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PubMed1999

Molecular characterization of mitochondria in asexual and sexual blood stages of Plasmodium falciparum.

Molecular mechanisms that regulate gene expression during development of asexual stage to sexual stage of Plasmodium falciparum in the human erythrocyte are largely unknown. There were apparent variations in ultrastructural characteristics of the mitochondrion between the two developing stages. The asexual stage's mitochondrion had developed less than that of the sexual stage. The respiratory complexes of the mitochondrial electron transport system in the asexual stage were approximately 8-10 times less active than those in the sexual stage. Using quantitative polymerase chain reaction to amplify the cytochrome b gene encoding a subunit of mitochondrial cytochrome c reductase, the amount of the cytochrome b gene of the sexual stage was calculated to be approximately 3 times higher than that obtained from the asexual stage. Moreover, using quantitative reverse-transcription polymerase chain reaction, a relatively high level of approximately 1.3-kb transcript mRNA of the cytochrome b gene was observed in the sexual stage compared to the asexual stage. A known single-copy chromosomal dihydrofolate reductase gene was found to have a similar amount in the two stages. These results suggest that the copy number of the mitochondrial gene, including transcriptional and translational mechanisms, plays a major regulatory role in differential expression during the development of the asexual to sexual stage of P. falciparum in the human cell.

Animals↗
PubMed2005

Admixed sexual and facultatively asexual aphid lineages at mating sites.

Cyclically parthenogenetic organisms may have facultative asexual counterparts. Such organisms, including aphids, are therefore interesting models for the study of ecological and genetic interactions between lineages differing in reproductive mode. Earlier studies on aphids have revealed major differences in the genetic outcomes of populations that are possibly resulting mostly either from sexual or from asexual reproduction. Besides, notable gene flow between sexual and asexual derivatives has been suspected, which could lead to the emergence of new asexual lineages. The present study examines the interplay between these lineages and is based on analyses of population structure of individuals that may contribute to the pool of sexual reproductive forms in the host alternating aphid Rhopalosiphum padi. Using a Bayesian assignment method, we first show that the sexual forms of R. padi on mating sites encompass two genetically distinct clusters of individuals in the western part of France. The first cluster included unique genotypes of sexual lineages, while the second cluster included facultatively asexual lineages in numerous copies, the reproductive mode of the two clusters being confirmed by reference clones. Sexual reproductive forms produced by sexual and facultatively asexual lineages are thus admixed at mating sites which gives a large opportunity for the two clusters to mate with each other. Nevertheless, this study also highlights, as previously demonstrated, that the two clusters retained high genetic differentiation. Possible explanations for the inferred limited genetic exchanges are advanced in the discussion, but further dedicated investigations are required to solve this paradox.

Animals↗
PubMed2006

Light regulation of asexual development in the rice blast fungus, Magnaporthe oryzae.

Light is a major environmental factor that influences many biological processes. We characterized the roles of light in asexual development (including the formation of aerial hyphae and conidiophore) in Magnaporthe oryzae, which is the causal agent of rice blast disease. Our data revealed a complex nature of light regulation in the asexual developments of M. oryzae. Asexual development of M. oryzae is suppressed by blue light in a light/dark cycling environment and asexual spore release is controlled by both blue and red light. We demonstrated that even very dim light, about 10 micromol m(-2), is sufficient to suppress spore-release behavior in M. oryzae. We also generated knockout strains of a blue light receptor, mgwc-1, the M. oryzae homolog of white collar-1 in Neurospora crassa, and demonstrated blue-light-specific regulation in the asexual development and spore release in M. oryzae. Our findings in this agriculturally important pathogen, M. oryzae, broaden our understanding of the roles of light in fungal development.

Blotting, Southern↗
PubMed2003

Coalescence times and the Meselson effect in asexual eukaryotes.

In asexual eukaryotes, the two allelic gene copies at a locus are expected to become highly divergent as a result of the independent accumulation of mutations in the absence of segregation. If sexual reproduction was abandoned millions of generations ago, intra-individual allelic divergences can be significantly larger than in species that reproduce sexually. Owing to the disputed existence of truly ancient asexual species, this so-called 'Meselson effect' has been put forward as a means of confirming the complete loss of sexual reproduction. Very few attempts have, however, been made at quantifying the effect of sexual reproduction on the degree of divergence between gene copies in an asexual population. Here, I describe how asexual reproduction can be regarded as a special case of population subdivision. Using a slightly modified version of the standard two-deme structured coalescent, I derive the expected coalescence time for a pair of gene copies in an asexual population and show that the Meselson effect is compatible with low rates of sexual reproduction.

