Effect of sexual recombination on population diversity in aflatoxin production by Aspergillus flavus and evidence for cryptic heterokaryosis

Aspergillus flavus is the major producer of carcinogenic aflatoxins (AFs) in crops worldwide. Natural populations of A. flavus show tremendous variation in AF production, some of which can be attributed to environmental conditions, differential regulation of the AF biosynthetic pathway and deletions or loss‐of‐function mutations in the AF gene cluster. Understanding the evolutionary processes that generate genetic diversity in A. flavus may also explain quantitative differences in aflatoxigenicity. Several population studies using multilocus genealogical approaches provide indirect evidence of recombination in the genome and specifically in the AF gene cluster. More recently, A. flavus has been shown to be functionally heterothallic and capable of sexual reproduction in laboratory crosses. In the present study, we characterize the progeny from nine A. flavus crosses using toxin phenotype assays, DNA sequence‐based markers and array comparative genome hybridization. We show high AF heritability linked to genetic variation in the AF gene cluster, as well as recombination through the independent assortment of chromosomes and through crossing over within the AF cluster that coincides with inferred recombination blocks and hotspots in natural populations. Moreover, the vertical transmission of cryptic alleles indicates that while an A. flavus deletion strain is predominantly homokaryotic, it may harbour AF cluster genes at a low copy number. Results from experimental matings indicate that sexual recombination is driving genetic and functional hyperdiversity in A. flavus. The results of this study have significant implications for managing AF contamination of crops and for improving biocontrol strategies using nonaflatoxigenic strains of A. flavus.     

Microbe‐mediated control of Aspergillus flavus in stored rice grains with a focus on aflatoxin inhibition and biodegradation

Biological control of mycotoxigenic fungi using antagonistic microbes is a promising alternative to agricultural chemicals for postharvest storage. In this study, we evaluated rice‐derived bacterial strains to identify biocontrol agents to inhibit Aspergillus flavus in stored rice grains. Consequently, we obtained three potential biocontrol strains (Microbacterium testaceum KU313, Bacillus megaterium KU143 and Pseudomonas protegens AS15) from 26 tested strains that were prescreened from the 460 strains isolated from rice grains. The three selected strains proved to be effective biocontrol agents showing antifungal activity against A. flavus and good colonisation ability on rice grains, along with inhibition of the fungal growth and aflatoxin production. In particular, P. protegens AS15 greatly inhibited the aflatoxins produced by A. flavus on rice grains to 8.68 (percent aflatoxin reduction relative to control = 82.9%) and 18.05 (68.3 %) ng g^?1 dry weight of rice grains, compared with the 50.89 and 56.97 ng g^?1 dry weight of rice grains of the MgSO₄ control at 1 and 2 weeks after inoculation, respectively. In addition, strain AS15 had a significant ability to not only degrade aflatoxin B₁ (the most harmful aflatoxin), but also utilise the toxin for bacterial growth in a nutrient‐deficient medium. Therefore, the selected bacterial strains could be environmentally sound alternatives for the management of A. flavus and aflatoxin production by reducing the fungal damage to stored rice grains. This would also reduce the human and animal health hazards associated with the consumption of fungus‐contaminated rice grains. To our knowledge, this is the first report of the potential of the bacterial species M. testaceum and P. protegens as biocontrol agents for controlling aflatoxigenic A. flavus on stored rice grains.     

Asexual evolution: do intragenomic parasites maintain sex?

Resolving the paradox of sex, with its twofold cost to genic transmission, remains one of the major unresolved questions in evolutionary biology. Counting this genetic cost has now gone genomic. In this issue of Molecular Ecology, Kraaijeveld et al. (2012) describe the first genome‐scale comparative study of related sexual and asexual animal lineages, to test the hypothesis that asexuals bear heavier loads of deleterious transposable elements. A much higher density of such parasites might be expected, due to the inability of asexual lineages to purge transposons via mechanisms exclusive to sexual reproduction. They find that the answer is yes—and no—depending upon the family of transposons considered. Like many such advances in testing theory, more questions are raised by this study than answered, but a door has been opened to molecular evolutionary analyses of how responses to selection from intragenomic parasites might mediate the costs of sex.     

