Viable Ascospores Research Articles

Genetically Isolated Population of <i>Saccharomyces bayanus</i> in New Zealand and Australia

The genetic relatedness of yeasts in the Saccharomyces bayanus complex has been studied using the methods of molecular and classical genetics. A divergent population of S. bayanus has been found in New Zealand and Australia. The S. bayanus complex includes four genetic populations: S. bayanus var. bayanus, S. bayanus var. uvarum, S. eubayanus and New Zealand population. The strains of the New Zealand population differ significantly in the nucleotide sequences of nuclear (FSY1, HIS3, MET2) and mitochondrial (FUN14, COX2) genes and form semi-sterile hybrids with other populations: viability of ascospores is 6.2–23.3%. There is no complete interspecific postzygotic isolation between S. bayanus var. bayanus, S. bayanus var. uvarum, S. eubayanus, and New Zealand populations: all hybrids showed regular meiotic segregation of control auxotrophic markers. According to the results obtained, four genetic populations belong to the same biological species with genomic divergence at the level of taxonomic varieties.

Overexpression of the Neurospora crassa Transcription Factor fsd‐1 Inhibits Mating

The filamentous fungus Neurospora crassa can undergo both asexual and sexual reproduction. However, little is known about the molecular mechanisms that regulate the N. crassa female sexual development cycle. Previous research has shown that deletion strains of the transcription factor fsd‐1 have delayed development of female reproductive structures and are sterile. fsd‐1 is transcribed into three different transcripts, which differ by the length and intron/exon structure of their 5’ untranslated region, and these transcripts vary in expression level during the N. crassa life cycle. fsd‐1 controls the development of female reproductive structures in N. crassa, which affects many downstream processes. The goal of this project is to phenotypically characterize the reproductive ability of strains overexpressing fsd‐1, for each of the three transcripts, and we hypothesize that fsd‐1 overexpression will affect the reproductive ability of these strains. Characterization of the changes in transcriptional expression caused by fsd‐1 overexpression will give us insight into the fsd‐1 regulon. Using Illumina RNA sequencing, we mapped the genome‐wide transcriptional changes that occur as a result of fsd‐1 overexpression. Analysis of the reproductive abilities of strains that overexpress fsd‐1 showed that these strains can produce spores, but that the spores do not germinate. Our RNAseq and phenotype experiments revealed that fsd‐1 regulates processes including melanin synthesis, cell wall formation, and ascospore viability, and plays a key role in sexual development.

Response of Resistant Hazelnut Cultivars to Eastern Filbert Blight in the Pacific Northwest

The hazelnut (Corylus avellana) cultivars ‘Ennis’, ‘Jefferson’, and ‘McDonald’ were inoculated with various doses of Anisogramma anomala ascospores to determine whether high concentrations of ascospores resulted in eastern filbert blight (EFB) cankers. Trees at budbreak or early shoot growth were inoculated with 0, 104, 105, 106, and 107 ascospores/ml, placed in a mist chamber for 72 h, and held for 1.5 years for canker development. Ennis trees became infected when the concentration was at least 104 ascospores/ml whereas resistant trees became infected only when the concentration was 106 or 107 ascospores/ml. The progression of EFB symptoms in a 10-ha Jefferson hazelnut orchard was also documented over a 5-year period from 2010 to 2014. EFB cankers were first observed fall 2010, averaged 7 cm in length, and were characterized by longitudinal cracks and flat or slightly sunken areas on trunks. Some cankers had a few small stromata with viable ascospores. Cankers on 143 trees were measured after leaf fall each year. Comparison between the initial canker size and the final size measured in 2014 indicated that 49% of the cankers continued to get larger but there was no indication of whether the cankers would increase or decrease in size over time. Growers should use fungicides the first few years after planting until trees are established and vigorously growing. Cankers that develop on resistant trees should be removed and destroyed prior to budbreak in the spring.

