Asexual Phase Research Articles

EtCRK2, a cyclin-dependent kinase gene expressed during the sexual and asexual phases of the Eimeria tenella life cycle

EtCRK2, a cyclin-dependent kinase from the coccidian parasite, Eimeria tenella is closely related to eukaryotic cyclin-dependent kinases that regulate progression of the cell cycle and to several cyclin-dependent kinases identified in the Apicomplexa. Northern blot analyses revealed that EtCRK2 is transcribed during both asexual (first-generation schizogony) and sexual (oocyst sporulation) replicative phases of the parasite life cycle. In addition, it appears to be transcriptionally regulated during meiosis. Recombinant EtCRK2 produced in Escherichia coli has kinase activity which is significantly stimulated by the addition of vertebrate cyclin A. This cyclin-dependent kinase may play a significant role in regulating critical cell cycle events during both asexual proliferation and sexual development of the parasite.

Ultrastructural and Immunochemical Changes During the In Vitro Development of Plasmopara halstedii

ABSTRACT The asexual phase of the life cycle of Plasmopara halstedii, the causal agent of downy mildew of sunflower, plays a key role in the propagation of the disease. We investigated the morphological and ultrastructural changes that occur during the asexual development of the pathogen. Direct examination of infected cotyledons confirmed the presence of sporangiophores. In contact with water, important ultrastructural changes occurred, affecting the surface of zoosporangia, which became smoother, and their cytoplasm, which differentiated into flagellate zoospores. The subsequent encystment of zoospores was characterized by the synthesis of a cell wall and the loss of the flagella. In addition, two monoclonal antibodies (MAbs) specific for P. halstedii were used to analyze the immunochemical changes associated with these modifications. MAb 16A6, which bound to a 48-kDa glycoprotein, mainly labeled the surface of mobile or encysted zoospores and of mother cells of germ tubes. Conversely, MAb 2F9, which recognized highly glycosylated antigens, labeled the surface of zoosporangia and of flagellate zoospores, but not the encysted zoospores. These results provide new insights into the morphological and ultrastructural changes associated with the release and the encystment of zoospores which may be interesting targets for the development of new antimicrobial products.

Identification of a second member of the ponticulin gene family and its differential expression pattern

We have identified a homologue ( ponB) of the ponticulin gene ( ponA), an F-actin binding protein, in the expressed sequence tag library generated to mRNA isolated from fusion-competent cells of Dictyostelium discoideum. PonB is predicted to have many of the same characteristics as ponticulin. Both proteins are predicted to possess a cleaved signal peptide, a glycosyl anchor, an amphipathic β-strand structure and six conserved cysteines. Because of the sequence similarity and predicted conserved structures, this gene constitutes the second member of a ponticulin gene family. Unlike ponticulin, ponB is not expressed in axenically grown cells or during the asexual reproductive phase of D. discoideum. PonB is expressed by cells grown on bacterial lawns and by cells induced to be fusion-competent, i.e., gametes. The expression of ponB correlates with the appearance of a new F-actin binding activity in cell lysates of bacterially grown ponA − cells. By immunofluorescence microscopy, ponB appears to be localized to vesicles and to the plasma membrane of bacterially grown cells. Because ponticulin is the major high-affinity link between the plasma membrane and the cytoskeleton, the ponticulin gene family is likely to be part of the redundant system of proteins involved in connecting the cytoskeleton to the plasma membrane.

Effects of Temperature and Water Vapor Pressure on Conidial Germination and Lesion Expansion of Sphaerotheca macularis f. sp. fragariae.

Conidial germination in vitro and foliar lesion expansion were studied for Sphaerotheca macularis f. sp. fragariae. Detached strawberry (Fragaria × ananassa) leaves were inoculated, then held in controlled environments of constant temperatures (4 to 36°C) and relative humidity (RH, 32 to 100%) representing the range of these variables observed under California commercial production conditions. Percent germination and lesion expansion rate were determined by destructive subsampling over time. Conidia germinated at all temperatures by 6 h and reached a maximum by 48 h, with the optimum near 20°C. Lesions were marked with the aid of a microscope and measured by computer-assisted image-analysis to determine expansion rate. Maximal rates occurred at 25°C. Several growth models were fit to the expansion rate data with high significance. Predicted optima from these models ranged from 22 to 27°C and/or 17 to 27 mm Hg VPwater@100%RH. Neither RH, partial vapor pressure of water (VPwater), nor vapor pressure deficit (VPD) correlated with lesion expansion rate, adding to studies minimizing the importance of RH and VPD as determinants of asexual phase powdery mildew growth other than specifically at spore germination.