Recombination and lineage‐specific gene loss in the aflatoxin gene cluster of Aspergillus flavus

Aflatoxins produced by Aspergillus flavus are potent carcinogens that contaminate agricultural crops. Recent efforts to reduce aflatoxin concentrations in crops have focused on biological control using nonaflatoxigenic A. flavus strains AF36 (=NRRL 18543) and NRRL 21882 (the active component of afla‐guard^®). However, the evolutionary potential of these strains to remain nonaflatoxigenic in nature is unknown. To elucidate the underlying population processes that influence aflatoxigenicity, we examined patterns of linkage disequilibrium (LD) spanning 21 regions in the aflatoxin gene cluster of A. flavus. We show that recombination events are unevenly distributed across the cluster in A. flavus. Six distinct LD blocks separate late pathway genes aflE, aflM, aflN, aflG, aflL, aflI and aflO, and there is no discernable evidence of recombination among early pathway genes aflA, aflB, aflC, aflD, aflR and aflS. The discordance in phylogenies inferred for the aflW/aflX intergenic region and two noncluster regions, tryptophan synthase and acetamidase, is indicative of trans‐species evolution in the cluster. Additionally, polymorphisms in aflW/aflX divide A. flavus strains into two distinct clades, each harbouring only one of the two approved biocontrol strains. The clade with AF36 includes both aflatoxigenic and nonaflatoxigenic strains, whereas the clade with NRRL 21882 comprises only nonaflatoxigenic strains and includes all strains of A. flavus missing the entire gene cluster or with partial gene clusters. Our detection of LD blocks in partial clusters indicates that recombination may have played an important role in cluster disassembly, and multilocus coalescent analyses of cluster and noncluster regions indicate lineage‐specific gene loss in A. flavus. These results have important implications in assessing the stability of biocontrol strains in nature.     

Deciphering the origin of Aspergillus flavus NRRL21882, the active biocontrol agent of Afla-Guard®

Single nucleotide polymorphisms (SNPs) of genome sequences of eight Aspergillus flavus and seven Aspergillus oryzae strains were extracted with Mauve, a multiple-genome alignment programme. A phylogenetic analysis with sequences comprised of concatenated total SNPs by the unweighted pair group method with arithmetic mean (UPGMA) of MAFFT adequately separated them into three groups, A. flavus S-morphotype, A. flavus L-morphotype and A. oryzae. Divergence time inferred for A. flavus NRRL21882, the active agent of the biocontrol product Afla-Guard^®, and S-morphotype was about 5·1 mya. Another biocontrol strain, A. flavus AF36, diverged from aflatoxigenic L-morphotype about 2·6–3·0 mya. Despite the close relatedness of A. oryzae to A. flavus, A. oryzae strains likely evolved from aflatoxigenic Aspergillus aflatoxiformans (=A. parvisclerotigenus). A survey of A. flavus populations implies that prior Afla-Guard^® applications are associated with prevalence of NRRL21882-type isolates in Mississippi fields. In addition, a few NRRL21882 relatives were identified. A. flavus Og0222, a biocontrol ingredient of Aflasafe™, was verified as a NRRL21882-type strain, having identical sequence breakpoints that led to deletion of aflatoxin and cyclopiazonic acid gene clusters. A similar UPGMA analysis suggests that the occurrence of NRRL21882-type strains is a more recent event.     