A single out-of-season fungicide application reduces the grape powdery mildew inoculum

Strategies for the control of grape powdery mildew, caused by Erysiphe necator, Schw., require consideration of the development of fungicide resistance. We investigated the effect of inoculum reduction by fungicides with low resistance risk (copper oxychloride, potassium bicarbonate, meptyldinocap, wettable sulphur) in the laboratory and as individual post-harvest and bud break treatments. Ascospore viability and release were analysed after a post-harvest treatment and disease incidence and severity of grapes were determined following a bud break treatment. In the laboratory all fungicides reduced the viability (spring 2018 p = 0.008; autumn 2018 p = 0.021; spring 2019 p = 0.010; autumn 2019 p = 0.484) and the release (spring 2018 p = 0.007; autumn 2018 < 0.001; spring 2019 < 0.001) of ascospores, although the decrease was more pronounced in spring compared to autumn. At Krems, the post-harvest field application of copper oxychloride, potassium bicarbonate, meptyldinocap and wettable sulphur in autumn 2017 reduced the percentage of viable chasmothecia significantly (p < 0.001) by 56–74%. In autumn 2018, the tested fungicides had no effect on the percentage of viable chasmothecia (p = 0.129). At Andau, the percentage of viable chasmothecia was only reduced by copper oxychloride (p = 0.029). The amount of viable ascospores was significantly (p = 0.002) lowered after a post-harvest field treatment with the fungicides by 46–70%, whereas for meptyldinocap only a trend could be determined. Disease incidence and severity were not influenced by sulphur applications at bud break (p = 0.689, p = 0.224) and by different levels of chasmothecia (p = 0.104, p = 0.353). A trend in one year, that a high amount of chasmothecia increased the disease incidence, was related to the weather conditions and the duration from the time of first ascosporic infection until flowering. The data indicates the importance of an effective control with fungicides during the pre-flowering stage in years with a long time from primary infection until flowering. Furthermore, this study shows that post-harvest sprays with low risk fungicides can help to reduce the number of fungicide treatments needed for powdery mildew control at the end of the in-season spray program. This measure could be integrated into an environmentally friendly pesticide reduction program for controlling powdery mildew in viticulture.

Powdery mildew epidemics in the nursery: role of chasmothecia in epidemic initiation and fungicide regimes to limit chasmothecia formation

Powdery mildew caused by Podosphaera cerasi is a detrimental fungal disease for the sweet cherry industry. Epidemics in orchards are initialized by ascospores that are released from chasmothecia generated in the fall of the previous year, however, the initiation of epidemics in nurseries has not been explored. The current study aimed to evaluate the origin of disease epidemics in Washington State cherry tree nurseries and to evaluate the efficacy of different fungicide regimes to suppress chasmothecia numbers and ascospore viability. Greenhouse and lab assays in the present study showed that chasmothecia from the nursery are not contributing to powdery mildew epidemics and the disease is originating from an external source such as wind-blown conidia from neighboring orchards. Additionally, intensive synthetic fungicide regimes and late-season JMS Stylet-Oil applications were found to be viable options to inhibit chasmothecia formation. As chasmothecia with a nursery origin are not the initial inoculum, late season fungicide applications to limit chasmothcia formation should only be applied in an orchard setting. Orchard managers can limit epidemics by implementing late-season fungicide applications into their IPM plan that target chasmothecia. Future research should evaluate the dispersal of conidia throughout the cherry tree growing industry of Washington and their role in powdery mildew epidemics in nurseries. Moreover, the physiological and genetic differences between chasmothecia in the nursery and the orchard should be evaluated.

Survival and viability of ascospores of Erysiphe necator in Austrian vineyards

Grape powdery mildew caused by Erysiphe necator is a limiting factor in grape production. In order to develop effective management strategies, the factors influencing the survival of the fungus need to be identified and characterized accordingly. For this purpose, we focused on the effect of weather conditions during overwintering on the survival and viability of ascospores. In spring 2017 and 2018, grape leaf litter and bark samples were collected and examined to determine the density of chasmothecia and the viability of ascospores in various Austrian vine growing regions. There were obvious differences in the amounts of chasmothecia between both years and all examined vineyards. Lower quantities of chasmothecia were detected on the exfoliating bark compared to leaf litter, with up to 37% of chasmothecia containing viable ascospores. In comparison, chasmothecia from leaf litter showed a lower viability (up to 5%). The number of viable ascospores per head of vine ranged from 0 to 351 and from 0 to 251 in 2017 and 2018, respectively, and showed partly a strong variation within one location in both years. The infectivity of ascospores on detached leaves was confirmed. In a survival experiment, chasmothecia, when incubated at 7 °C, released more viable ascospores than chasmothecia incubated at 17 °C. After an incubation period of 30 weeks, only chasmothecia stored at the lower temperature contained viable ascospores. However, the mean temperature differences of 0.1 to 1.2 °C during the period of formation of chasmothecia to bud break in both years and six investigated areas did not explain differences in the viability of the ascospores. Differences in vineyard management seem to be of particular importance here.

