Asexual Cells Research Articles

Schizosaccharomyces lindneri sp. nov., a fission yeast occurring in honey.

Two strains of fission yeast were isolated from honey. They differ from the type strain of Schizosaccharomyces octosporus by three substitutions in the D1/D2 domain of the nuclear 26S large subunit ribosomal RNA(rRNA) gene sequence, resulting in a 99.5% identity. In the internal transcribed spacer (ITS) region (consisting of ITS1, 5.8S rDNA, and ITS2), the strains differ from S. octosporus by 16 gaps and 91 substitutions, which is equivalent to an identity of 88.1%. Genome sequencing on one of the new strains revealed that the average nucleotide identity (ANI) between its genome and the reference genome of S. octosporus is 90.43% and there exist major genome rearrangements between the two genomes. Mating analysis revealed that S. octosporus and one of the new strains are completely reproductively separated. A strong prezygotic barrier exists and the few mating products consist of diploid hybrids that do not form recombinant ascospores. In the new strains, asci are either zygotic, arising from conjugation, or they develop without conjugation from asexual cells (azygotic). Compared to the currently recognized Schizosaccharomyces species, the spectrum of nutrients that are assimilated by the new strains is restricted. Of the 43 carbohydrates that were included in the physiological standard tests, only 7 were assimilated. According to the results of the genome sequence analysis, the mating trials, and the phenotypic characterization, the new species Schizosaccharomyces lindneri is described to accommodate the two strains(holotype: CBS 18203T and ex-type: MUCL 58363; MycoBank no.: MB 847838).

Intermediate Thermoresistance in Black Yeast Asexual Cells Variably Increases with Culture Age, Promoting Survival and Spoilage in Thermally Processed Shelf-Stable Foods

Black yeasts are a functional group that has caused spoilage in cold-filled and hot-filled beverages, as well as other water activity-controlled food products. We established quantitative thermoresistance parameters for the inactivation of 12 Aureobasidium and Exophiala isolates through isothermal experiments and a challenge study. Culture age (2 versus 28 days) variably affected the thermoresisitance among the black yeast strains. Variation in thermoresistance exists within each genus, but the two most resistant strains were the Exophiala isolates. The two most heat-resistant isolates were Exophiala phaeomuriformis FSL-E2-0572, with a D60-value of 7.69 ± 0.63 min in 28-day culture and Exophiala dermatitidis YB-734, with a D60-value of 16.32 ± 2.13 min in 28-day culture. Although these thermoresistance levels were, in some cases, greater than those for conidia and vegetative cells from other common food spoilage fungi, they were much more sensitive than the ascospores of heat-resistant molds most associated with spoilage of hot-filled products. However, given that black yeasts have caused spoilage in hot-filled products, we hypothesized that this intermediate degree of thermoresistance may support survival following introduction during active cooling before package seals have formed. A challenge study was performed in an acidic (apple cider) and water activity-controlled (maple syrup) product to evaluate survival. When apple cider was hot filled at 82°C, black yeast counts were reduced by 4.1 log CFU/mL 24 h after the heat treatment, but the survivors increased up to 6.7 log CFU/mL after 2 weeks. In comparison, the counts were below the detection limit after both 24 h and 14 days of shelf life in both products when filled at the boiling points. This suggests that ensuring water microbial quality in cooling tunnels and nozzle sanitation may be essential in mitigating the introduction of these fungi.

Mitochondrial complementation: a possible neglected factor behind early eukaryotic sex.

Sex is ancestral in eukaryotes. Meiotic sex differs from bacterial ways of exchanging genetic material by involving the fusion of two cells. We examine the hypothesis that fusion evolved in early eukaryotes because it was directly beneficial, rather than a passive side effect of meiotic sex. We assume that the uptake of (proto)mitochondria into eukaryotes preceded the evolution of cell fusion and that Muller's ratchet operating within symbiont lineages led to the accumulation of lineage-specific sets of mutations in asexual host cells. We examine whether cell fusion, and the consequent biparental inheritance of symbionts, helps to mitigate the effects of this mutational meltdown of mitochondria. In our model, host cell fitness improves when two independently evolved mitochondrial strains co-inhabit a single cytoplasm, mirroring mitochondrial complementation found in modern eukaryotes. If fusion incurs no cost, we find that an allele coding for fusion can invade a population of nonfusers. If fusion is costly, there are two thresholds. The first describes a maximal fusing rate (probability of fusion per round of cell division) that is able to fix. An allele that codes for a rate above this threshold can reach a polymorphic equilibrium with nonfusers, as long as the rate is below the second threshold, above which the fusion allele is counter-selected. Whenever it evolves, fusion increases the population-wide level of heteroplasmy, which allows mitochondrial complementation and increases population fitness. We conclude that beneficial interactions between mitochondria are a potential factor that selected for cell fusion in early eukaryotes.