Data Interpretation, Statistical↗
PubMed1990

Commitment of the malaria parasite Plasmodium falciparum to sexual and asexual development.

Blood-stage malaria parasites in the vertebrate host can develop either into the asexual, multiplying forms, called schizonts, or into gametocytes, the sexual stages of the parasite. In the present work we studied the differentiation into asexual parasites or gametocytes of the progeny of single, isolated schizonts of the clone 3D7A of Plasmodium falciparum, using monoclonal antibodies specific for the sexual or asexual stages of the parasite. We observed that schizonts obtained from a continuous culture undergoing serial cycles of growth and dilution with fresh red blood cells produced either only gametocytes or only asexual parasites, showing a high degree of commitment to one or the other developmental pathway. The relative proportion of schizonts which produced gametocytes was very low at low parasite densities in culture, while at high parasite densities a much greater proportion of schizonts produced gametocytes. Nevertheless, at both low and high parasite densities individual schizonts were almost always fully committed to producing only gametocytes or only asexual parasites.

Animals↗
PubMed2003

Shared ITS DNA substitutions in isolates of opposite mating type reveal a recombining history for three presumed asexual species in the filamentous ascomycete genus Alternaria.

About 15,000 species of ascomycete fungi lack a known sexual state. For fungi with asexual states in the anamorph genera Embellisia, Ulocladium, and Alternaria, six species have known sexual states but more than 50 species do not. In sexual filamentous ascomycetes, opposite mating type information at the MAT1 locus regulates mating and the opposite mating type genes each have a clonal, non-recombining phylogenetic history. We used PCR to amplify and sequence fragments of the opposite mating type genes from three supposedly asexual species, A. brassicae, A. brassicicola and A. tenuissima. Each haploid fungal isolate had just one mating type, but both mating types were present in all the three species. We sequenced the ribosomal ITS regions for isolates of opposite mating type, for the three asexual species and four known related sexual species. In a phylogenetic analysis including other ITS sequences from GenBank, the three asexual species were not closely related to any of the known sexual species. Isolates of opposite mating type but the same species had identical ITS sequences. During any period of asexual evolutionary history, lineages of each mating type would have had a separate evolutionary history and any ITS substitutions shared between isolates of opposite mating type would have had to accumulate by convergence. Allowing for varying substitution rates and assuming a Poisson distribution of substitutions, the probability that isolates of opposite mating type shared an ITS substitution through convergence was low. This suggests that isolates of opposite mating type of A. brassicae, A. brassicicola and A. tenuissima were exchanging substitutions through sexual or parasexual reproduction while the ITS was evolving. If sexuality was lost, it was lost after the period of evolutionary history represented by the shared substitutions.

Alternaria↗
PubMed1996

Heterozygosity, heteromorphy, and phylogenetic trees in asexual eukaryotes.

Little attention has been paid to the consequences of long-term asexual reproduction for sequence evolution in diploid or polyploid eukaryotic organisms. Some elementary theory shows that the amount of neutral sequence divergence between two alleles of a protein-coding gene in an asexual individual will be greater than that in a sexual species by a factor of 2tu, where t is the number of generations since sexual reproduction was lost and u is the mutation rate per generation in the asexual lineage. Phylogenetic trees based on only one allele from each of two or more species will show incorrect divergence times and, more often than not, incorrect topologies. This allele sequence divergence can be stopped temporarily by mitotic gene conversion, mitotic crossing-over, or ploidy reduction. If these convergence events are rare, ancient asexual lineages can be recognized by their high allele sequence divergence. At intermediate frequencies of convergence events, it will be impossible to reconstruct the correct phylogeny of an asexual clade from the sequences of protein coding genes. Convergence may be limited by allele sequence divergence and heterozygous chromosomal rearrangements which reduce the homology needed for recombination and result in aneuploidy after crossing-over or ploidy cycles.

Eukaryotic Cells↗
PubMed2006

The fate of transposable elements in asexual populations.

Sexual reproduction and recombination are important for maintaining a stable copy number of transposable elements (TEs). In sexual populations, elements can be contained by purifying selection against host carriers with higher element copy numbers; however, in the absence of sex and recombination, asexual populations could be driven to extinction by an unchecked proliferation of TEs. Here we provide a theoretical framework for analyzing TE dynamics under asexual reproduction. Analytic results show that, in an infinite asexual population, an equilibrium in copy number is achieved if no element excision is possible, but that all TEs are eliminated if there is some excision. In a finite population, computer simulations demonstrate that small populations are driven to extinction by a Muller's ratchet-like process of element accumulation, but that large populations can be cured of vertically transmitted TEs, even with excision rates well below transposition rates. These results may have important consequences for newly arisen asexual lineages and may account for the lack of deleterious retrotransposons in the putatively ancient asexual bdelloid rotifers.

