Anther Smut Fungus Microbotryum Violaceum Research Articles

Disease Status and Population Origin Effects on Floral Scent: Potential Consequences for Oviposition and Fruit Predation in A Complex Interaction Between A Plant, Fungus, and Noctuid Moth

In the Silene latifolia-Hadena bicruris nursery pollination system, the Hadena moth is both pollinator and seed predator of its host plant. Floral scent, which differs among S. latifolia individuals and populations, is important for adult Hadena to locate its host. However, the success of moth larvae is strongly reduced if hosts are infected by the anther smut fungus Microbotryum violaceum, a pathogen that is transmitted by flower visitors. There were no qualitative differences between the scent of flowers from healthy and diseased plants. In addition, electroantennographic measurements showed that Hadena responded to the same subset of 19 compounds in samples collected from healthy and diseased plants. However, there were significant quantitative differences in scent profiles. Flowers from diseased plants emitted both a lower absolute amount of floral scent and had a different scent pattern, mainly due to their lower absolute amount of lilac aldehyde, whereas their amount of (E)-beta-ocimene was similar to that in healthy flowers. Dual choice behavioral wind tunnel tests using differently scented flowers confirmed that moths respond to both qualitative and quantitative aspects of floral scent, suggesting that they could use differences in floral scent between healthy and infected plants to discriminate against diseased plants. Population mean fruit predation rates significantly increased with population mean levels of the emission rates of lilac aldehyde per flower, indicating that selection on floral scent compounds may not only be driven by effects on pollinator attraction but also by effects on fruit predation. However, variation in mean emission rates of scent compounds per flower generally could not explain the higher fruit predation in populations originating from the introduced North American range compared to populations native to Europe.

First Report of Anther Smut Caused by Microbotryum violaceum on Forked Catchfly (Silene dichotoma) in Turkey.

Forked catchfly (Silene dichotoma Ehrh.), family Caryophyllaceae, is a common and native plant in rangelands and pastures in Turkey. It is also an introduced plant that is widely distributed in North America. In May of 2007, approximately 20 forked catchfly plants on the campus of Ondokuz Mayis University in Samsun, Turkey were found diseased with the anther smut fungus Microbotryum violaceum (Pers.:pers.) G. Deml & Oberw. (Basidiomycota, Microbotryomycetes, Microbotryales [3], Microbotryaceae). All anthers in all flowers of diseased plants were smutted. Diseased flowers were collected, air dried, and sent to the quarantine facility of the Foreign Disease-Weed Science Research Unit (FDWSRU), USDA/ARS, Fort Detrick, MD. Teliospores within the flowers were extracted and observed microscopically. Teliospores were globose, 6 to 9 μm (mean 6.5 μm) in diameter, pale violet, with reticulate walls, and matching the description of M. violaceum (4). Nucleotide sequences for the internal transcribed spacers (ITS 1 and 2) and 5.8S ribosomal region (GenBank Accession No. EU122308) were aligned with other sequences in GenBank with the BLAST algorithm. Sequences of this isolate aligned 99% with sequences of other isolates of M. violaceum, M. lychnidis-dioicae (A.P. de Candolle ex J.I. Liro) G. Deml & F. Oberwinkler, and M. silenes-inflatae (A.P. de Candolle ex J.I. Liro) G. Deml & F. Oberwinkler and clustered with other M. violaceum isolates. M. violaceum is an obligate parasite of many plant species in the Caryophyllaceae family, and the fungus has been widely studied as a model for population genetics and evolutionary biology (2). To our knowledge, this is the first report of M. violaceum parasitizing forked catchfly in Turkey, and is the only report of this fungus-plant interaction in Asia Minor (1). The fungus has not been reported from this plant in North America (1). A voucher specimen has been deposited with the U.S. National Fungus Collections (BPI 878235) and living spores are being maintained at FDWSRU.