Antigenic Variation in Plasmodium falciparum.

Plasmodium falciparum is the protozoan parasite that causes most malaria-associated morbidity and mortality in humans with over 500,000 deaths annually. The disease symptoms are associated with repeated cycles of invasion and asexual multiplication inside red blood cells of the parasite. Partial, non-sterile immunity to P. falciparum malaria develops only after repeated infections and continuous exposure. The successful evasion of the human immune system relies on the large repertoire of antigenically diverse parasite proteins displayed on the red blood cell surface and on the merozoite membrane where they are exposed to the human immune system. Expression switching of these polymorphic proteins between asexual parasite generations provides an efficient mechanism to adapt to the changing environment in the host and to maintain chronic infection. This chapter discusses antigenic diversity and variation in the malaria parasite and our current understanding of the molecular mechanisms that direct the expression of these proteins.

Gene expression analysis of parthenogenetic embryonic development of the pea aphid, Acyrthosiphon pisum, suggests that aphid parthenogenesis evolved from meiotic oogenesis.

Aphids exhibit a form of phenotypic plasticity, called polyphenism, in which genetically identical females reproduce sexually during one part of the life cycle and asexually (via parthenogenesis) during the remainder of the life cycle. The molecular basis for aphid parthenogenesis is unknown. Cytological observations of aphid parthenogenesis suggest that asexual oogenesis evolved either through a modification of meiosis or from a mitotic process. As a test of these alternatives, we assessed the expression levels and expression patterns of canonical meiotic recombination and germline genes in the sexual and asexual ovaries of the pea aphid, Acyrthosiphon pisum. We observed expression of all meiosis genes in similar patterns in asexual and sexual ovaries, with the exception that some genes encoding Argonaute-family members were not expressed in sexual ovaries. In addition, we observed that asexual aphid tissues accumulated unspliced transcripts of Spo11, whereas sexual aphid tissues accumulated primarily spliced transcripts. In situ hybridization revealed Spo11 transcript in sexual germ cells and undetectable levels of Spo11 transcript in asexual germ cells. We also found that an obligately asexual strain of pea aphid produced little spliced Spo11 transcript. Together, these results suggest that parthenogenetic oogenesis evolved from a meiosis-like, and not a mitosis-like, process and that the aphid reproductive polyphenism may involve a modification of Spo11 gene activity.

Egress ofPlasmodium bergheigametes from their host erythrocyte is mediated by the MDV-1/PEG3 protein

Malaria parasites invade erythrocytes of their host both for asexual multiplication and for differentiation to male and female gametocytes - the precursor cells of Plasmodium gametes. For further development the parasite is dependent on efficient release of the asexual daughter cells and of the gametes from the host erythrocyte. How malarial parasites exit their host cells remains largely unknown. We here report the characterization of a Plasmodium berghei protein that is involved in egress of both male and female gametes from the host erythrocyte. Protein MDV-1/PEG3, like its Plasmodium falciparum orthologue, is present in gametocytes of both sexes, but more abundant in the female, where it is associated with dense granular organelles, the osmiophilic bodies. Deltamdv-1/peg3 parasites in which MDV-1/PEG3 production was abolished by gene disruption had a strongly reduced capacity to form zygotes resulting from a reduced capability of both the male and female gametes to disrupt the surrounding parasitophorous vacuole and to egress from the host erythrocyte. These data demonstrate that emergence from the host cell of male and female gametes relies on a common, MDV-1/PEG3-dependent mechanism that is distinct from mechanisms used by asexual parasites.