On the independence of barrage formation and heterokaryon incompatibility in Neurospora crassa

A barrage is a line or zone of demarcation that may develop at the interface where genetically different fungi meet. Barrage formation represents a type of nonself recognition that has often been attributed to the heterokaryon incompatibility system, which limits the co-occurrence of genetically different nuclei in the same cytoplasm during the asexual phase of the life cycle. While the genetic basis of the heterokaryon incompatibility system is well characterized in Neurospora crassa, barrage formation has not been thoroughly investigated. In addition to the previously described Standard Mating Reaction barrage, we identified at least three types of barrage in N. crassa; dark line, clear zone, and raised aggregate of hyphae. Barrage formation in N. crassa was evident only when paired mycelia were genetically different and only when confrontations were carried out on low nutrient growth media. Barrages were observed to occur in some cases between strains that were identical at all major heterokaryon incompatibility ( het) loci and the mating-type locus, mat, which acts as a heterokaryon incompatibility locus during the vegetative phase of N. crassa. We also found examples where barrages did not form between strains that had genetic differences at het-6, het-c, and/or mat. Taken together, these results suggest that the genetic control of barrage formation in N. crassa can operate independently from that of heterokaryon incompatibility and mating type. Surprisingly, barrages were not observed to form when wild-collected strains of N. crassa were paired. However, an increase in the frequency of pairings that produced barrages was observed among strains obtained by back-crossing wild strains to laboratory strains, or through successive rounds of inbreeding of wild-derived strains, suggesting the presence in wild strains of genes that suppress barrage.

The influence of UV-B radiation on the reproductive cells of the intertidal macroalga, Enteromorpha intestinalis

The relative sensitivity of the reproductive cells of the seaweed Enteromorpha intestinalis to UV-B was assessed by measuring in vivo chlorophyll fluorescence ( F v variable fluorescence), germination success and growth rates. Zoospores (the asexual reproductive cells) exhibit up to a 6 fold higher sensitivity to UV-B exposure than the mature thalli (measured as chlorophyll fluorescence, F v), and differences in growth rates were also found. Consistent patterns emerged throughout these experiments in variable fluorescence, germination success and growth rates, indicating a greater sensitivity in the sexual reproductive phase of the life cycle compared with the asexual phase. Inhibition of germination success (up to 50%) and growth rates (up to 16.4%) of settled gametes and zoospores after 1-h exposures to elevated levels of UV-B (equivalent to 27 and 31% ozone depletion) showed that damage to the reproductive cells was irreversible. In conclusion, the ecological significance of elevated UV-B exposure in the marine environment may be seriously under-estimated if effects on the early lifestages of algae are not considered.

The Neurospora crassa colonial temperature-sensitive 3 (cot-3) gene encodes protein elongation factor 2.

At elevated temperatures, the Neurospora crassa mutant colonial, temperature-sensitive 3 (cot-3) forms compact, highly branched colonies. Growth of the cot-3 strain under these conditions also results in the loss of the lower molecular weight (LMW) isoform of the Ser/Thr protein kinase encoded by the unlinked cot-1 gene, whose function is also involved in hyphal elongation. The unique cot-3 gene has been cloned by complementation and shown to encode translation elongation factor 2 (EF-2). As expected for a gene with a general role in protein synthesis, cot-3 mRNA is abundantly expressed throughout all asexual phases of the N. crassa life cycle. The molecular basis of the cot-3 mutation was determined to be an ATT to AAT transversion, which causes an Ile to Asn substitution at residue 278. Treatment with fusidic acid (a specific inhibitor of EF-2) inhibits hyphal elongation and induces hyperbranching in a manner which mimics the cot-3 phenotype, and also leads to a decrease in the abundance of the LMW isoform of COT1. This supports our conclusion that the mutation in cot-3 which results in abnormal hyphal elongation/branching impairs EF-2 function and confirms that the abundance of a LMW isoform of COT1 kinase is dependent on the function of this general translation factor.