Diversity and Postzygotic Evolution of the Mitochondrial Genome in Hybrids of Saccharomyces Species Isolated by Double Sterility Barrier.

Eukaryotic species are reproductively isolated by sterility barriers that prevent interspecies fertilization (prezygotic sterility barrier) or the fertilization results in infertile offspring (postzygotic sterility barrier). The Saccharomyces species are isolated by postzygotic sterility barriers. Their allodiploid hybrids form no viable gametes (ascospores) and the viable ascospores of the allotetraploids cannot fertilize (conjugate). Our previous work revealed that this mechanism of reproductive isolation differs from those operating in plants and animals and we designated it double sterility barrier (the failure of homeologous chromosomes to pair and the repression of mating by mating-type heterozygosity). Other studies implicated nucleo-mitochondrial incompatibilities in the sterility of the Saccharomyces hybrids, a mechanism assumed to play a central role in the reproductive isolation of animal species. In this project the mitochondrial genomes of 50 cevarum (S. cerevisiae × S. uvarum) hybrids were analyzed. 62% had S. cerevisiae mitotypes, 4% had S. uvarum mitotypes, and 34% had recombinant mitotypes. All but one hybrid formed viable spores indicating that they had genomes larger than allodiploid. Most of these spores were sterile (no sporulation in the clone of vegetative descendants; a feature characteristic of allodiploids). But regardless of their mitotypes, most hybrids could also form fertile alloaneuploid spore clones at low frequencies upon the loss of the MAT-carrying chromosome of the S. uvarum subgenome during meiosis. Hence, the cevarum alloploid nuclear genome is compatible with both parental mitochondrial genomes as well as with their recombinants, and the sterility of the hybrids is maintained by the double sterility barrier (determined in the nuclear genome) rather than by nucleo-mitochondrial incompatibilities. During allotetraploid sporulation both the nuclear and the mitochondrial genomes of the hybrids could segregate but no correlation was observed between the sterility or the fertility of the spore clones and their mitotypes. Nucleo-mitochondrial incompatibility was manifested as respiration deficiency in certain meiotic segregants. As respiration is required for meiosis-sporulation but not for fertilization (conjugation), these segregants were deficient only in sporulation. Thus, the nucleo-mitochondrial incompatibility affects the sexual processes only indirectly through the inactivation of respiration and causes only partial sterility in certain segregant spore clones.

DnmA and FisA Mediate Mitochondria and Peroxisome Fission, and Regulate Mitochondrial Function, ROS Production and Development in Aspergillus nidulans.

The dynamin-like protein Drp1 and its receptor Fis-1 are required for mitochondria and peroxisome fission in animal and yeast cells. Here, we show that in the fungus Aspergillus nidulans the lack of Drp1 and Fis-1 homologs DnmA and FisA has strong developmental defects, leading to a notable decrease in hyphal growth and asexual and sexual sporulation, with some of these defects being aggravated or partially remediated by different carbon sources. Although both DnmA and FisA, are essential for mitochondrial fission, participate in peroxisomal division and are fully required for H2O2-induced mitochondrial division, they also appear to play differential functions. Despite their lack of mitochondrial division, ΔdnmA and ΔfisA mutants segregate mitochondria to conidiogenic cells and produce viable conidia that inherit a single mitochondrion. During sexual differentiation, ΔdnmA and ΔfisA mutants develop fruiting bodies (cleistothecia) that differentiate excessive ascogenous tissue and a reduced number of viable ascospores. ΔdnmA and ΔfisA mutants show decreased respiration and notably high levels of mitochondrial reactive oxygen species (ROS), which likely correspond to superoxide. Regardless of this, ΔdnmA mutants can respond to an external H2O2 challenge by re-localizing the MAP kinase-activated protein kinase (MAPKAP) SrkA from the cytoplasm to the nuclei. Our results show that ROS levels regulate mitochondrial dynamics while a lack of mitochondrial fission results in lower respiration, increased mitochondrial ROS and developmental defects, indicating that ROS, mitochondrial division and development are critically interrelated processes.