Colonial Rhizocephalans (Crustacea, Rhizocephala): Asexual Reproduction, Stem Cells, and Reproductive Strategy by V. V. Isaeva and A. I. Sukalyuk, Moscow: Nauka, 2007

Colonial Rhizocephalans (Crustacea, Rhizocephala): Asexual Reproduction, Stem Cells, and Reproductive Strategy by V. V. Isaeva and A. I. Sukalyuk, Moscow: Nauka, 2007

Oxylipin studies expose aspirin as antifungal

The presence of aspirin-sensitive 3-hydroxy fatty acids (i.e. 3-OH oxylipins) in yeasts was first reported in the early 1990s. Since then, these oxidized fatty acids have been found to be widely distributed in yeasts. 3-OH oxylipins may: (1) have potent biological activity in mammalian cells; (2) act as antifungals; and (3) assist during forced spore release from enclosed sexual cells (asci). A link between 3-OH oxylipin production, mitochondria and aspirin sensitivity exists. Research suggests that: (1) 3-OH oxylipins in some yeasts are probably also produced by mitochondria through incomplete beta-oxidation; (2) aspirin inhibits mitochondrial beta-oxidation and 3-OH oxylipin production; (3) yeast sexual stages, which are probably more dependent on mitochondrial activity, are also characterized by higher 3-OH oxylipin levels as compared to asexual stages; (4) yeast sexual developmental stages as well as cell adherence/flocculation are more sensitive to aspirin than corresponding asexual growth stages; and (5) mitochondrion-dependent asexual yeast cells with a strict aerobic metabolism are more sensitive to aspirin than those that can also produce energy through an alternative anaerobic glycolytic fermentative pathway in which mitochondria are not involved. This review interprets a wide network of studies that reveal aspirin to be a novel antifungal.

Re-instatement of Leptophytum and its type Leptophytum laeve: taxonomy and biogeography of the genera Leptophytum and Phymatolithon (Corallinales, Rhodophyta)

In a recently published examination of the newly re-discovered holotype of Leptophytum (Lithophyllum) laeve, the characters of the holotype specimen were judged to be taxonomically inadequate. Consequently, an epitype was selected which was determined to belong to Phymatolithon lenormandii, rendering Leptophytum and its type L. laeve junior synonyms of Phymatolithon and P. lenormandii, respectively. We have re-studied the holotype specimen in Stockholm, and in this paper we demonstrate that it is consistent with the protologue and exhibits features that are fully consistent with the modern circumscription of Leptophytum laeve, based on population studies. Our results demonstrate that the holotype specimen is taxonomically adequate and that the selection of an epitype was not necessary. Accordingly, we propose the reinstatement of Leptophytum Adey 1966, based on its type L. laeve (Foslie) Adey (basionym: Lithophyllum lenormandii f. laeve Foslie 1891). The genera Leptophytum and Phymatolithon Foslie are similar in some anatomical respects (such as in having diminutive sub-epithallial initials, i.e. meristem cells, a polystromatic non-coaxial hypothallium, and roundish to flattened epithallial cells in transverse section). The genera differ somewhat in morphology and markedly in most reproductive features (such as spermatangial systems, origin of gonimoblasts, position of conceptacle primordia, and the presence or absence of distinct asexual conceptacle pore cells). The taxonomic, ecological and biogeographic characteristics of the two genera lead us to conclude that the similarities between Phymatolithon and Leptophytum have very likely resulted from parallel evolution.

Asexual reproduction: 'midwives' assist dividing amoebae.

Asexual cells are normally able to reproduce entirely by themselves. But we have discovered that in about one-third of the dividing cells of Entamoeba invadens contraction of the cleavage furrow1 may stop before separation is complete. We show here that the connected daughter cells overcome this problem by calling upon a neighbouring amoeba to help them achieve the final stage of division. The 'midwife' cell is chemotactically recruited for this mechanical intervention in what is a surprising example of primitive cooperation.

TESTING A HYPOTHESIS FOR THE EVOLUTION OF SEX

An asexual set of primitive bacteria is simulated with a bit-string Penna model with a Fermi function for survival. A recent hypothesis by Jan, Stauffer, and Moseley on the evolution of sex from asexual cells as a strategy for trying to escape the effects of deleterious mutations is checked. This strategy is found to provide a successful scenario for the evolution of a stable macroscopic sexual population.

