fe and Zn use efficiency. Growth at elevated CO₂ changed the stoichiometric relationships among elements in C. velia cells, but had no effect on the relative abundance of the main organic pools. The high CO₂ in the animal tissue surrounding the photosynthetic cells may therefore facilitate C. velia life in symbiosis.     

High photochemical trapping efficiency in Photosystem I from the red clade algae Chromera velia and Phaeodactylum tricornutum

In the present work, we report the first comparative spectroscopic investigation between Photosystem I (PSI) complexes isolated from two red clade algae. Excitation energy transfer was measured in PSI from Chromera velia, an alga possessing a split PsaA protein, and from the model diatom Phaeodactylum tricornutum. In both cases, the estimated effective photochemical trapping time was in the 15–25 ps range, i.e. twice as fast as higher plants. In contrast to green phototrophs, the trapping time was rather constant across the whole emission spectrum. The weak wavelength dependence was attributed to the limited presence of long-wavelength emitting chlorophylls, as verified by low temperature spectroscopy. As the trapping kinetics of C. velia PSI were barely distinguishable from those of P. tricornutum PSI, it was concluded that the scission of PsaA protein had no significant impact on the overall PSI functionality. In conclusion, the two red clade algae analysed here, carried amongst the most efficient charge separation so far reported for isolated Photosystems.     

Sterol composition and biosynthetic genes of the recently discovered photosynthetic alveolate, Chromera velia (chromerida), a close relative of apicomplexans

Chromera velia is a recently discovered, photosynthetic, marine alveolate closely related to apicomplexan parasites, and more distantly to perkinsids and dinoflagellates. To date, there are no published studies on the sterols of C. velia. Because apicomplexans and perkinsids are not known to synthesize sterols de novo, but rather obtain them from their host organisms, our objective was to examine the composition of the sterols of C. velia to assess whether or not there is any commonality with dinoflagellates as the closest taxonomic group capable of synthesizing sterols de novo. Furthermore, knowledge of the sterols of C. velia may provide insight into the sterol biosynthetic capabilities of apicomplexans prior to loss of sterol biosynthesis. We have found that C. velia possesses two primary sterols, 24-ethylcholesta-5,22E-dien-3β-ol, and 24-ethylcholest-5-en-3β-ol, not common to dinoflagellates, but rather commonly found in other classes of algae and plants. In addition, we have identified computationally three genes, SMT1 (sterol-24C-methyltransferase), FDFT1 (farnesyl diphosphate farnesyl transferase, squalene synthase), and IDI1 (isopentenyl diphosphate Δ-isomerase), predicted to be involved in sterol biosynthesis by their similarity to analogous genes in other sterol-producing eukaryotes, including a number of algae.     

玉米花粉单倍体植株染色体上异染色质的变异

我们用Giemsa BSG C-带技术检查了玉米花药培养获得的花粉单倍体植株根尖细胞染色体上异染色质的变异,观察结果表明,有的植株所显示的C-带数目是与供体植株的相一致,有的植株所显示的C-带数目则发生了显著变化,其中有的增加,有的减少。并讨论了异染色质发生变异的可能原因。还相应地观察到间期核中染色中心的变化是与中期染色体上C-带数目的变化相一致。     

继代培养的玉米花粉胚性细胞系的倍性及其染色体上组成异染色质变异

来源于八趟白的细胞系No.1,经过六年半继代培养,显示出随着继代培养时间的延长,染色体数目变异的频率减低,其单倍性愈趋稳定。尽管继代培养时间的延长并不影响这个细胞系倍性的稳定性,但却能引起单倍体细胞中第4和第9染色体上的组成异染色质发生显著变异。     

Dynamics of holm oak (Quercus ilex L.) coppices after clearcutting in southern France

Holm oak (Quercus ilex L.) forest is one of the most widespread biocoenoses in the southern part of France. Until recently, clearcutting of wood for domestic use was carried out every 20 years or so in these coppice stands. In order to study coppice stand dynamics after cutting this paper presents results of observations of some items describing the vegetation structure, the floral composition, and the life cycles in a holm oak coppice stand during a six year period after clearcutting. One of the questions addressed in this study was whether these coppice stands reach a steady state based on auto-succession, or if they continue to change. The results of the floristic changes after cutting suggest that the model of auto-succession best fits with the vegetation dynamics, even if changes in the density and life cycles of herbaceous species still exist six years after clearcutting.     