Beneficial rhizospheric microorganisms mediated plant growth promotion and suppression of aflatoxigenic fungal and aflatoxin contamination in groundnut seeds

The potential of root‐colonising antagonistic microbial biocontrol agents was evaluated for their ability to improve plant growth and suppress aflatoxigenic fungal and aflatoxin contamination in groundnut. By considering root colonisation of groundnut seedlings, plant growth promotion and antagonism against aflatoxigenic Aspergillus flavus as preliminary criteria, eight rhizobacteria and nine Trichoderma spp. were selected and characterised for their beneficial traits. These strains gave varying results for IAA production, phosphate solubilisation, ACC deaminase, chitinase and siderophore production. Under laboratory and greenhouse conditions, these strains significantly (P < 0.05) suppressed seed‐borne and rhizospheric population of A. flavus and improved seed quality variables. However, cdELISA results revealed that none of the biocontrol strains were effective in reducing aflatoxin level in seed. Based on the overall performance, Pseudomonas fluorescens 2bpf, Bacillus sp. Bsp‐3/aM and Trichoderma atroviride UMDBT‐Dha.Tat8 were used for field trials in the form of talcum powder formulations. Under field conditions, biocontrol agents improved seedling emergence, plant biomass and pod yield. Seeds harvested from plots treated with biocontrol agents showed significant (P < 0.05) reduction in A. flavus infection and aflatoxin production after 6 months' storage. Use of microbial strains with multiple beneficial traits is advantageous in bioformulation development. Hence, in future, these formulations will play a major role as biofertilisers and biopesticides, which can reduce the usage of agrochemicals up to greater extents in groundnut production.     

Use of microsatellite markers to assess the competitive ability of nontoxigenic Aspergillus flavus strains in studies on biocontrol of aflatoxins in maize in Thailand

Many nontoxigenic strains of Aspergillus flavus have been used in studies on biocontrol by competitive exclusion, but assessing their competitive ability is difficult. This paper reports on the use of a microsatellite marker technique for assessing competitiveness. The chosen microsatellite markers were able to differentiate, at an individual level, between the four biocontrol strains used in a study on the biocontrol of aflatoxins in maize in Thailand. The microsatellite markers were then used to determine which of the four biocontrol strains used were identical with 86 nontoxigenic strains of A. flavus taken from dried maize samples produced in that study. Fifty-one of the 86 strains (59%) were identified as one of the four biocontrol strains, with another four likely to be so. Analysis of microsatellites in A. flavus strains taken from dried samples at the conclusion of a field trial was shown to be of value in understanding the competitive ability of the specific strains used for biocontrol. This method provides an objective assessment of the competitiveness of biocontrol strains.     

Molecular evidence for host-adapted races of the fungal endophyte Epichloë bromicola after presumed host shifts

Abstract.— Host shifts of plant‐feeding insects and parasites promote adaptational changes that may result in the formation of host races, an assumed intermediate stage in sympatric speciation. Here, we report on genetically differentiated and host‐adapted races of the fungal endophyte Epichloë bromicola, which presumably emerged after a shift from the grass Bromus erectus to other Bromus hosts. Fungi of the genus Epichloë (Ascomycota) and related anamorphs of Neotyphodium are widespread endophytes of cool‐season grasses. Sexually reproducing strains sterilize the host by formation of external fruiting structures (stromata), whereas asexual strains are asymptomatic and transmitted via seeds. In E. bromicola, strains infecting B. erectus are sexual, and strains from two woodland species, B. benekenii and B. ramosus, are asexual and seed transmitted. Analyses of amplified fragment length polymorphism fingerprinting and of intron sequences of the tub2 and tef1 genes of 26 isolates from the three Bromus hosts collected at natural sites in Switzerland and nearby France demonstrated that isolates are genetically differentiated according to their host, indicating that E. bromicola does not form a single, randomly mating population. Phylogenetic analyses of sequence data did not unambiguously resolve the exact origin of asexual E. bromicola strains, but it is likely they arose from within sexual populations on B. erectus. Incongruence of trees derived from different genes may have resulted from recombination at some time in the recent history of host strains. Reciprocal inoculations of host plant seedlings showed that asexual isolates from B. benekenii and B. ramosus were incapable of infecting B. erectus, whereas the sexual isolates from B. erectus retained the assumed ancestral trait of broad compatibility with Bromus host seedlings. Because all isolates were interfertile in experimental crosses, asexual strains may not be considered independent biological species. We suggest that isolates infecting B. benekenii and B. ramosus represent long‐standing host races or incipient species that emerged after host shifts and that may evolve through host‐mediated reproductive isolation toward independent species.     