A transmission ratio distortion and the ‘max-4’ ascus phenotype: Do both reflect the same Bateson-Dobzhansky-Muller Incompatibility emerging during trans-species introgression of translocations in Neurospora?

The T(EB4)Nta, T(IBj5)Nta, and T(B362i)NtA strains were constructed by introgressing the insertional translocations EB4, IBj5, and B362i from Neurospora crassa into the related species N. tetrasperma. The progeny from crosses of T(IBj5)Nta and T(B362i)NtA with opposite mating-type derivatives of the standard N. tetrasperma strain 85 exhibited a unique and unprecedented transmission ratio distortion (TRD) that disfavored homokaryons produced following alternate segregation relative to those produced following adjacent-1 segregation. The TRD was not evident among the [mat A + mat a] dikaryons produced following either segregation. Further, crosses of the T(IBj5)Nta and T(B362i)NtA strains with the Eight spore (E) mutant showed an unusual ascus phenotype called 'max-4'. We propose that the TRD and the max-4 phenotype are manifestations of the same Bateson-Dobzhansky-Muller incompatibility (BDMI). Since the TRD selects against 2/3 of the homokaryotic progeny from each introgression cross, the BDMI would have enriched for the dikaryotic progeny in the viable ascospores, and thus, paradoxically, facilitated the introgressions.

Identification and pathogenicity of Fusarium species associated with pokkah boeng of sugarcane in Brazil

Brazil is the world’s biggest producer of sugarcane (Saccharum spp. hybrids). Pokkah boeng is an important fungal disease in this crop caused by members of the Fusarium fujikuroi species complex (FFSC) and characterized by deformation of the aerial part of the plant and stem rot. While the occurrence of symptoms has been reported in plantations in Brazil, no official reports of the disease exist. In this study, species of the FFSC were identified that are associated with sugarcane plants with symptoms of pokkah boeng in Brazil. This was achieved using two‐loci molecular phylogeny, sexual compatibility and analysis of morphological markers. The ability of strains to cause disease in plants of sugarcane, maize, sorghum and millet was also evaluated. The 39 isolates studied were identified as F. sacchari, F. proliferatum and another, still unknown, phylogenetic lineage that is sister to F. andiyazi. Crossing field isolates of F. sacchari and F. proliferatum with their respective tester strains produced fertile perithecia and viable ascospores. All three species induced symptoms of pokkah boeng on inoculated sugarcane plants and caused stem rot in maize, sorghum and millet. Symptoms on sugarcane are chlorosis and necrosis of leaves, punctured lesions, twisted leaves, reduction of the total leaf area, death of the top of the plant and stalk rot. The findings confirmed the aetiology of the disease in Brazil, generated basic knowledge for the development of strategies for diagnosis and monitoring of the disease and support breeding programmes for selecting resistant germplasm.

HMGB proteins are required for sexual development in Aspergillus nidulans.

Aspergillus nidulans has three high mobility group box (HMGB) proteins, HmbA, HmbB and HmbC that are chromatin-associated architectural proteins involved in DNA-related functions. By creating and studying deletion strains in both veA+ and veA1 background, we have characterized the role of HmbA, HmbB and HmbC in sexual development. Expression of the mating-type MAT1-1 and MAT1-2 coding genes were found to be extremely down-regulated in all three mutants on day 4 of sexual development, which results in deficient ascospore production and/or ascospore viability in the mutants. In addition, we found that HmbA and HmbB play also a role in sensing of and response to environmental signals, while HmbC functionally interacts with VeA, a key regulator of the coordination of asexual and sexual development, as well as of secondary metabolism.