Animals↗
PubMed2003

The lipid body protein, PpoA, coordinates sexual and asexual sporulation in Aspergillus nidulans.

The coexistence of sexual and asexual reproductive cycles within the same individual is a striking phenomenon in numerous fungi. In the fungus Aspergillus nidulans (teleomorph: Emericella nidulans) endogenous oxylipins, called psi factor, serve as hormone-like signals that modulate the timing and balance between sexual and asexual spore development. Here, we report the identification of A. nidulans ppoA, encoding a putative fatty acid dioxygenase, involved in the biosynthesis of the linoleic acid derived oxylipin psiBalpha. PpoA is required for balancing anamorph and teleomorph development. Deletion of ppoA significantly reduced the level of psiBalpha and increased the ratio of asexual to sexual spore numbers 4-fold. In contrast, forced expression of ppoA resulted in elevated levels of psiBalpha and decreased the ratio of asexual to sexual spore numbers 6-fold. ppoA expression is mediated by two developmental regulators, VeA and the COP9 signalosome, such that ppoA transcript levels are correlated with the initiation of asexual and sexual fruiting body formation. PpoA localizes in lipid bodies in these tissues. These data support an important role for oxylipins in integrating mitotic and meiotic spore development.

Aspergillus nidulans↗
PubMed2004

Asexuality: prevalence and associated factors in a national probability sample.

I used data from a national probability sample (N > 18,000) of British residents to investigate asexuality, defined as having no sexual attraction to a partner of either sex. Approximately 1% (n = 195) of the sample indicated they were asexual. A number of factors were related to asexuality, including gender (i.e., more women than men), short stature, low education, low socioeconomic status, and poor health. Asexual women also had a later onset of menarche relative to sexual women. The results suggest that a number of pathways, both biological and psychosocial, contribute to the development of asexuality.

Adolescent↗
PubMed1978

Malaria immunization in Rhesus monkeys. A vaccine effective against both the sexual and asexual stages of Plasmodium knowlesi.

Rhesus monkeys were immunized with a preparation of Plasmodium knowlesi parasites containing principally microgametes with lesser numbers of macrogametes and asexual trophozoites. The antigen mixture was emulsified in Freund's complete adjuvant (FCA) and administered intramuscularly. After one or two inoculations of from 10(5) to 10(7) microgametes in FCA, monkeys showed high levels of circulating anti-gamete antibodies as demonstrated by various in vitro microgamete immobilization or transmission blocking tests. After challenge with P. knowlesi, immunized monkeys developed low level asexual parasitemias and were not infectious to feeding mosquitoes as measured by growth of the parasite on the mosquito gut. Control monkeys developed rapidly rising, usually fatal infections and were highly infectious to mosquitoes. Anti-gamete antibodies appear to neutralize the sexual parasites and prevent mosquito infection within the gut of the recently fed mosquito vector. Suppression of asexual parasitemia in immunized monkeys may be due to the presence of asexual trophozoites in the antigen mixture or to antigens common to both sexual and asexual stages of the parasite. A vaccine effective as a single injection capable of interrupting malaria transmission from man to man whereas reducing the severity of the disease in infected individuals offers a new approach to the control of one of the major diseases affecting man.

Animals↗
PubMed2000

Avoiding the cost of males in obligately asexual Daphnia pulex (Leydig).

Asexual organisms are thought to gain an advantage by avoiding the cost of producing males. In the cladoceran Daphnia pulex (Leydig), male production is determined by the environment and is independent of the origin of the asexual obligate parthenogens from the sexual cyclical parthenogens. If there is a cost to producing males, successful obligate parthenogens should have reduced or eliminated male production. Field and laboratory observations showed that obligate parthenogens have much-reduced male production compared to cyclical parthenogens. Although the reduction or elimination of males in the obligate parthenogens suggests that the cost of males is avoided, the coexistence of sexual and asexual forms of D. pulex may be partially explained by cyclical parthenogens compensating for the cost of males by having greater fecundity. In addition, the absence of a mating constraint for the obligate parthenogens may favour an increased allocation to asexual diapausing eggs earlier in the season compared to the cyclical parthenogens which require mating with males to produce sexual diapausing eggs. No difference in the production of diapausing eggs was observed, probably because males were abundant in populations of cyclical parthenogens and do not appear to limit the production of sexual diapausing eggs. D. pulex is a useful system for determining the ecological consequences of abandoning sexual reproduction and explaining the coexistence of sexual and asexual forms of a species.