Sexual and asexual reproduction of the holothurian Stichopus chloronotus (Echinodermata): a comparison between La Réunion (Indian Ocean) and east Australia (Pacific Ocean)

Summary

Stichopus chloronotus (Brandt, 1835) is one among nine aspidochirotide holothurian species known to reproduce both sexually by broadcast spawning and asexually by transverse fission. New data on the sexual cycle of this species in La Réunion are presented here and information available on sexual and asexual reproduction in this species is summarised. Sexual reproduction on La Réunion shows a distinct seasonality with a main spawning period in the warm season (November-February). The spawning period the Great Barrier Reef appears to be at the same time. Some intriguing deviations from unity in sex-ratio, usually biased towards more male individuals, have been observed in both geographic regions (sex ratio at La Reunion 31:1). New data on the asexual reproduction of this species in La Réunion confirm the high rates of fission. The peak of asexual reproduction in both the Indian and Pacific Ocean was observed in winter (June-July). Thus, asexual reproduction in this species occurs outside the season for sexual reproduction. The rate of asexual reproduction appears to vary between sample locations. However, results of population genetic studies on S. chloronotus (Uthicke et al., 1999; Uthicke et al., 2001) indicated that in most populations investigated a maximum of about 60% of all individuals may be derived from sexual recruitment. Cluster analyses on genetic distances between populations grouped populations within Oceans together, with the exception of one sample from a nearshore reef of the GBR. Although genetic differences between the two regions exist, these are relatively small regarding the large geographic distance. We conclude that asexual reproduction in S. chloronotus is important to maintain local population sizes, but that larval exchange between populations mediated by sexual reproduction is important for colonisation of new areas and to provide connectivity between populations. Here, we present the first synthesis of these phenomena for a holothurian species.     

Evaluation of different genotypes of nontoxigenic Aspergillus flavus for their ability to reduce aflatoxin contamination in peanuts

Aflatoxins produced by the fungus Aspergillus flavus are potent carcinogens and account for large monetary losses worldwide in peanuts, maize, and cottonseed. Biological control in which a nontoxigenic strain of A. flavus is applied to crops at high concentrations effectively reduces aflatoxins through competition with native aflatoxigenic populations. In this study, eight nontoxigenic strains of A. flavus belonging to different vegetative compatibility groups and differing in deletion patterns within the aflatoxin gene cluster were evaluated for their ability to reduce aflatoxin B₁ when paired with eight aflatoxigenic strains on individual peanut seeds. Inoculation of wounded viable peanut seeds with conidia demonstrated that nontoxigenic strains differed in their ability to reduce aflatoxin B₁. Reductions in aflatoxin B₁ often exceeded expected reductions based on a 50:50 mixture of the two A. flavus strains, although one nontoxigenic strain significantly increased aflatoxin B₁ when paired with an aflatoxigenic strain. Therefore, nontoxigenicity alone is insufficient for selecting a biocontrol agent and it is also necessary to test the effectiveness of a nontoxigenic strain against a variety of aflatoxigenic strains.     

