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PubMed2004

Aspergillus fumigatus rasA and rasB regulate the timing and morphology of asexual development.

Expression of rasA plays an important role in conidial germination in Aspergillus nidulans. Conidial germination is required to initiate both infection and asexual development in the opportunistic pathogen Aspergillus fumigatus. Therefore, we sought to determine the requirements for Ras proteins in conidial germination and asexual development of A. fumigatus. A second homolog, rasB, has been identified that characterizes a new subclass of Ras genes. Dominant active (DA) and dominant negative (DN) mutations of each gene were introduced into protoplasts as transgenes. DArasA expression led to reduced conidiation, malformed conidiophores, and altered mitotic progression, whereas expression of DNrasA caused a significant reduction in the rate of conidial germination. In contrast, expression of DNrasB slightly delayed the initiation of germination and caused the development of conidiophores in submerged culture. DArasB expression led to reduced conidiation. RasA and RasB appear to play different, but overlapping, roles in the vegetative growth and asexual development of A. fumigatus.

Aspergillus fumigatus↗
PubMed2004

The role of tetraploids in the sexual-asexual cycle in dandelions (Taraxacum).

Apomictic plants often produce pollen that can function in crosses with related sexuals. Moreover, facultative apomicts can produce some sexual offspring. In dandelions, Taraxacum, a sexual-asexual cycle between diploid sexuals and triploid apomicts, has been described, based on experimental crosses and population genetic studies. Little is known about the actual hybridization processes in nature. We therefore studied the sexual-asexual cycle in a mixed dandelion population in the Netherlands. In this population, the frequencies of sexual diploids and triploids were 0.31 and 0.68, respectively. In addition, less than 1% tetraploids were detected. Diploids were strict sexuals, triploids were obligate apomicts, but tetraploids were most often only partly apomictic, lacking certain elements of apomixis. Tetraploid seed fertility in the field was significantly lower than that of apomictic triploids. Field-pollinated sexual diploids produced on average less than 2% polyploid offspring, implying that the effect of hybridization in the 2x-3x cycle in Taraxacum will be low. Until now, 2x-3x crosses were assumed to be the main pathway of new formation of triploid apomicts in the sexual-asexual cycle in Taraxacum. However, tetraploid pollen donors produced 28 times more triploid offspring in experimental crosses with diploid sexuals than triploid pollen donors. Rare tetraploids may therefore act as an important bridge in the formation of new triploid apomicts.

Hybridization, Genetic↗
PubMed2005

Does insecticide resistance alone account for the low genetic variability of asexually reproducing populations of the peach-potato aphid Myzus persicae?

The typical life cycle of aphids includes several parthenogenetic generations and a single sexual generation (cyclical parthenogenesis), but some species or populations are totally asexual (obligate parthenogenesis). Genetic variability is generally low in these asexually reproducing populations, that is, few genotypes are spread over large geographic areas. Both genetic drift and natural selection are often invoked to account for this low genetic variability. The peach-potato aphid, Myzus persicae, which encompasses both cyclical and obligate parthenogens, has developed several insecticide resistance mechanisms as a consequence of intense insecticide use since the 1950s. We collected asexually reproducing M. persicae from oilseed rape and examined genetic variability at eight microsatellite loci and three insecticide resistance genes to determine whether their genetic structure was driven by drift and/or selection. We identified only 16 multilocus microsatellite genotypes among 255 individuals. One clone, which combined two insecticide resistance mechanisms, was frequently detected in all populations whatever their location over a large geographical area (the northern half of France). These unexpected findings suggest that drift is not the unique cause of this low variability. Instead, the intensification of both insecticide treatments and oilseed rape cultivation may have favored a few genotypes. Thus, we propose that selective pressures resulting from human activities have considerably modified the genetic structure of M. persicae populations in northern France in a relatively short period of time.

Alleles↗
PubMed2003

Genetic variation in organisms with sexual and asexual reproduction.

The genetic variation in a partially asexual organism is investigated by two models suited for different time scales. Only selectively neutral variation is considered. Model 1 shows, by the use of a coalescence argument, that three sexually derived individuals per generation are sufficient to give a population the same pattern of allelic variation as found in fully sexually reproducing organisms. With less than one sexual event every third generation, the characteristic pattern expected for asexual organisms appear, with strong allelic divergence between the gene copies in individuals. At intermediary levels of sexuality, a complex situation reigns. The pair-wise allelic divergence under partial sexuality exceeds, however, always the corresponding value under full sexuality. These results apply to large populations with stable reproductive systems. In a more general framework, Model 2 shows that a small number of sexual individuals per generation is sufficient to make an apparently asexual population highly genotypically variable. The time scale in terms of generations needed to produce this effect is given by the population size and the inverse of the rate of sexuality.