Modulation of Protein Kinase C Activity in Plasmodium falciparum – Infected Erythrocytes

Infection of human erythrocytes with the malaria parasite Plasmodium falciparum induces many morphological and biochemical changes in the host cell. Host serine/threonine protein kinases could be involved in some of these processes. The aim of this study was to determine the effect of infection on red blood cell protein kinase C (PKC) and establish the importance of this enzyme in parasite growth and sexual stage differentiation. Phorbol myristate acetate (PMA)-induced translocation of erythrocyte PKC activity is impaired in erythrocytes enriched for mature asexual stage infected cells. Western blotting shows that this is due to a relative reduction in membrane PKC protein levels rather than inhibition of enzyme activity and analysis of PKC activity isolated from whole cell lysates by DE52 chromatography suggests that total activatable PKC levels are lower in infected erythrocytes. A reduction in PMA-induced activation is also observed in PKC assays performed in situ. Downregulation of erythrocyte PKC by overnight incubation with PMA before infection causes a significant decrease in the rate of the asexual growth, suggesting that the enzyme, although lost later in infection, may be important in the earlier development of the parasite. By contrast, the lack of PKC had no effect on the production of sexual stage parasites.

Modulation of Protein Kinase C Activity in Plasmodium falciparum – Infected Erythrocytes

AbstractInfection of human erythrocytes with the malaria parasite Plasmodium falciparum induces many morphological and biochemical changes in the host cell. Host serine/threonine protein kinases could be involved in some of these processes. The aim of this study was to determine the effect of infection on red blood cell protein kinase C (PKC) and establish the importance of this enzyme in parasite growth and sexual stage differentiation. Phorbol myristate acetate (PMA)-induced translocation of erythrocyte PKC activity is impaired in erythrocytes enriched for mature asexual stage infected cells. Western blotting shows that this is due to a relative reduction in membrane PKC protein levels rather than inhibition of enzyme activity and analysis of PKC activity isolated from whole cell lysates by DE52 chromatography suggests that total activatable PKC levels are lower in infected erythrocytes. A reduction in PMA-induced activation is also observed in PKC assays performed in situ. Downregulation of erythrocyte PKC by overnight incubation with PMA before infection causes a significant decrease in the rate of the asexual growth, suggesting that the enzyme, although lost later in infection, may be important in the earlier development of the parasite. By contrast, the lack of PKC had no effect on the production of sexual stage parasites.

Origin of Sex for Error Repair II. Rarity and Extreme Environments

In a previous paper we studied the simultaneous, and at times conflicting, needs of coping with DNA damage, efficient cell replication, and the avoidance of cell mortality. These selective factors operated on sexual and asexual haploid and diploid populations that were reproductively isolated from one another. We concluded, in part, that a sexual type of cell could not expand from extreme rarity in populations dominated by asexual haploid and diploid cells. In the present paper we show that it is relatively easy for a rare sexual mutant to expand in a population dominated by asexual haploid cells if some matings occur between sexual and asexual cell types. We also study the persistence of sex in high mortality, high damage environments, in which neither the asexual diploid nor haploid can survive. The diploid cannot survive because its lower birth rate cannot overcome mortality and the haploid cannot survive because its birth rate cannot overcome gene damage. Sex can persist in these punishing environments by tuning the parameters of the sexual cycle, and the fusion and splitting rates, into a specified region, thereby reaping both benefits of damage repair and efficient replication.

Genetic error, sex, and diploidy.