Genetic variation and asexual reproduction in the facultatively parthenogenetic cockroach Nauphoeta cinerea: implications for the evolution of sex

Asexual reproduction could offer up to a two‐fold fitness advantage over sexual reproduction, yet higher organisms usually reproduce sexually. Even in facultatively parthenogenetic species, where both sexual and asexual reproduction is sometimes possible, asexual reproduction is rare. Thus, the debate over the evolution of sex has focused on ecological and mutation‐elimination advantages of sex. An alternative explanation for the predominance of sex is that it is difficult for an organism to accomplish asexual reproduction once sexual reproduction has evolved. Difficulty in returning to asexuality could reflect developmental or genetic constraints. Here, we investigate the role of genetic factors in limiting asexual reproduction in Nauphoeta cinerea, an African cockroach with facultative parthenogenesis that nearly always reproduces sexually. We show that when N. cinerea females do reproduce asexually, offspring are genetically identical to their mothers. However, asexual reproduction is limited to a nonrandom subset of the genotypes in the population. Only females that have a high level of heterozygosity are capable of parthenogenetic reproduction and there is a strong familial influence on the ability to reproduce parthenogenetically. Although the mechanism by which genetic variation facilitates asexual reproduction is unknown, we suggest that heterosis may facilitate the switch from producing haploid meiotic eggs to diploid, essentially mitotic, eggs.     

BIOGEOGRAPHY OF ASEXUAL AND SEXUAL REPRODUCTION IN CALOGLOSSA (DELESSERIACEAE,RHODOPHYTA) FROM AUSTRALIA AND NEW ZEALAND

Caloglossa species are widely distributed in mangroves and salt marshes around the world and their life history patterns are being investigated in laboratory culture. In Australia all isolates of C. monosticha, C. postiae and C. ogasawaraensis have Polysiphonia‐type (P‐type) sexual life histories. Among the 70 C. leprieurii isolates from Australia and New Zealand P‐type sexual reproduction also is dominant. However, ten isolates of C. leprieurii from the Spencer Gulf and the Gulf of St. Vincent in South Australia give rise to successive tetrasporphyte generations without gametophytes. Moreover, one isolate from Queensland is asexual. Only one South Australia isolate, obtained from Lake Alexandrina at the mouth of the Murray River, is sexual. South Australia and Pacific Mexico are two regions in which asexual reproduction is dominant. In another mangrove dwelling red alga Bostrychia moritiziana (Rhodomelaceae) non‐sexual reproduction also is frequent in Australia, New Caledonia and Bali (Indonesia). This asexual reproductive pattern of tetrasporophytic recycling appears to have arisen independently among individual populations of various red algal species in different regions. Investigations are underway on the molecular phylogeny of the Caloglossa leprieurii isolates.     

Carbonylated proteins are eliminated during reproduction in C. elegans

Oxidatively damaged proteins accumulate with age in many species (Stadtman (1992) Science 257 , 1220–1224). This means that damage must be reset at the time of reproduction. To visualize this resetting in the roundworm Caenorhabditis elegans, a novel immunofluorescence technique that allows the detection of carbonylated proteins in situ was developed. The application of this technique revealed that carbonylated proteins are eliminated during C. elegans reproduction. This purging occurs abruptly within the germline at the time of oocyte maturation. Surprisingly, the germline was markedly more oxidized than the surrounding somatic tissues. Because distinct mechanisms have been proposed to explain damage elimination in yeast and mice (Aguilaniu et al. (2003) Science 299 , 1751–1753; Hernebring et al. (2006) Proc Natl Acad Sci USA 103 , 7700–7705), possible common mechanisms between worms and one of these systems were tested. The results show that, unlike in yeast (Aguilaniu et al. (2003) Science 299 , 1751–1753; Erjavec et al. (2008) Proc Natl Acad Sci USA 105 , 18764–18769), the elimination of carbonylated proteins in worms does not require the presence of the longevity‐ensuring gene, SIR‐2.1. However, similar to findings in mice (Hernebring et al. (2006) Proc Natl Acad Sci USA 103 , 7700–7705), proteasome activity in the germline is required for the resetting of carbonylated proteins during reproduction in C. elegans. Thus, oxidatively damaged proteins are eliminated during reproduction in worms through the proteasome. This finding suggests that the resetting of damaged proteins during reproduction is conserved, therefore validating the use of C. elegans as a model to study the molecular basis of damage elimination.     