Gene Frequency↗
PubMed2006

Sexual selection and its effect on the fixation of an asexual clone.

Sexual selection is a powerful and ubiquitous force in sexual populations. It has recently been argued that sexual selection can eliminate the twofold cost of sex even with low genomic mutation rates. By means of differential male mating success, deleterious mutations in males become more deleterious than in females, and it has been shown that sexual selection can drastically reduce the mutational load in a sexual population, with or without any form of epistasis. However, any mechanism that claims to maintain sexual reproduction must be able to prevent the fixation of an asexual mutant clone with a twofold fitness advantage. Here, I show that despite very strong sexual selection, the fixation of an asexual mutant cannot be prevented under reasonable genomic mutation rates. Sexual selection can have a strong effect on the average mutational load in a sexual population, but as it cannot prevent the fixation of an asexual mutant, it is unlikely to play a key role on the maintenance of sexual reproduction.

Animals↗
PubMed2001

The importance of sexual and asexual reproduction in the recent evolution of Allium vineale.

In the weedy plant species Allium vineale (wild garlic), individuals may simultaneously produce sexually and asexually derived offspring, by seed and bulbils, respectively. In this study, genetic and genotypic diversity was determined in samples from 14 European A. vineale populations using nuclear (RAPD) and cytoplasmic (PCR-RFLP of cpDNA) markers to investigate the importance of the different reproductive modes. In the whole sample, 77 nuclear multilocus genotypes and four chloroplast haplotypes (chlorotypes) were found. Populations exhibited a high degree of subdivision for nuclear and cytoplasmic markers as estimated from hierarchical F-statistics; at the same time, identical chlorotypes could be found in populations separated by large distances. Genotypic diversity was significantly lower than expected under free recombination in almost all populations, indicating that recruitment into populations is mostly by asexually produced offspring. Nevertheless, within each chlorotype, the distribution of markers from pairs of nuclear loci was incompatible with a purely clonal structure, suggesting that many multilocus genotypes have originated by sexual recombination rather than by mutation within asexual lineages. It is argued that the weedy habit of A. vineale is likely to have favored bulbil reproduction, whereas sexually generated genotypes may have facilitated local adaptation during the species' expansion across Europe.

Allium↗
PubMed2006

TmpA, a member of a novel family of putative membrane flavoproteins, regulates asexual development in Aspergillus nidulans.

Asexual reproduction (conidiation) in Aspergillus nidulans is induced by environmental signals like exposure to air or nutrient starvation, and depends on brlA gene activation. The study of 'fluffy' mutants showing delayed asexual development and reduced brlA expression has defined the fluG pathway, involved in regulation of this differentiation process. Genetic characterization of a 'fluffy' mutant identified tmpA as a new gene involved in regulation of conidiation. TmpA defines a new family of putative transmembrane proteins of unknown function, widespread in filamentous fungi and plants, with homologues showing similarity to non-ribosomal peptide synthetases. The deletion of tmpA resulted in decreased brlA expression and conidiation in air-exposed colonies. This defect was suppressed when DeltatmpA mutants were grown next to wild-type or DeltafluG mutant colonies, even without direct contact between hyphae. In liquid culture, tmpA was essential for conidiation induced by nitrogen but not by carbon starvation, whereas the overexpression of different tmpA tagged alleles resulted in conidiation. The overexpression of fluG-induced conidiation independently of tmpA and DeltatmpADeltafluG double mutants showed an additive 'fluffy' phenotype, indicating that tmpA and fluG regulate asexual sporulation through different pathways. TmpA and its homologues appear to have diverged from the ferric reductase family, retaining overall transmembrane architecture, NAD(P), flavin adenine dinucleotide (FAD) and possibly haem-binding domains. Based on our results, we propose that TmpA is a membrane oxidoreductase involved in the synthesis of a developmental signal.

Aspergillus nidulans↗
PubMed2003

The cytogenetics of thelytoky in a predominantly asexual parasitoid wasp with covert sex.

Asexual lineages in the parasitoid wasp Lysiphlebus fabarum (Hymenoptera: Braconidae: Aphidiinae) have previously been shown to have occasionally undergone sexual reproduction and recombination with males from related sexual populations. In the present study, the cytogenetic system of asexual females in this species is shown by 4',6-diamidino-2-phenylindole dihydrochloride (DAPI) staining to be central fusion automixis. This system has the potential to allow occasional sex and recombination without leading to an elevation of ploidy and with the maintenance of at least some heterozygosity. No evidence of the bacterial symbiont Wolbachia was found, and the observed system was compared with that in parasitoid wasps where asexuality is the result of Wolbachia infection.