Animals↗
PubMed2005

Genetic diversity in populations of asexual and sexual bag worm moths (Lepidoptera: Psychidae).

BACKGROUND: Despite the two-fold cost of sex, most of the higher animals reproduce sexually. The advantage of sex has been suggested to be its ability, through recombination, to generate greater genetic diversity than asexuality, thus enhancing adaptation in a changing environment. We studied the genetic diversity and the population structure of three closely related species of bag worm moths: two strictly sexual (Dahlica charlottae and Siederia rupicolella) and one strictly asexual (D. fennicella). These species compete for the same resources and share the same parasitoids. RESULTS: Allelic richness was comparable between the sexual species but it was higher than in the asexual species. All species showed high heterozygote deficiency and a large variation was observed among FIS values across loci and populations. Large genetic differentiation was observed between populations confirming the poor dispersal ability of these species. The asexual species showed lower genotype diversity than the sexual species. Nevertheless, genotype diversity was high in all asexual populations. CONCLUSION: The three different species show a similar population structure characterised by high genetic differentiation among populations and low dispersal. Most of the populations showed high heterozygote deficiency likely due to the presence of null alleles at most of the loci and/or to the Wahlund effect. Although the parthenogenetic D. fennicella shows reduced genetic diversity compared to the sexual species, it still shows surprisingly high genotype diversity. While we can not totally rule out the presence of cryptic sex, would explain this high genotype diversity, we never observed sex in the parthenogenetic D. fennicella, nor was there any other evidence of this. Alternatively, a non-clonal parthenogenetic reproduction, such as automictic thelytoky, could explain the high genotypic diversity observed in D. fennicella.

Animals↗
PubMed1997

Dynamics of the emergence of genetic resistance to biocides among asexual and sexual organisms.

A stochastic, agent based, evolutionary algorithm, modeling mating, reproduction, genetic variation, phenotypic expression and selection was used to study the dynamic interactions affecting a multiple-gene system. The results suggest that strong irreversible constraints affect the evolution of resistance to biocides. Resistant genes evolve differently in asexual organisms compared with sexual ones in response to various patterns of biocide applications. Asexual populations (viruses and bacteria) are less likely to develop genetic resistance in response to multiple pesticides or if pesticides are used at low doses, whereas sexual populations (insects for example) are more likely to become resistant to pesticides if susceptibility to the pesticide relates to mate selection. The adaptation of genes not related to the emergence of resistance will affect the dynamics of the evolution of resistance. Increasing the number of pesticides reduces the probability of developing resistance to any of them in asexual organisms but much less so in sexual organisms. Sequential applications of toxins, were slightly less efficient in slowing emergence of resistance compared with simultaneous application of a mix in both sexual and asexual organisms. Targeting only one sex of the pest speeds the development of resistance. The findings are consistent to most of the published analytical models but are closer to known experimental results, showing that nonlinear, agent based simulation models are more powerful in explaining complex processes.

Algorithms↗
PubMed1996

Neuronal signal substances in asexual multiplication and development in flatworms.

1. The phenomenon of asexual multiplication is rare in the animal kingdom. It occurs, however, in all main flatworm taxa. Flatworms are characterized by an extensive versatility, ranging from the different types of asexual multiplication to the different orthogonal plans for the nervous system. The role of the nervous system in the asexual multiplication taking place in flatworms is pointed out and discussed. 2. Immunocytochemical studies of the changes in the flatworm neuroanatomy show that the nervous system, particularly the main never cords, has a central role during asexual development. 3. Antibodies to different neuronal substances yield different immunoreactivity patterns and develop according to different time schedules. Serotoninergic nervous elements seem to have a leading role. 4. Substances produced by the nervous system influence fissioning and subsequent regeneration in free-living flatworms in the following ways. (a) A function as a wound hormone has been suggested for the neuropeptide RF-amide. (b) Mitogenic effects have been shown for several biogenic amines and neuropeptides. (c) Inhibitory roles are suggested for somatostatin and melatonin in connection with cell proliferation respective fissioning. 5. Growth factors have been observed both in free-living and parasitic flatworms. 6. Cells reactive to antibodies against epidermal growth factor increase in number in parallel with increases in mitotic activity in the gull tapeworm and occur in regions with high mitotic activity. A correlation between these two phenomena is suggested.