Mathematical models and experiments on transformation are reported testing the hypothesis that sex and diploidy evolved as a DNA repair system. The models focus on the origin of diploidy and sex by studying selection between asexual haploids, sexual haploids, and diploids. Haploid cells are efficient replicators, while diploid cells are resistance to damage. A sexual haploid may combine the advantages of both: spending much of its life cycle in the haploid state, then temporarily fusing to become diploid, followed by splitting to the haploid state. During the diploid state DNA damage can be repaired, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. Five basic rate parameters are employed: birth and death; genomic damage (for the haploids alone); and, for the sexual cell, fusion and splitting. Parameter space bifurcation diagrams for the equilibria are drawn, and solutions of the equations are described in terms of these diagrams. Each type of cell has a region of the parameter space that it occupies exclusively (given its initial presence in the competition). The haploid wins in environments characterized by low damage. The diploid wins in environments characterized by high damage, low mortality, and abundant resources. In general, only a single type of cell occupies a given portion of the space. We find, however, that competitive coexistence of an asexual diploid and sexual haploid is possible in spite of the fact that they are competing for a single resource (nucleotide building blocks). Sex can increase from rarity if matings occur with asexual cells. Only sex can cope with both high mortality and high damage. We then turn to natural bacterial transformation as a model system for the experimental study of sex. Natural transformation in distributed widely, but apparently sparsely, in all bacterial groups. A very preliminary phylogenetic analysis of the bacilli and related species indicates that transformation is probably not a diversifying force in bacterial evolution. However, it is difficult to be sure because of the ambiguity surrounding negative data. Experiments with the bacterium Bacillus subtilis indicate that transformation frequencies respond adaptively to DNA damage if homologous donor DNA is used. Several specific hypotheses for this response are considered. Recent work in other labs on the evolution of transformation is discussed from the point of view of the hypothesis that transformation functions in DNA repair.

Nuclear functions in postconjugational development of Paramecium caudatum: Ability to form food vacuoles analyzed by nuclear elimination and implantation

AbstractParamecium caudatum loses the ability to form food vacuoles at the crescent stage of the micronucleus from 5 to 6 hr after the initiation of conjugation and regains it immediately after the third division of the zygotic nucleus. To assess the micronuclear function in the development of the oral apparatus after coniugation, prezygotic micronuclei was removed from cells at various stages of conjugation, and their ability to form food vacuoles were examined. (1) When all of the prezygotic micronuclear derivatives were eliminated before the stage of formation of the zygotic nucleus, the exconjugant did not regain its ability. (2) When a zygotic nucleus or postzygotic nuclei were removed, in some cases the cell formed as many food vacuoles as did nonoperated cells after conjugation, while in other operated cells the number of food vacuoles was subnormal. (3) When a micronucleus from a cell at vegetative phase (G1) was transplanted into a cell of an amicronucleate mating pair at the stage between 8 and 9 hr after the initiation of conjugation, the implanted cell regained the ability to form food vacuoles. However, no cell regained the ability when the implantation was carried out within 1 hr after the separation of the mates. The results show that the micronucleus plays an indispensable role in the development of the oral apparatus at the stages of exchange of gametic nuclei and fertilization and that the micronucleus transplanted from asexual cells can fulfill this function. On the other hand, removal of the macronucleus from exconjugants showed that the maternal macronucleus also has an indispensable function in regaining the ability to form food Vacuoles. © 1992 Wiley‐Liss, Inc.

DNA repair and the evolution of transformation in Bacillus subtilis. III. Sex with damaged DNA.

Natural genetic transformation in the bacterium Bacillus subtilis provides an experimental system for studying the evolutionary function of sexual recombination. The repair hypothesis proposes that during transformation the exogenous DNA taken up by cells is used as template for recombinational repair of damages in the recipient cell's genome. Earlier results demonstrated that the population density of transformed cells (i.e., sexual cells) increases, relative to nontransformed cells (primarily asexual cells), with increasing dosage of ultraviolet irradiation, provided that the cells are transformed with undamaged homologous DNA after they have become damaged. In nature, however, donor DNA for transformation is likely to come from cells that are as damaged as the recipient cells. In order to better simulate the effects of transformation in natural populations we conducted similar experiments as those just described using damaged donor DNA. We document in this report that transformants continue to increase in relative density even if they are transformed with damaged donor DNA. These results suggest that sites of transformation are often damaged sites in the recipient cell's genome.