Animals↗
PubMed2004

The GanB Galpha-protein negatively regulates asexual sporulation and plays a positive role in conidial germination in Aspergillus nidulans.

We isolated the ganB gene encoding the Galpha-protein homolog from Aspergillus nidulans. To investigate the cellular function of GanB, various mutant strains were isolated. Deletion of constitutively inactive ganB mutants showed conidiation and derepressed brlA expression in a submerged culture. Constitutive activation of GanB caused a reduction in hyphal growth and a severe defect in asexual sporulation. We therefore propose that GanB may negatively regulate asexual sporulation through the BrlA pathway. In addition, deletion or constitutive inactivation of GanB reduced germination rate while constitutive activation led to precocious germination. Furthermore, conidia of a constitutively active mutant could germinate even without carbon source. Taken together, these results indicated that GanB plays a positive role during germination, possibly through carbon source sensing, and negatively regulates asexual conidiation in A. nidulans.

Amino Acid Sequence↗
PubMed2000

Morphogenesis during asexual reproduction in Pygospio elegans Claparede (Annelida, Polychaeta).

The spionid Pygospio elegans reproduces both asexually and sexually. Using scanning electron and bright field microscopy, we examined morphogenesis following asexual reproduction to determine how "lost" body regions were regenerated after a worm spontaneously divided. Asexual reproduction occurred through transverse fission and divided the parent worm into 2 to 6 fragments (architomy). All fragments retained their original anterior-posterior polarity. Regeneration in all fragments followed a specific series of events: wound healing (day 1); extension of the blastema to generate lost body regions-specifically, the head and thorax for posterior fragments and the tail and pygidium for anterior fragments (days 2-3); segmentation (days 3-6); and differentiation of segment- or region-specific structures (days 4-8). This pattern occurred regardless of where the original division took place. Subsequent growth occurred through addition of terminal setigers anterior to the pygidium followed by differentiation of tail setigers into abdominal setigers, leaving the tail region about 6 to 10 setigers in size. Division rates were compared in worms from three populations in Nova Scotia, Canada. Worms from two populations (Conrad's Beach, Starr's Point) divided more frequently (about 1.2 and 1.3 weeks between divisions, respectively) than worms from Bon Portage Island (3.5 weeks between divisions). Fragments containing the original head (original mouth intact, generally much larger fragment) had a higher survivorship than fragments containing the original tail.

Animals↗
PubMed2002

A novel invertebrate trophic factor related to invertebrate neurotrophins is involved in planarian body regional survival and asexual reproduction.

Trophic factors are a heterogeneous group of molecules that promote cell growth and survival. In freshwater planarians, the small secreted protein TCEN49 is linked to the regional maintenance of the planarian central body region. To investigate its function in vivo, we performed loss-of-function and gain-of-function experiments by RNA interference and by the implantation of microbeads soaked in TCEN49, respectively. We show that TCEN49 behaves as a trophic factor involved in central body region neuron survival. In planarian tail regenerates, tcen49 expression inhibition by double-stranded RNA interference causes extensive apoptosis in various cell types, including nerve cells. This phenotype is rescued by the implantation of microbeads soaked in TCEN49 after RNA interference. On the other hand, in organisms committed to asexual reproduction, both tcen49 mRNA and its protein are detected not only in the central body region but also in the posterior region, expanding from cells close to the ventral nerve chords. In some cases, the implantation of microbeads soaked in TCEN49 in the posterior body region drives organisms to reproduce asexually, and the inhibition of tcen49 expression obstructs this process, suggesting a link between the central nervous system, TCEN49, regional induction, and asexual reproduction. Finally, the distribution of TCEN49 cysteine and tyrosine residues also points to a common evolutionary origin for TCEN49 and molluscan neurotrophins.

Amino Acid Sequence↗
PubMed1998

Plasmodium falciparum: asexual erythrocytic stages synthesize two structurally distinct free and protein-bound glycosylphosphatidylinositols in a maturation-dependent manner.