EVOLUTION OF ASEXUALITY VIA DIFFERENT MECHANISMS IN GRASS THRIPS (THYSANOPTERA: Aptinothrips)

Asexual lineages can derive from sexual ancestors via different mechanisms and at variable rates, which affects the diversity of the asexual population and thereby its ecological success. We investigated the variation and evolution of reproductive systems in Aptinothrips, a genus of grass thrips comprising four species. Extensive population surveys and breeding experiments indicated sexual reproduction in A. elegans, asexuality in A. stylifer and A. karnyi, and both sexual and asexual lineages in A. rufus. Asexuality in A. stylifer and A. rufus coincides with a worldwide distribution, with sexual A. rufus lineages confined to a limited area. Inference of molecular phylogenies and antibiotic treatment revealed different causes of asexuality in different species. Asexuality in A. stylifer and A. karnyi has most likely genetic causes, while it is induced by endosymbionts in A. rufus. Endosymbiont‐community characterization revealed presence of Wolbachia, and lack of other bacteria known to manipulate host reproduction. However, only 69% asexual A. rufus females are Wolbachia‐infected, indicating that either an undescribed endosymbiont causes asexuality in this species or that Wolbachia was lost in several lineages that remained asexual. These results open new perspectives for studies on the maintenance of mixed sexual and asexual reproduction in natural populations.     

Genes involved in Dictyostelium discoideum sexual reproduction

Macrocyst formation in the cellular slime moulds is a sexual process induced under dark and humid conditions. Normal development life cycle in these organisms involves proliferation by cell division and, upon starvation, formation of multicellular aggregates and fruiting bodies, consisting of spores and stalk cells. Macrocyst formation, cell division by binary fission and spore formation are thus three alternative modes of reproduction, for which it is of interest to understand how a choice is made. The genetic basis of asexual development and fruiting body formation is well known, by contrast information on the genetic control of sexual reproduction during macrocyst formation is scarce. In Dictyostelium discoideum, the most widely used species, several cell-surface proteins relevant to sexual cell fusion have been identified using cell fusion-blocking antibodies, but isolation of the relevant genes has been unsuccessful. Analysis of sexually deficient mutants, some of which are normal for asexual development, has shown that sexual reproduction is regulated by both specific genes and genes that are also involved in asexual development. Reverse genetic analysis of 24 genes highly enriched in a gamete-specific subtraction library has revealed four genes involved in the regulation of sexual cell interactions. One of them was found to be a novel regulator of the cAMP signalling pathway specific to sexual development. Studies on the molecular genetic control of the sexual cycle will be reviewed and their contribution to our understanding of the organization and function of the D. discoideum genome as a whole discussed.     

Role of parasite load and differential habitat preferences in maintaining the coexistence of sexual and asexual competitors in fish of the Cobitis taenia hybrid complex

In the context of the paradoxical ubiquity of sex, we tested whether stable coexistence of sexual and asexual fish of the genus Cobitis is mediated by parasites, as asexual fish suffer more from parasitic infections because of their lower genetic variability [the Red Queen hypothesis (RQH)], or by partial niche shift of the two strains differing in mode of reproduction. We did not find a clear correlation between infection risk with a helminth parasite and the proportion of sexuals, and we found similar infection rates among sexual females and co‐occurring asexuals in general, including the most frequent clone in particular. These results suggest that the mechanisms of the RQH are not directly engaged in stabilizing this asexual complex. On the other hand, the temporally stable gradient in sexual/asexual proportions along the river correlated with gradients in environmental parameters (physicochemical water parameters, velocity, and shading of the habitat) and turnover in the fish assemblage structure. Sexual and asexual forms thus appear to prefer different habitats. The Cobitis teania asexual complex thus contributes to the view that persistence of sex may, as in many taxa, be driven by case‐specific processes. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 220–235.     