Variation in Breeding System Traits within and among Populations ofMicrobotryum violaceumonSilene dioica

Breeding systems exert profound effects on the amount and distribution of genetic diversity within and among populations. Knowledge of breeding systems is also important for understanding dynamics between coevolving organisms, e.g., pathogen‐host interactions. Here we study the breeding system of the obligate anther smut Microbotryum violaceum on Silene dioica. Microbotryum violaceum is capable of both inbreeding and outcrossing, but several recent studies on other host races have indicated that automixis via intrapromycelial mating is the predominant breeding system. Compared with conjugations between cells from different meioses, automixis results in slower loss of heterozygosity and faster production of infectious hypha. However, high rates of intrapromycelial matings have been suggested to invoke a fitness cost due to production of fewer infectious dikaryons. Working with single strains under standardized laboratory conditions, we studied traits that could influence the distribution of genetic variability and pathogen fitness. We found that intrapromycelial mating is the dominant conjugation form for M. violaceum var. dioica but that the breeding system varies, partly because of genetic differences, both within and among populations. Further, we did not find the predicted fitness reduction for intrapromycelial matings, suggesting that intrapromycelial mating is a highly favorable breeding system for M. violaceum.

Host Fidelity of the Pollinator Guilds of Silene dioica and Silene latifolia: Possible Consequences for Sympatric Host Race Differentiation of a Vectored Plant Disease

Host fidelity can play an important role in sympatric host race formation of phytophagous insects by providing a mechanism for prezygotic reproductive isolation. Similarly, but less recognized, host fidelity of insects could provide a mechanism for maintaining host‐specific differentiation among insect‐vectored pathogens. We studied the transfer of fluorescent dye–mimicking spores of the pollinator‐transmitted anther smut fungus Microbotryum violaceum in experimental plots of two of its closely related hosts, Silene dioica and Silene latifolia. Mean rates of diurnal interspecific transfer were 26% from S. latifolia to S. dioica and 34% in the reverse direction, suggesting that host fidelity of pollinators per se cannot account for the maintenance of genetically differentiated host races of this fungus observed in sympatry. In a large natural sympatric population of the two hosts, S. dioica flowers experienced a sixfold higher probability of receiving spores from conspecifics than from heterospecifics during the time frame of susceptibility to successful systemic infection due to their earlier onset of flowering. Also, spores from heterospecifics were an order of magnitude farther away than spores from conspecifics. We therefore speculate that the observed sympatric host‐specific differentiation is due to a combination of factors, including vector behavior and host spatial and temporal structure.

Mimicry in plant-parasitic fungi

Mimicry is the close resemblance of one living organism (the mimic) to another (the model), leading to misidentification by a third organism (the operator). Similar to other organism groups, certain species of plant-parasitic fungi are known to engage in mimetic relationships, thereby increasing their fitness. In some cases, fungal infection can lead to the formation of flower mimics (pseudo flowers) that attract insect pollinators via visual and/or olfactory cues; these insects then either transmit fungal gametes to accomplish outcrossing (e.g. in some heterothallic rust fungi belonging to the genera Puccinia and Uromyces) or vector infectious spores to healthy plants, thereby spreading disease (e.g. in the anther smut fungus Microbotryum violaceum and the mummy berry pathogen Monilinia vaccinii-corymbosi). In what is termed aggressive mimicry, some specialized plant-parasitic fungi are able to mimic host structures or host molecules to gain access to resources. An example is M. vaccinii-corymbosi, whose conidia and germ tubes, respectively, mimic host pollen grains and pollen tubes anatomically and physiologically, allowing the pathogen to gain entry into the host's ovary via stigma and style. We review these and other examples of mimicry by plant-parasitic fungi and some of the mechanisms, signals, and evolutionary implications.