Characterization of Sclerotinia sclerotiorum airborne inoculum, the widespread agent of white mould disease

A means to rationalize the use of fungicides for crop protection and to make agriculture friendlier to environment and human health is to develop disease-risk forecasting systems based on the assessment of airborne inoculum abundance. Sclerotinia sclerotiorum, the pandemic agent of white mould disease, is disseminated via the atmosphere in the form of ascospores. These airborne spores are the primary sources of inoculum initiating the majority of epidemics. However, for witloof chicory (Cichorium intybus var. foliosum L.), there is no data about airborne inoculum of S. sclerotiorum, which makes it difficult to develop a forecasting model. In the present study, we characterized the temporal evolution of the abundance and of the genetic characteristics of S. sclerotiorum inoculum on a witloof chicory field located in Northern France over a 3-year period. To our knowledge, this study provides the first quantification of viable airborne populations of this fungus in witloof chicory crops. Moreover, it provides the first genetic characterization of S. sclerotiorum airborne inoculum. The results show that viable ascospores were present through 80% of the sampling dates. A significant correlation between abundance of airborne ascospores and local relative humidity suggests a local origin of inoculum. However, the existence of a slight genetic differentiation between isolates carried by air masses coming from the West and from the North-West is compatible with the hypothesis of a distant origin of S. sclerotiorum inoculum. We discuss the additional studies that are envisioned to clarify the origin of S. sclerotiorum airborne inoculum in witloof chicory fields.

Assessing the Aerial Interconnectivity of Distant Reservoirs of Sclerotinia sclerotiorum.

Many phytopathogenic fungi are disseminated as spores via the atmosphere from short to long distances. The distance of dissemination determines the extent to which plant diseases can spread and novel genotypes of pathogens can invade new territories. Predictive tools including models that forecast the arrival of spores in areas where susceptible crops are grown can help to more efficiently manage crop health. However, such models are difficult to establish for fungi with broad host ranges because sources of inoculum cannot be readily identified. Sclerotinia sclerotiorum, the pandemic agent of white mold disease, can attack >400 plant species including economically important crops. Monitoring airborne inoculum of S. sclerotiorum in several French cropping areas has shown that viable ascospores are present in the air almost all the time, even when no susceptible crops are nearby. This raises the hypothesis of a distant origin of airborne inoculum. The objective of the present study was to determine the interconnectivity of reservoirs of S. sclerotiorum from distant regions based on networks of air mass movement. Viable airborne inoculum of S. sclerotiorum was collected in four distinct regions of France and 498 strains were genotyped with 16 specific microsatellite markers and compared among the regions. Air mass movements were inferred using the HYSPLIT model and archived meteorological data from the global data assimilation system (GDAS). The results show that up to 700 km could separate collection sites that shared the same haplotypes. There was low or no genetic differentiation between strains collected from the four sites. The rate of aerial connectivity between two sites varied according to the direction considered. The results also show that the aerial connectivity between sites is a better indicator of the probability of the incoming component (PIC) of inoculum at a given site from another one than is geographic distance. We identified the links between specific sites in the trajectories of air masses and we quantified the frequencies at which the directional links occurred as a proof-of-concept for an operational method to assess the arrival of airborne inoculum in a given area from distant origins.

Sexuality and compatibility of Bipolaris sorokiniana and segregation pattern in teleomorph (Cochliobolus sativus): geographic origin and segregation ratio

Bipolaris sorokiniana causes leaf blotch of wheat, inflicting substantial grain yield losses in worldwide. Knowledge of sexual recombination of the fungal pathogen can help to implement effective disease management strategies. Randomly selected 51 isolates of B. sorokiniana was studied to test compatibility with all possible combinations. Six most fertile crosses were found, among them two combinations (BS-81 x BS-83 and BS-60 x BS-134) produced abundant pseudothecia with mature and viable ascospores. This result disclosed that most compatible isolates belonging in the Lower and High Ganges river floodplain area of Bangladesh. One hundred and fifty ascospores were examined to evaluate morphological segregation pattern. Morphological characters (ascospores color, shape, size, Septation number and colony color) of ascospores reject the null hypothesis. A clear 3:1 and 9:7 segregation ratios for ascospores color, mycelial colony color, shape of ascospores; and size of ascospores, septation number of ascospores were found in F2 progeny. RAPD technique was tested to determine molecular segregation pattern in 75 F2 progeny following reproducible polymorphic prominent bands. After test of independence, each trait showed significant and fit to the 1:1 segregation ratio expected for Mendelian trait.