Biology of the threespine stickleback,Gasterosteus aculeatus,and the blackspotted stickleback,G. wheatlandi,during their marine pelagic phase in the Bay of Fundy,Canada

Synopsis Two species ofGasterosteus were collected from the surface waters of 48 stations in the Bay of Fundy from 1979–1982. Densities varied widely according to locality and time of year (range: 1 fish 0.2 m^–2 to 1 fish 26.3 m^–2 of sea surface) but peaked within 30 km of the New Brunswick/Maine coast in July and August. A few adults were caught as far as 100 km from the coast in winter. Guts ofG. wheatlandi were significantly fuller (63.3 ± 0.6%) than those ofG. aculeatus (44.6 ± 0.1%) but there were no differences between male and female guts within species. Quantitatively, gut contents were very similar between species, consisting primarily of (99.4%): calanoid copepods (Centropages typicus Kroyer andEurytemora cfamericana Williams), copepod nauplii, cladocerans (Podon leuckarti Sars andP. polyphemoides Leuckart) and planktonic eggs (in decreasing order of importance).Gasterosteus wheatlandi ate significantly more calanoids thanG. aculeatus but all other prey types were eaten in similar amounts. Within species, there were no differences in the numbers of each prey type eaten over time. The most abundant calanoid species caught alongside the fishes in the sampling nets did not occur in the sticklebacks' diets; probably they were too large to be ingested.Gasterosteus aculeatus bred in inshore, brackish water ponds in late April. By late July most juveniles had migrated out into the Bay but a few remained until early September.Gasterosteus wheatlandi bred later in the brackish water ponds (late May) and juveniles began migrating into the Bay in early August; a few remained in the ponds until early September. Adults were found in the Bay alongside juveniles suggesting that individuals of both species may survive for a second year. In both species, growth rates of juveniles were greater in brackish water than in the sea.     

Biology of slugs (Agriolimacidae and Arionidae: Mollusca) in New Zealand hill country pastures

Summary Life cycles of the slugs Deroceras reticulatum and Arion intermedius were studied over a 2.5-year period in a sheep-grazed pasture in the Kaimai Range, New Zealand. D. reticulatum approximated a bivoltine phenology, with intervals between consecutive generations ranging from 4 to 7 months and maximum life span from 8 to 12 months. Egg laying occurred in autumn and spring-early summer. Periods of egg hatching were followed by a phase of juvenile growth characterised by linear increase in the logarithm of population mean body weights. This was followed by a phase of reproductive activity. A. intermedius had an annual life cycle. Eggs laid during late summer and autumn hatched during autumn-winter. Immature slugs were characterised by a period of low growth rate during winter followed by a period of rapid growth culminating in reproductive maturity. In both species, the phase of rapid growth rate was associated with enlargement of the hermaphrodite gland. The size of the hermaphrodite gland relative to body weight reached a maximum during the spermatozoon stage, generally coincident with maximum body weight. During the reproductive period of D. reticulatum and A. intermedius the body weight remained relatively stable but the hermaphrodite gland became progressively smaller as the slugs approached the post-reproductive stage. The albumen gland attained maximum weight at the oocyte stage of the hermaphrodite gland, at the onset of reproductive activity. The pasture, strongly dominated by grasses, was heterogenous in slope, vegetation and treading/grazing influences from sheep due to stratification of the habitat into contour tracks, associated with sheep movement around the slope contours, and intertrack areas. High population densities of D. reticulatum were associated with the track edges (kerbs) and the upper intertrack slopes, while densities of A. intermedius were highest for the intertrack slopes. Low densities of both species occurred on the tracks and at sheep campsites. Analysis of cohort life tables indicated that these dispersion patterns were important in the dynamics of the populations. The stage mortality contributing most to the variance in reproductive populations of both species was that between hatching and onset of reproduction; most of the mortality occurred shortly after hatching on south-facing slopes, but tended to occur later in the life cycle on north slopes. For D. reticulatum this mortality was inversely related to initial density and associated, at least in part, with predation by carabids and birds and with treading by sheep. Density relationships, both at whole plot and plot stratum level, indicated that variations in natality had a stabilizing influence on D. reticulatum populations.     