Extreme genetic diversity in asexual grass thrips populations

The continuous generation of genetic variation has been proposed as one of the main factors explaining the maintenance of sexual reproduction in nature. However, populations of asexual individuals may attain high levels of genetic diversity through within‐lineage diversification, replicate transitions to asexuality from sexual ancestors and migration. How these mechanisms affect genetic variation in populations of closely related sexual and asexual taxa can therefore provide insights into the role of genetic diversity for the maintenance of sexual reproduction. Here, we evaluate patterns of intra‐ and interpopulation genetic diversity in sexual and asexual populations of Aptinothrips rufus grass thrips. Asexual A. rufus populations are found throughout the world, whereas sexual populations appear to be confined to few locations in the Mediterranean region. We found that asexual A. rufus populations are characterized by extremely high levels of genetic diversity, both in comparison with their sexual relatives and in comparison with other asexual species. Migration is extensive among asexual populations over large geographic distances, whereas close sexual populations are strongly isolated from each other. The combination of extensive migration with replicate evolution of asexual lineages, and a past demographic expansion in at least one of them, generated high local clone diversities in A. rufus. These high clone diversities in asexual populations may mimic certain benefits conferred by sex via genetic diversity and could help explain the extreme success of asexual A. rufus populations.     

Fluorescent Viability Stains to Probe the Metabolic Status of Aflatoxigenic Fungus in Dual Culture of <Emphasis Type="Italic">Aspergillus flavus</Emphasis> and <Emphasis Type="Italic">Pichia anomala</Emphasis>

The metabolic activity of the aflatoxigenic fungus, Aspergillus flavus co-cultured with the biocontrol yeast, Pichia anomala was examined using several viability stains. Both the FUN-1 stain and the combined use of DiBAC₄(5) with CDFA-AM stains were applied in this study. The results suggest that the ATP-generating system in A. flavus was inactivated as the ratio of yeasts to fungi increased in the dual culture. A decrease in hyphal membrane potential and esterase activity was substantiated by the combined stains of DiBAC₄(5) and CDFA-AM. Reduced metabolic function in conjunction with cell wall damage of A. flavus hindered the growth and biomass production of this fungus. Viability stains such as FUN-1 and DiBAC₄(5) with CDFA-AM may assist in elucidating the biocontrol mechanism by allowing for the visualization of the antagonistic effect of yeast species on target fungi in situ, as well as for screening potent biocontrol yeast agents against fungal pathogens.     

Characterization of oxylipins and dioxygenase genes in the asexual fungus Aspergillus niger

Background  

Aspergillus niger is an ascomycetous fungus that is known to reproduce through asexual spores, only. Interestingly, recent genome analysis of A. niger has revealed the presence of a full complement of functional genes related to sexual reproduction [1]. An example of such genes are the dioxygenase genes which in Aspergillus nidulans, have been shown to be connected to oxylipin production and regulation of both sexual and asexual sporulation [2–4]. Nevertheless, the presence of sex related genes alone does not confirm sexual sporulation in A. niger.     

Mitochondrial DNA diversity and geographical distribution of sexual and asexual strains of the braconid parasitoid Meteorus pulchricornis

Asexuality is an important tool with regard to the use of parasitoid wasps as biocontrol agents. Asexual (apomictic thelytokous) strains of Meteorus pulchricornis (Wesmael) (Hymenoptera: Braconidae), a polyphagous endoparasitoid of lepidopteran larvae, are sympatric with sexual (arrhenotokous) strains in Japan. The results of phylogenetic analyses of mitochondrial cytochrome c oxidase subunit I (COI) sequences indicated two major haplotype groups on the Japanese islands. The northern group I predominantly contained sexual strains, whereas the southern group II contained both sexual and asexual strains. Most asexual strains were likely derived within group II. An asexual strain recently established in New Zealand has the identical haplotype to a strain in Japan and was proven to have originated from East Asia. Three hypotheses on the evolution of asexuality are discussed for this parasitoid wasp: recessive gene, hybridization, and cytoplasmic element.