Animals↗
PubMed2007

The gynogenetic reproduction of diploid and triploid hybrid spined loaches (Cobitis: Teleostei), and their ability to establish successful clonal lineages--on the evolution of polyploidy in asexual vertebrates.

Polyploidisation is assumed to have played a significant role in the evolution of hybrid asexual lineages. The virtual absence of natural asexual systems in which more than a single ploidy level successfully establishes successful independent clonal lineages is generally explained by the strong effects of polyploidisation on fitness. Experimental crosses were made between diploid and triploid asexual Cobitis elongatoides x C. taenia hybrids (female) and both parental spined loach species (male). Genotyping of the progeny using allozymes and multilocus DNA fingerprinting, along with flow cytometric measurement of ploidy level, demonstrated the occurrence of gynogenetic reproduction in both female biotypes. The incorporation of the sperm genome occurred in some progeny, giving rise to a higher ploidy level, but the rate of polyploidisation differed significantly between the diploid and triploid females. These outcomes are consistent with the existence of developmental constraints on tetraploidy, which determine the rarity of tetraploids in natural populations. No cases of ploidy level reduction were observed. Since diploid and triploid hybrid populations occur where the lack of potential progenitor excludes the possibility of de novo origin, it is probable that both diploid and triploid females can establish successful clonal lineages. Spined loaches represent a unique example, among asexual vertebrates, where more than one ploidy level can establish persistent clonal lineages, which are reproductively independent of one another.

Animals↗
PubMed1983

Parthenogenesis and asexual multiplication among parasitic platyhelminths.

Among flatworms with parasitic and commensal modes of existence, parthenogenesis and asexual multiplication appear to be largely confined to the Digenea and Cestoda, the only parasitic platyhelminths that routinely utilize indirect life-cycles. Parthenogenesis is apparently restricted to a minority of adult digeneans and cestodes inhabiting their final hosts, and a survey is made of the particular modes of parthenogenesis (i.e. apomictic, automictic and generative) which are employed by such adults. Asexual (amictic) multiplication, in the form of fissioning, is demonstrated by young adults of the cyclophyllidean cestode, Mesocestoides corti, but is otherwise not exhibited by adult cestodes or digeneans, other than in the perplexing phenomenon of proglottid formation in polyzoic tapeworms. Secondary multiplication is of ubiquitous occurrence in digenean life-cycles in the form of the proliferation which takes place within sporocysts and rediae (germinal sacs) located in the first intermediate host. The controversy concerning the nature of this multiplication is reconsidered in the context of recent findings which have centred on cellular aspects. On the basis of present evidence germinal sac multiplication should be regarded as an asexual rather than a parthenogenetic process. The cestode asexual multiplication which occurs in intermediate hosts is a function of the metacestode stage of development. Metacestode proliferation is only known from about 20 species and 6 families of polyzoic cestodes with approximately half the described instances occurring in the family Taeniidae. The organization of these proliferative metacestodes, findings concerning their totipotent stem cells and the ontogeny of buds and new scolices are all reviewed. Finally, the capacity for population expansion of multiplicative larval digeneans and metacestodes are compared, while the ecological roles and the genetical consequences of both parthenogenesis and amictic multiplication in the two taxa are also examined.

Animals↗
PubMed2003

Evolutionary history of asexual hybrid loaches (Cobitis: Teleostei) inferred from phylogenetic analysis of mitochondrial DNA variation.

Reconstruction of the evolutionary history of asexual lineages undermines their suitability as models for the studies of evolutionary consequences of sexual reproduction. Using molecular tools we addressed the origin, age and maternal ancestry of diploid and triploid asexual lineages arisen through the hybridization between spiny loaches Cobitis elongatoides, C. taenia and C. tanaitica. Reconstructions of the phylogenetic relationships among mitochondrial DNA (mtDNA) haplotypes, revealed by sequence analyses, suggest that both hybrid complexes (C. elongatoides-taenia and C. elongatoides-tanaitica) contained several asexual lineages of independent origin. Cobitis elongatoides was the exclusive maternal ancestor of all the C. elongatoides-tanaitica hybrids, whereas within the C. elongatoides-taenia complex, hybridization was reciprocal. In both complexes the low haplotype divergences were consistent with a recent origin of asexual lineages. Combined mtDNA and allozyme data suggest that the triploids arose through the incorporation of a haploid sperm genome into unreduced ova produced by diploid hybrids.

Animals↗