Heterothallic sexual reproduction in three canker-inducing tree pathogens within the Fusarium torreyae species complex

ABSTRACTFusarium zanthoxyli and F. continuum are sister taxa that are the etiological agents of canker disease of prickly ash (Zanthoxylum bungeanum) in northern China. These two pathogens, together with F. torreyae, the causal agent of canker disease of the critically endangered conifer Florida torreya (Torreya taxifolia) from northern Florida and southwestern Georgia, constitute a novel clade, the F. torreyae species complex. To assess their reproductive mode, a polymerase chain reaction (PCR) assay targeting the MAT1-1 and MAT1-2 idiomorphs was designed and validated, using MAT sequences mined from the whole-genome sequence of the three F. torreyae clade pathogens and several closely related fusaria. Results of the MAT idiomorph PCR assay indicated that isolates of the three pathogens were MAT1-1 or MAT1-2. When MAT1-1 and MAT1-2 isolates of each species were crossed on carrot agar, all of the F. zanthoxyli (N = 30) and F. continuum (N = 3) isolates tested were female fertile, yielding mature perithecia with viable ascospores. By comparison, only one pairing of the five different isolates of F. torreyae produced perithecia; however, the majority of the asci in this cross aborted or produced fewer than eight ascospores. Of the three temperatures tested (i.e., 22, 25, and 27 C), the optimal temperature for perithecium production was 22–25 C in F. zanthoxyli and 25 C in F. continuum and F. torreyae. Ascospore progeny from three separate crosses of F. zanthoxyli and F. continuum and one cross of F. torreyae were genotyped to assess whether they were the products of genetic recombination and sexual reproduction. Genotyping of 34–40 progeny from the F. zanthoxyli and F. continuum crosses confirmed that they were the products of sexual reproduction. However, only 36% of the progeny in the F. torreyae cross were recombinant, which was roughly half of the nonparental progeny expected with three markers segregating.

Inert Materials as Long-Term Carriers and Disseminators of Viable Leptosphaeria maculans Ascospores and Wider Implications for Ascomycete Pathogens.

The viability of ascospores of the Phoma stem canker (blackleg) pathogen, Leptosphaeria maculans, was tested on a range of carrier materials, including metals, fabrics, woods, and plastics, and under different temperature conditions of 23 and 4, 36 and 14, and 45 and 15°C day and night, respectively. At 23 and 4°C (day and night, respectively), ascospores remained viable for up to 240 days on Tasmanian oak (Eucalyptus regnans) and pine wood (Pinus radiata). At 36 and 14°C (day and night, respectively), ascospores remained viable on pine wood for up to 180 days. At 45 and 15°C (day and night, respectively), ascospores remained viable up to 60 days on jute. There were also significant differences (P < 0.001) between carrier materials in their abilities to retain ascospores following washing. At least 30% of intact ascospores recovered from inert carrier materials were able to germinate on artificial growth media within 48 h of recovery and some ascospores were still viable after 240 days. These findings confirm that L. maculans ascospores remain viable for a much longer time in the absence of a host than previously considered. This demonstrates the importance of inert materials as long-term and long-distance carriers of viable L. maculans ascospores, and highlights their potential role for spread of L. maculans races to new regions and countries via farming equipment, clothing, and other associated materials. Local, national, and international biosecurity agencies need to be aware that the risks of spread of ascomycete plant, animal, and human pathogens via inert materials are significantly greater than currently assessed.

Population structure, genetic diversity, and sexual state of the rice brown spot pathogen Bipolaris oryzae from three Asian countries

Bipolaris oryzae causes brown spot in rice (Oryza sativa) inflicting substantial grain yield losses worldwide. Knowledge of the population structure, genetic diversity and sexual recombination of the fungal pathogen can help to implement effective disease management strategies. In this study, B. oryzae isolates sampled from Iran, the Philippines and Japan were analysed with 12 simple‐sequence repeat (SSR) markers, newly developed from the genome sequence of the fungus. Among the 288 B. oryzae isolates genotyped, 278 unique haplotypes were identified. High genotype numbers (richness) with even distribution (evenness) were found within the collection sites. Both mating types, MAT1‐1 and MAT1‐2, were present in each collection area, and the sexual state was induced under controlled conditions with production of viable ascospores. However, the tests of linkage disequilibrium rejected of the hypothesis of random mating. Discriminant analysis of principal components (DAPC) revealed that the B. oryzae collection formed three clusters, each consisting of isolates from different collection sites. Analysis of molecular variance (amova) showed that genetic variation among clusters was 18.7%, with the rest of the variation distributed within clusters (RST = 0.187, P &lt; 0.001). Statistically significant pairwise genetic differentiation was found between the clusters. These results show that Asian B. oryzae isolates are genetically diverse, and, overall, distributed in three groups. These findings will be helpful in managing the disease and guide the use of representative isolates needed for selection of resistant rice varieties.