Studies on Cell Number and Nuclei in Spores and on Ploidy Level in Ascochyta rabiei Isolates

Isolates of the phytopathogenic ascomycete Ascochyta rabiei (Pass.) Labr. were stained with the DNA-specific fluorochrome 4',6-diamidino-2-phenylindole (DAPI) and compared for differences in number of nuclei per pycnidiospore and the ploidy level. Microscopic analyses revealed that within the examined isolates five different combinations of cell number and number of nuclei in spores exist. A one-celled spore may contain one, two and four nuclei, respectively, and in the case of two-celled spores there exist types with one and two nuclei in one cell. Microfluorometric analyses of wild types and benomyl-treated isolates revealed differences in ploidy level among the wild types.     

Genetic Architecture of Natural Variation of Telomere Length in Arabidopsis thaliana

Telomeres represent the repetitive sequences that cap chromosome ends and are essential for their protection. Telomere length is known to be highly heritable and is derived from a homeostatic balance between telomeric lengthening and shortening activities. Specific loci that form the genetic framework underlying telomere length homeostasis, however, are not well understood. To investigate the extent of natural variation of telomere length in Arabidopsis thaliana, we examined 229 worldwide accessions by terminal restriction fragment analysis. The results showed a wide range of telomere lengths that are specific to individual accessions. To identify loci that are responsible for this variation, we adopted a quantitative trait loci (QTL) mapping approach with multiple recombinant inbred line (RIL) populations. A doubled haploid RIL population was first produced using centromere-mediated genome elimination between accessions with long (Pro-0) and intermediate (Col-0) telomere lengths. Composite interval mapping analysis of this population along with two established RIL populations (Ler-2/Cvi-0 and Est-1/Col-0) revealed a number of shared and unique QTL. QTL detected in the Ler-2/Cvi-0 population were examined using near isogenic lines that confirmed causative regions on chromosomes 1 and 2. In conclusion, this work describes the extent of natural variation of telomere length in A. thaliana, identifies a network of QTL that influence telomere length homeostasis, examines telomere length dynamics in plants with hybrid backgrounds, and shows the effects of two identified regions on telomere length regulation.     

Trichinella spiralis: Growth of the intracellular (muscle) larva

Newborn larvae of Trichinella spiralis were infective when injected directly into the thigh muscle of mice and rats. Infections initiated in this manner resulted in synchronously growing populations of muscle larvae, thereby permitting a detailed study of larval growth to be carried out.In mice, the mean larval growth, as measured by increase in larval volume, occurred in three phases; an initial growth phase (Day 0–1), a lag phase (Days 1–3), and an exponential growth phase (Days 3–19). Larvae grew an average of 39% per day during the exponential phase.No further increase in larval volume was noted after Day 19. There was no statistically significant difference found in the rate of larval growth among individual mice for any given day. The larval growth rate was the same in rats as in mice.     

Effects of ovarian maturation and insemination on the length of intermoult in Thermobia domestica

ABSTRACT. A study of the individual variability in the length of intermoult periods allows correlations to be established between reproductive and moulting cycles in adult females of Thermobia domestica (Packard) (Thysanura, Lepismatidae). By keeping the females without or with males and by changing the day of insemination, it is shown that the intermoult periods vary with the rate of ovarian maturation from the beginning of each stadium. Females with slow oocyte growth are never inseminated, even in the presence of males; they have short intermoults. Females with rapid oocyte growth can be inseminated and the timing of insemination regulates the length of the intermoults. It appears that the variability in duration of the intermoult only concerns the first part of the stadium (postecdysial period of the reproductive cycle=period of intense vitellogenesis), whereas the second part of the stadium (pre-ecdysial period=previtellogenesis) has an almost constant length. The endocrine mechanisms involved are considered, taking into account the cyclic changes in hormonal levels already described in other papers.