Heat shock protein 80 ofNeurospora crassa: Sequence analysis of the gene and expression during the asexual phase

Heat shock protein 80 (Hsp80) of Neurospora crassa, a member of the stress-90 protein family, is a cytosolic molecular chaperone that interacts directly with Hsp70 to form a hetero-oligomeric complex. The complete nucleotide sequence of the gene encoding this protein, along with the 5'- and 3'-flanking DNA, is reported. The coding sequence is interrupted by two introns, 61 and 30 nucleotides, respectively, in length. The deduced amino acid sequence corresponds to a 695-residue polypeptide with a calculated molecular mass of 78,894 Da and an average pI of 4.94. Primer extension experiments demonstrated two transcription start sites, a major and a minor one. No sequence motifs resembling the standard eukaryotic heat shock elements were evident in the putative promoter region. Immunoblot analysis showed Hsp80 protein to be present in the mature, dormant conidia, while the hsp80 transcripts were not detected. Both the transcripts and the protein were present in the germinating conidia in the absence of externally applied stress.

Neutral genetic differentiation in an island model with cyclical parthenogenesis

Abstract Unusually high levels of genetic differentiation are often observed between populations of Daphnia (Crustacea: Cladocera) and other cladocerans. Selection and departure from migration–mutation–drift equilibrium have been invoked to explain this fact. However, the null model of neutral genes at equilibrium has not been explicitly stated. Using a simple island model that takes into account the main characteristics of the cyclical parthenogenetic life-cycle, we show that a high level of differentiation can be obtained for neutral genes at equilibrium if the migration rate during the asexual phase is low and the effective size over the season is reduced, or if mating within clones is common. Recurrent population bottlenecks have a large effect on differentiation and the time to reach equilibrium. Such fluctuating clonal dynamics could be brought about by limitations in the number of clones that hatch and establish themselves each season, or by random associations between neutral and selected genes, as previously suggested in the literature.

Heat shock protein 80 of <i>Neurospora crassa</i>: Sequence analysis of the gene and expression during the asexual phase

Heat shock protein 80 of <i>Neurospora crassa</i>: Sequence analysis of the gene and expression during the asexual phase

Migration of a marine oligochaete: induction of dispersal and microhabitat choice

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 207:89-96 (2000) - doi:10.3354/meps207089 Migration of a marine oligochaete: induction of dispersal and microhabitat choice Per G. Nilsson*, Jeffrey S. Levinton, Josepha P. Kurdziel Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook, New York 11794, USA *Present address: Tjärnö Marine Biological Laboratory, 45296 Strömstad, Sweden. E-mail: per.nilsson@tmbl.gu.se ABSTRACT: We present evidence for migration in the asexual phase of the life cycle of Paranais litoralis (Müller), a marine oligochaete that reproduces asexually by fission. Depletion of resources triggers a swimming response by some worms (Œmigrators¹). Migrating worms are longer, thinner and have more segments than non-migrating worms, indicating that migrating worms postpone reproduction. Experiments show that migrators swim more actively than non-migrators, and that this causes them to stay suspended in the water column longer than non-migrators. Worms avoid sediment where resources are exhausted, both when burrowing and when returning to the bottom after swimming. Based on laboratory cultures and field observations, we hypothesize that local depletion of resources is a common phenomenon for P. litoralis, and that migration is important for the persistence of P. litoralis populations. KEY WORDS: Migration · Swimming · Colonization · Asexual reproduction · Spatial population model Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 207. Online publication date: November 22, 2000 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2000 Inter-Research.

The teleomorph stage, Mycosphaerella graminicola, in epidemics of septoria tritici blotch on winter wheat in the UK