Natural selection on floral traits of female Silene dioica by a sexually transmitted disease

Floral traits endowing high reproductive fitness can also affect the probability of plants contracting sexually transmitted diseases. We explore how variations in floral traits influence the fitness of Silene dioica females in their interactions with pollinators carrying pollen or spores of the sterilizing anther-smut fungus Microbotryum violaceum. We collected healthy and infected plants in a highly diseased population and grew them under conditions that 'cure' infected individuals, and used standard regression methods to detect natural selection on floral traits. Narrow-sense heritabilities, coefficients of additive genetic variation (CV(A)) and genetic correlations among traits were estimated from paternal half-sib groups. Pollinator preferences imposed strong direct and directional selection on traits affecting female attractiveness and pollen-/spore-capturing abilities. Levels of additive genetic variance were high in these traits, suggesting that rapid responses to selection are possible. By considering our results in the light of spatial and temporal heterogeneity resulting from the colonization dynamics typical for this species, we suggest that the conflicting selective effects of pollen/spore loads lead to the maintenance of genetic variation in these traits.

Selfing Propensity under Choice Conditions in a Parasitic Fungus, Microbotryum violaceum, and Parameters Influencing Infection Success in Artificial Inoculations

The anther smut fungus Microbotryum violaceum has a mixed mating system, being able to either self or outcross. The infection success may be influenced by selfing or outcrossing. It will affect the amount and distribution of genetic variation within natural populations and, in particular, in this system, may explain the maintenance of common highly deleterious alleles linked to the mating‐type locus. Using artificial inoculations with two different mixtures of strains that have different identifiable genotypes on the host plant Silene latifolia, we estimate infection success and selfing propensity of this fungus. Fungal spores were placed in flowers, mimicking natural spore deposition by pollinators. Successful infection was found in 31.5% of inoculated plants, and infection success depended on the number of inoculations and host plant sex, females having a higher per‐contact risk but males a higher global probability of infection. Many host individuals were infected by more than one fungal genotype, and ...

Sex‐specific effect of Microbotryum violaceum (Uredinales) spores on healthy plants of the gynodioecious Gypsophila repens (Caryophyllaceae)

Females of gynodioecious species need to have reproductive advantages to compensate for their lack of male function and assure their maintenance in natural populations. Females may be more fecund than hermaphrodites because they reallocate resources from male to female function, avoid selfing and inbreeding depression, or produce higher quality offspring by screening arriving pollen better. A novel type of female advantage is proposed that may be important for several species of gynodioecious Caryophyllaceae. The anther smut fungus Microbotryum violaceum generally sterilizes its infected hosts but even without infection, spore deposition can negatively affect the reproduction of healthy individuals. In Gypsophila repens, flowers of hermaphrodite plants experimentally pollinated with both fungal spores and pollen produced significantly fewer fruits and seeds than those that received pollen alone, whereas female reproduction was unaffected by spore deposition. This unexplored reproductive advantage of females is probably due to the larger stigmatic surface in pistillate flowers, that allows pollen germination despite the presence of spores. Because longer stigmas may also lead to increased spore deposition, these results raise questions regarding the infection probability for each sex, possible sex-specific defense mechanisms, and the general role of pathogens in the maintenance of females in natural gynodioecious populations.

PERFORMANCE OF LYCHNIS FLOS-CUCULI FROM FRAGMENTED POPULATIONS UNDER EXPERIMENTAL BIOTIC INTERACTIONS

To study genetic effects of habitat fragmentation on plant performance and plant response to biotic interactions, we performed a greenhouse study with plants from 27 populations of the common plant Lychnis flos-cuculi differing in size, isolation, and microsatellite heterozygosity. We germinated seeds of 449 plants and grew up to nine offspring per maternal plant in single pots assigned to a factorial competition-by-pathogen infection treatment. We applied competition by sowing seeds of the grass Anthoxanthum odoratum into half of the pots. Moreover, half of the plants were inoculated with infective sporidia of the anther smut Microbotryum violaceum. Significant variation among populations in most size measures indicated genetic differentiation between populations. Plants from smaller populations developed fewer flowers than plants from larger populations indicating a genetic Allee effect. A decrease in flower number was also observed for populations with decreased microsatellite heterozygosity, suggesting higher inbreeding depression. Competition and pathogen infection reduced plant size independently from one another and independent from the fragmentation of the habitats of plant origin. While pathogen infection increased the total number of flowers per plant, it decreased the number of uninfected flowers per plant. This study demonstrates that even common species are negatively affected by habitat fragmentation. At the same time, it suggests little effect of habitat fragmentation on plant response to experimental competition and pathogen infection.