Glycosylphosphatidylinositols represent the predominant class of glycolipids synthesized by the asexual, intraerythrocytic stages of Plasmodium falciparum. These glycolipids have been implicated as malarial toxins involved in parasite-induced release of cytokines, such as tumor necrosis factor-alpha and interleukin-1. Two potential glycosylphosphatidylinositol membrane-anchor precursors with the structures ethanolamine phosphate (mannose-alpha 1,2)mannose-alpha 1,2-mannose-alpha 1,6-mannose-alpha 1,4-glucosamine-inositol(acyl)phosphate diacylglycerol (P.f.alpha) and ethanolamine-phosphate-mannose-alpha 1,2-mannose-alpha 1,6-mannose-alpha 1,4-glucosamine-inositol(acyl)phosphate diacylglycerol (P.f.beta) have been described in P. falciparum. Only one (P.f.alpha) has been demonstrated to serve as an anchor for merozoite surface protein-1 and merozoite surface protein-2. In this report we present data showing that asexual, intraerythrocytic stages of P. falciparum use both glycosylphosphatidylinositols to anchor proteins. The synthesis of the two glycosylphosphatidylinositol membrane anchor precursors and the protein-bound glycosylphosphatidylinositol anchors is tightly regulated and varies throughout the intraerythrocytic development of the asexual stages of P. falciparum. The glycosylphosphatidylinositol membrane-anchor precursor P.f.beta is synthesized and transferred to protein predominantly in trophozoite stages (about 30 h).

Animals↗
PubMed1987

Genotypic characteristics of cyclic parthenogens and their obligately asexual derivatives.

Among extant taxa it appears that cyclic parthenogenesis has evolved on fewer than ten occasions. Evidence suggests that these groups are all derived from bisexual ancestors with genetic systems possessing specific preadaptations for the breeding system shift. Its apparently independent origin in two cecidomyiid lineages suggests that, when such preadaptations exist, the adoption of cyclic parthenogenesis may be a highly probable event. The genetic consequences of this transition are not dramatic; the genotypic characteristics of most cyclic parthenogens are similar to those of bisexuals. Specifically, genotypic frequencies at polymorphic loci are generally in Hardy-Weinberg equilibrium and linkage disequilibrium between variants at different loci is rare. Levels of variation in local populations of cyclic parthenogens are lower than those typical of sexual outcrossers, but the lack of variation appears to be a consequence of founder effects, as regional gene pool diversity is not similarly impoverished. An unusual genotypic structure occasionally develops in populations from habitats in which sustained parthenogenetic reproduction is possible and sexual recruitment is low. Under these circumstances low fitness genotypes are winnowed from the population by natural selection. The truncation of genotypic diversity often produces Hardy-Weinberg deviations and linkage disequilibrium. The depletion of clonal diversity rarely leads to the domination of a habitat by a single clone; an array of ecologically diversified clonal variants remains. The level of clonal variation in such populations is occasionally augmented by sexual recruitment, but the persistence of clonal diversity in local populations of obligate parthenogens makes it clear that clonal coexistence is not a consequence of such recruitment. Cyclic parthenogens have made the transition to obligate asexuality with high frequency, but there is little evidence to support the argument (Williams, 1975) that such shifts result from the relaxation of the short-term selection pressures supposedly necessary to sustain the sexual phase of the life cycle. Maynard Smith (1978) has recognized that in groups such as the aphids, which have never evolved the ability to produce their diapausing eggs asexually, sexuality may simply be retained as a consequence of its role in the formation of these eggs. However, he accepted that short-term selection was required to maintain sexuality in groups such as the cladocerans in which asexual forms have demonstrated their ability to retain life cycle complexity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

The asexual ploidy cycle and the origin of sex.

Sex involves syngamy (gamete fusion), which doubles the amount of DNA in a cell, and meiosis, which halves it. The result is a 'ploidy cycle' of alternating diploid and haploid phases. Asexual reproduction does not require changes of ploidy, and yet asexual forms may have ploidy cycles. Here I show that such cycles lessen the mutation load, compared with permanent diploidy or polyploidy, and are thus likely to evolve in cases where it is always advantageous to have more than one copy of the genome per cell. The asexual ploidy cycle could have facilitated the origin of sex, by providing a means of orderly genetic reduction available immediately after the origin of syngamy.

Animals↗
PubMed2004

The role of sexual and asexual reproduction in structuring high latitude populations of the reef coral Pocillopora damicornis.

The genotypic composition of populations of the asexually viviparous coral Pocillopora damicornis varies in a manner that challenges classical models of the roles of sexual and asexual reproduction. On the geographically isolated Hawaiian reefs and high latitude reefs in Western Australia, P. damicornis populations are highly clonal although it has been argued that sexual reproduction via broadcast spawning generates widely dispersed colonists. In contrast, on eastern Australia's tropical Great Barrier Reef populations show little evidence of clonality. Here, we compare the genotypic diversity of adult and juvenile colonies of P. damicornis at seven sites on eastern Australia's high latitude Lord Howe Island reefs to determine if levels of clonality vary with habitat heterogeneity and age of colonies (as predicted by theory) or alternatively if clonality is again always high as for other isolated reef systems. We found 55-100% of the genotypic diversity expected for random mating at all seven sites and little evidence of asexual recruitment irrespective of habitat heterogeneity (sheltered versus wave exposed) or colony age. We found reduced levels of genetic diversity compared with tropical reefs (2.75 versus 4 alleles/locus), which supports earlier findings that Lord Howe Island is an isolated reef system. Furthermore, heterozygote deficits coupled with significant genetic subdivision among sites (FST=0.102+/-0.03) is typical of populations that have limited larval connections and are inbred. We conclude that the genetic structure of P. damicornis at Lord Howe Island reflects populations that are maintained through localised recruitment of sexually produced larvae.