Physiological Reorganization and Post-Traumatic Regeneration inStylonychia mytilus:Reflections on Developmental Constraints and the Evolutionary Origin of Alternative Modes of Asexual Morphogenesis

We investigated development of cortical ciliature in Stylonychia mytilus during starvation-induced physiological reorganization, and during regeneration following amputation of the anterior part of the cell. Cortical reorganization in the two processes is generally similar. The posterior part of the adoral zone of membranelles is resorbed and replaced with newly assembled membranelles. The pre-existing set of ventral cirri and dorsal bristles is entirely resorbed and replaced with new ones. Regenerants exhibit posterior displacement of the frontal-ventral-transverse cirri primordium and the undulating membrane primordium, and recruit basal bodies from ectopic locations for the development of these ciliature. This illustrates flexibility in the initiation site of ciliary primordia, and opportunism in utilizing building blocks. Such morphogenetic versatility of hypotrichs provides the basis for the operation of a global control of pattern formation, which governs cortical reorganization in dividers, and additionally, in the absence of the prerequisites for binary fission, alternative modes of cortical development such as physiological reorganization or regeneration. These considerations suggest that the three processes are homologous and that physiological reorganization and regeneration have evolved from binary fission. In physiological reorganization and regeneration, the micro- and macronuclei reorganize to resemble that in binary fission; these nuclear events are considered evolutionary relics of the nuclear development of binary fission. Tetrahymena also exhibits such morphogenetic flexibility; stomatogenesis is under global control, so that asexual cells can replace its oral apparatus without undergoing binary fission. Paramecium, on the other hand, adopts a more rigid strategy in relying heavily on pre-existing structures for morphogenetic cues; this could have imposed constraints in the exploration of alternative modes of asexual development.

Patterns of organellar and nuclear inheritance among progeny of two geographically isolated strains of Volvox carteri

Strains of Volvox carteri forma nagariensis derived from Japanese and Indian isolates ("J" and "I" strains, respectively) exhibited length differences (RFLPs) for approximately 90% of the restriction fragments detected by hybridization with a variety of unique-sequence, small-gene-family and repetitive-element probes, including heterologous probes of chloroplast and mitochondrial origin. Extensive post-zygotic mortality was observed among the zygotes produced by crossing J and I strains, suggesting some form of genetic incompatability between them. Most of the viable progeny exhibited recombinant patterns of nuclear inheritance and maternal inheritance of mitochondrial and chloroplast markers. However, many progeny exhibited exclusively uniparental (usually maternal, but in one case paternal) inheritance of both nuclear and organellar markers. Some of these non-recombinant individuals may be derived from "parthenospores" (dormant asexual cells resembling zygospores). Others may be a result of "pseudogamy," in which one of the parental pronuclei is excluded from the zygote, followed by selective exclusion of both the mitochondrial and the chloroplast genomes derived from that same parent. When segregation patterns for 44 nuclear markers were analyzed in 90 recombinant progeny, statistically significant, locus-specific deviations from expected Mendelian transmission ratios were observed for a sizeable fraction of all markers in both reciprocal crosses: some markers were preferentially transmitted by the J strain, while others were preferentially transmitted by the I strain. It is speculated that these transmission distortions may be related to the regions of inter-isolate genetic incompatibility, and may complicate the use of J x I crosses to establish a RFLP-based linkage map for the species.

DNA repair and the evolution of transformation in the bacterium Bacillus subtilis.

The purpose of the work reported here is to test the hypothesis that natural genetic transformation in the bacterium Bacillus subtilis has evolved as a DNA repair system. Specifically, tests were made to determine whether transformation functions to provide DNA template for the bacteria] cell to use in recombinational repair. The survivorship and the homologous transformation rate as a function of dose of ultraviolet irradiation (UV) was studied in two experimental treatments, in which cells were either transformed before (DNA-UV), or after (UV-DNA), treatment with UV. The results show that there is a qualitative difference in the relationship between the survival of transformed cells (sexual cells) and total cells (primarily asexual cells) in the two treatments. As predicted by the repair hypothesis, in the UV-DNA treatment, transformed cells had greater average survivorship than total cells, while in the DNA-UV treatment this relationship was reversed. There was also a consistent and qualitative difference between the UV-DNA and DNA-UV treatments in the relationship between the homologous transformation rate (transformed cells/total cells) and UV dosage. As predicted by the repair hypothesis, the homologous transformation rate increases with UV dose in the UV-DNA experiments but decreases with UV dose in the DNA-UV treatments. However, the transformation rate for plasmid DNA does not increase in a UV-DNA treatment. These results support the DNA repair hypothesis for the evolution of transformation in particular, and sex generally.