Earlier studies have shown that primary infections of winter wheat crops by septoria tritici blotch are initiated in autumn by air‐borne ascospores of the teleomorph stage, Mycosphaerella graminicola, which originate from sources outside the crop. Subsequent disease development within the crop, and damage to the upper leaves in summer, were commonly ascribed to the anamorph stage, Septoria tritici, which first arises from primary, ascosporic lesions and develops to give rise to splash‐borne pycnidiospores produced in pycnidia. Trapping studies, using a Burkard volumetric trap sited adjacent to field trials of winter wheat, showed that ascospores are released from pseudothecia throughout the year; peak release is not restricted to the autumn and early winter when the primary infections occur. As M. graminicola ascospores can be confused with those of other species, their authenticity was established by incubating ascospores on trap tapes for 24–48 h before counting. Examination of wheat plants cv. Riband, removed from unsprayed crops just prior to harvest and exposed to external weather conditions over the winter in 1995 and 1996, showed that pseudothecia mature on residues of the upper leaf layers between August and March. However, peak development occurred in December/January, when there was a corresponding decline in pycnidial formation. On regenerated wheat in set‐aside (left uncultivated) fields, sources of pseudothecia of M. graminicola were exhausted by February of the following year. Natural development of the teleomorph stage was monitored in an unsprayed crop of winter wheat cv. Riband during the 1996–97 season, and was present on leaf 8 at growth stage 32 (24 April) and on leaf 3 at growth stage 85 (8 July), then a week later, on both leaf 2 and the flag leaf. The temporal development of M. graminicola asexual and sexual phases was also studied in inoculation experiments under external weather conditions on pot‐grown seedlings of winter wheat cv. Longbow, using inoculum from two single‐ascospore cultures.

Reproductive strategies of Mycalecontarenii (Porifera: Demospongiae)

The sexual and asexual phases of the reproductive cycle of the sponge Mycalecontarenii (Martens, 1824) were studied from samples collected over a period of 2 years (June 1994 to May 1996) in a Mediterranean coastal basin (Porto Cesareo, southwestern Apulia). The species is viviparous; year round it shows a discontinuous oocyte production. No males were found, possibly owing to the very short period of spermatogenesis. M.␣contarenii produces asexual buds during autumn and winter, and this process seems to be related to changes in water temperature. Even though, at present, we cannot evaluate the incidence of larval recruitment, we postulate that asexual reproduction in M.contarenii plays a crucial role in maintaining the sponge population.

Sexual reproduction in the fungal parasite, Zygorhizidium planktonicum

The fungal parasite Zygorhizidium planktonicum (Chytridiales) exhibits a haplontic life cycle characterized by an asexual infection phase and the formation of sexual resting spores following gameta...

Sexual Reproduction in the Fungal Parasite, Zygorhizidium planktonicum

The fungal parasite Zygorhizidium planktonicum (Chytridiales) exhibits a haplontic life cycle characterized by an asexual infection phase and the formation of sexual resting spores following gametangial conjugation. Uninucleate gametangia fuse via an epibiotic conjugation tube formed by the smaller, donor thallus. Plasmogamy entails allogamous and autogamous fusions; however, mechanisms for sexual expression remain unresolved. Karyogamy occurs within the recipient gametangium as documented by a single nucleus following deposition of a thick, multi-layered wall. The zygotic resting spore remains undifferentiated throughout dormancy at 4 C. Upon activation, a single germination pore is formed, and a thin-walled, operculate meiosporangium emerges as an extension of the inner spore wall. The nucleus migrates into the meiosporangium during premeiotic interphase or prophase of meiosis I. Meiotic nuclei are evidenced by synaptonemal complex formation 24–48 h after germination. Nuclear division and meiosis II proceed in rapid succession followed by at least one mitotic division prior to zoosporogenesis. Confirmation of zygotic meiosis in Z. planktonicum further substantiates the prevalence of haplontic life cycles in the Chytridiomycetes. The taxonomic implications of sexual reproduction are reevaluated with particular attention to plasmogamous mechanisms and nuclear cycles.

Sexual and asexual reproduction in two species of Tethya (Porifera: Demospongiae) from a Mediterranean coastal lagoon

The sexual and asexual phases of reproductive cycles of two sponges, Tethya citrina and T. aurantium, living sympatrically in a Mediterranean coastal lagoon (Stagnone di Marsala, NW Sicily) were studied from samples collected over an 18-mo period. Both species are oviparous and gonochoric. They have a summer, partially overlapping, period of oocyte production, although T. citrina appear to mature earlier. No males were found, possibly due to the very short period of spermatogenesis. Both species produce asexual buds during the autumn/winter months. However, they seem to follow different reproductive strategies, with T. citrina showing a significantly lower production of buds than T. aurantium; by contrast, egg production is significantly lower in the latter species. The difference in the reproductive resource allocation is consistent with data reported in the literature on the anatomy features, genetic population structure and ecological distribution.