Animals↗
PubMed1990

Genotypic variation within asexual lineages of Taraxacum officinale.

Restriction site variation in DNA that encodes rRNA (rDNA) was surveyed among 714 offspring within 31 lineages (26 genotypes) of obligate asexually reproducing Taraxacum officinale (dandelions). Although clonal offspring are expected, plants with nonparental rDNA were produced from two parents that were themselves siblings (same genotype). The variation is best characterized by the loss of an EcoRI restriction site that maps to the spacer region in the parental rDNA and is most likely involved in amplification of rare or unique rDNA repeats. In one family, 41 surveyed offspring lacked the EcoRI site. In the other family, only 1 of 26 offspring lost the EcoRI site. Other classes of DNA surveyed, chloroplast DNA and the alcohol dehydrogenase 2 gene (Adh2), showed no variation. However, offspring with nonparental rDNA also had nonparental alcohol dehydrogenase 1 (Adh1) restriction fragments. Because somatic mutations in plants can be incorporated into reproductive tissue, we propose that somatic events affecting at least both multicopy rDNA and DNA homologous to the maize Adh1 gene occurred at different developmental times in the two families. An event early in development would result in all variant offspring; an event late in development would result in a single variant offspring. These results support the view that mutation (in the broad sense) influences the level of genotypic variation in asexual organisms, which may facilitate adaptive evolution of asexual species.

Alcohol Dehydrogenase↗
PubMed1992

Asexual metazoans undergo senescence.

August Weismann popularized the notion that metazoans have a potentially immortal germ line separated from a mortal soma, and evolutionary biologists regard senescence as an evolved characteristic of the soma. Many have claimed that metazoans that do not sequester their germ line have no clear distinction between germ line and soma, and consequently they should lack senescence. Here we present experimental evidence that senescence occurs in the asexually reproducing marine oligochaete Paranais litoralis. We also analyze data reported in Sonneborn's classical study and show that the rhabdocoel Stenostomum incaudatum undergoes senescence. We argue that the stability of commitment to somatic function and the fact that asexual metazoans form their germ cells from undifferentiated stem cells are sufficient to allow for senescence of the asexual metazoan's soma. Thus the evolution of somatic differentiation, and not germ-line sequestration, would be the necessary condition for the evolution of senescence.

Journal Article↗
PubMed1996

Molecular evidence that the asexual industrial fungus Trichoderma reesei is a clonal derivative of the ascomycete Hypocrea jecorina.

The relationship of the important cellulase producing asexual fungus Trichoderma reesei to its putative teleomorphic (sexual) ancestor Hypocrea jecorina and other species of the Trichoderma sect. Longibrachiatum was studied by PCR-fingerprinting and sequence analyses of the nuclear ribosomal DNA region containing the internal transcribed spacers (ITS-1 and ITS-2) and the 5.8S rRNA gene. The differences in the corresponding ITS sequences allowed a grouping of anamorphic (asexual) species of Trichoderma sect. Longibrachiatum into Trichoderma longibrachiatum, Trichoderma pseudokoningii, and Trichoderma reesei. The sexual species Hypocrea schweinitzii and H. jecorina were also clearly separated from each other. H. jecorina and T. reesei exhibited identical sequences, suggesting close relatedness or even species identity. Intraspecific and interspecific variation in the PCR-fingerprinting patterns supported the differentiation of species based on ITS sequences, the grouping of the strains, and the assignment of these strains to individual species. The variations between T. reesei and H. jecorina were at the same order of magnitude as found between all strains of H. jecorina, but much lower than the observed interspecific variations. Identical ITS sequences and the high similarity of PCR-fingerprinting patterns indicate a very close relationship between T. reesei and H. jecorina, whereas differences of the ITS sequences and the PCR-fingerprinting patterns show a clear phylogenetic distance between T. reesei/H. jecorina and T. longibrachiatum. T. reesei is considered to be an asexual, clonal line derived from a population of the tropical ascomycete H. jecorina.

Ascomycota↗