Genetic relationship and biological status of the industrially important yeast Saccharomyces eubayanus Sampaio et al.

The genomes of the recently discovered yeast Saccharomyces eubayanus and traditional S. cerevisiae are known to be found in the yeast S. pastorianus (syn. S. carlsbergensis), which are essential for brewing. The cryotolerant yeast S. bayanus var. uvarum is of great importance for production of some wines. Based on ascospore viability and meiotic recombination of the control parental markers in hybrids, we have shown that there is no complete interspecies post-zygotic isolation between the yeasts S. eubayanus, S. bayanus var. bayanus and S. bayanus var. uvarum. The genetic data presented indicate that all of the three taxa belong to the same species.

Crosses Heterozygous for Hybrid Neurospora Translocation Strains Show Transmission Ratio Distortion Disfavoring Homokaryotic Ascospores Made Following Alternate Segregation.

By introgressing Neurospora crassa translocations into N. tetrasperma, we constructed heterokaryons bearing haploid nuclei of opposite mating types, and either the translocation and normal sequence chromosomes (i.e., [T + N]) or a duplication and its complementary deficiency (i.e., [Dp + Df]). The [T + N] heterokaryons result from alternate segregation of homologous centromeres, whereas adjacent-1 segregation generates [Dp + Df]. Self-cross of either heterokaryon produces [T + N] and [Dp + Df] progeny. Occasionally during N. tetrasperma ascus development, a pair of smaller homokaryotic ascospores replaces a heterokaryotic ascospore. Crosses with the Eight-spore mutant increase such replacement, and can generate asci with eight homokaryotic ascospores, either 4T + 4N from alternate segregation, or 4Dp + 4Df from adjacent-1 segregation. Crosses of some of the introgressed translocation strains with normal sequence N. tetrasperma produced more Dp than T or N homokaryotic progeny. We suggest this is due to an insufficiency for a presumptive ascospore maturation factor, which increases the chance that, in asci with > 4 viable ascospores, none properly mature. Since only four viable ascospores (Dp or [Dp + Df]) share the limiting factor following adjacent-1 segregation, whereas four to eight ascospores compete for it following alternate segregation, this would explain why Dp homokaryons outnumber T and N types, whereas the heterokaryons are not as affected. We believe that this novel form of transmission ratio distortion is caused by a Bateson–Dobzhansky–Muller Incompatibility (BDMI) triggered by an N. crassa gene in the N. tetrasperma background. Heterokaryons tend not to out-cross, and crosses of Dp strains are barren, thus the BDMI impedes interspecies gene flow.

Germination of Fusarium graminearum Ascospores and Wheat Infection are Affected by Dry Periods and by Temperature and Humidity During Dry Periods.

The effects of temperature and relative humidity (RH) on germination of Fusarium graminearum ascospores, and of dry periods (DP) of different lengths and of temperature and RH during DP on ascospore survival were studied both in vitro and in planta. Optimal temperatures for ascospore germination at 100% RH were 20 and 25°C; germination was ≤5% when ascospores were incubated at 20°C and RH ≤ 93.5%. Viable ascospores were found at all tested combinations of DP duration (0 to 48 h) × temperature (5 to 40°C) or RH (32.5 to 100% RH). Germination declined as DP duration and temperature increased. Germination was lower for ascospores kept at 65.5% RH during the DP than at 76.0, 32.5, or 93.5% RH. Equations were developed describing the relationships between ascospore germination, DP duration and temperature or RH during DP. Durum wheat spikes were inoculated with ascospores and kept dry for 0 to 48 h at approximately 15°C and 65% RH; plants were then kept in saturated atmosphere for 48 h to favor infection. Fungal biomass, measured as F. graminearum DNA by quantitative polymerase chain reaction, declined as DP increased to 24 and 48 h at 3 and 9 days postinfection but not in spikes at maturity.