Eimeria coecicola Cheissin 1947: endogenous development in gut-associated lymphoid tissue.

Coccidia-free rabbits were inoculated with different doses of a pure strain of Eimeria coecicola and samples of gut were taken at 80, 96, 112, 128, 144, and 160 h postinoculation. The use of a very low infective dose (2-20 oocysts) was sufficient to study the last merogony. The number of merozoites in meronts increased when the infective dose decreased. Only the first merogony of this coccidium in lymphocytes or M-cells of gut-associated lymphoid tissue (GALT) has previously been described. Three other generations of meronts are described herein. All these endogenous stages were observed in the epithelium of the vermiform appendix, sacculus rotundus, and Peyer's patches, especially at the bases of the domes. However, in heavily infected tissues the gamonts were seen throughout the epithelium of the GALT. The third- and fourth-generation meronts were of two types. As in other eimerian species of the rabbit, type A meronts produced thick polynucleated merozoites, whereas type B meronts gave rise to large numbers of thin merozoites with one nucleus. Microgamonts were polynucleated and less numerous than macrogamonts. Type A meronts were also polynucleated and less numerous at the end of the merogony. Therefore, types A and B could correspond to a sexual phenotype differentiation occuring during the two asexual phases of multiplication.

Adaptive significance of changes in morph production during the transition from parthenogenetic to sexual reproduction in the aphid Rhopalosiphum padi (Homoptera: Aphididae)

AbstractMany aphid species have life cycles which alternate between asexual reproduction during spring and summer and sexual reproduction in autumn. This paper describes the sequences of morphs produced by clones of Rhopalosiphum padi (Linnaeus) in response to a range of photoperiods and temperatures in the laboratory, and under semi-natural conditions at the time of year when the transition from parthenogenetic to sexual reproduction occurs. By using similarities in the order in which the various morphs were born, clones of R. padi were found to produce eight progeny sequence types. A clone of R. padi will produce these sequence types sequentially as it changes from parthenogenetic to fully sexual reproduction. The progeny sequence produced by clones under conditions corresponding to those of spring and summer consist of parthenogenetic females only. During late summer, production of parthenogenetic females is followed by males. Later in the year, a clone produces progeny sequences consisting of parthenogenetic females, gynoparae and males. The asexual phase of reproduction is terminated in late autumn when only gynoparae and males are born. A minority of clones terminate parthenogenetic reproduction by producing only gynoparae. The function of these changes in progeny sequence in maximizing the reproductive success of a clone of R. padi is discussed.

Transmission and overseasoning of Diaporthe adunca on Plantago lanceolata

To obtain a better understanding of the onset and the progress of epidemics caused by Diaporthe adunca on Plantago lanceolata we investigated the role of the asexual phase of the fungus not only in the transmission of the disease from season to season, but also in the spread of the fungus within the same season. Splash dispersal of conidia during rain episodes proved to be an effective method for the spread of the fungus. Conidial production by pycnidia present on scapes left on the ground during winter was delayed in comparison to that of pycnidia produced on greenhouse infected scapes, indicating that the fungus overwinters with immature pycnidia. Latent infections occurred in 8.3% of the seedlings which emerged from seeds originating from infected spikes. A scenario for the transmission and spread of the fungus is proposed.

Fungi and the evolution of growth form

Fungi belong to the modular class of organisms characterized by an iterative, indeterminate mode of growth; internal age structure; localized rather than generalized senescence; cellular totipotency; and passive rather than active mobility. Growth form in the fungi therefore reflects their absorptive, sessile lifestyle. Aggregation/differentation of a few basic cell types gives rise to multicellular complexes associated with foraging, reproductive, survival, or dispersal activities. The morphological plasticity of fungi is analogous to but surpasses that of other modular creatures such as the benthic invertebrates and plants. For instance, in response to environmental signals, fungi can vary the timing, extent, and mode of differentiation; interconvert among different growth forms; and decouple the sexual and asexual phases of the life cycle. Evolutionary determinants of their form are phylogenetic constraint, adaptation, developmental constraint, ecophenotypic factors, and chance. Physiological determinants of their form relate primarily to morphogenesis of the cell wall and include the cytoskeleton and cytosis. Key words: morphology, morphogenesis, shape, allometry, modular, evolutionary.