Patterns of within population dispersal and mating of the fungus Microbotryum violaceum parasitising the plant Silene latifolia.

This study explores the patterns of dispersal and mating of the anther smut Microbotryum violaceum, a model species in genetics and evolutionary biology. A French metapopulation of the fungus collected from its caryophyllaceous host Silene latifolia was analysed using microsatellites. The genetic diversity was low, populations were strongly differentiated, and there was no pattern of isolation by distance among populations. There was a strong deficit in heterozygotes, confirming the high self-fertilisation rates suggested by previous studies. Within populations there was a strong pattern of isolation by distance, with identical genotypes being highly clustered. This indicates that fungal spores are dispersed mostly between adjacent plants, and such local dispersal is important for understanding the dynamics and evolution of this disease. Local clusters of identical heterozygous genotypes did not contain significantly fewer individuals than did clusters of homozygous genotypes. As selfing between products of independent meiotic events (intertetrad selfing) rapidly reduces heterozygosity, this suggests that intratetrad matings are frequent, which helps to explain the puzzling maintenance of a sex-ratio distortion in M. violaceum.

The Effect of the Sterilising Anther-Smut Fungus Microbotryum Violaceum on the Genetic Composition of Silene Dioica Populations

Populations of the outbreeding perennial herb Silene dioica, growing on the islands of an archipelago in the Baltic Sea, are often diseased by the sterilising anther-smut fungus Microbotryum violaceum (synonym Ustilago violaceu). Since diseased host plants are sterilised, island populations with high disease incidences experience large reductions in the number of reproductive individuals. Here, we compared the genetic composition of diseased plants at six allozyme loci with that of the remaining reproductive individuals to determine whether this reduction changes the genetic composition of S. dioica island populations. We found a significant but small difference in genetic composition between diseased and healthy samples using analyses of multiple allelic types. We discuss our results in relation to changes in genetic differentiation among island populations and the impact of disease in the archipelago.

No Evidence of Sex‐Differential Pollen Limitation at the Flower Level in the Gynodioecious Gypsophila repens Infected by Microbotryum violaceum

Because in gynodioecious species females are expected to be more pollen limited than hermaphrodites, we tested for sex-differential pollen limitation at the flower level, comparing fruit production and fecundity between open-pollinated and hand-pollinated flowers of Gypsophila repens (Caryophyllaceae) over two years. No sex difference in pollen limitation was found. Hand-pollination increased fruit and seed production in the first year of the study but not in the second, and it actually decreased fruit set in the latter year, although overall fecundity was higher in that year. Ovule number differed between years, with perfect flowers containing more ovules in the second year. Gypsophila repens harbors the anther smut fungus Microbotryum violaceum, which infects and sterilizes many species of Caryophyllaceae. In G. repens, however, some infected plants are capable of seed production, although their fecundity is far lower than that of healthy plants. Seed production was not pollen limited even with the high percentage of infected individuals that could reduce the quantity of pollen available at the population level.

INTRASPECIFIC COMPETITION AND MATING BETWEEN FUNGAL STRAINS OF THE ANTHER SMUT MICROBOTRYUM VIOLACEUM FROM THE HOST PLANTS SILENE LATIFOLIA AND S. DIOICA

We studied intraspecific competition and assortative mating between strains of the anther smut fungus Microbotryum violaceum from two of its host species, Silene latifolia and S. dioica. Specifically, we investigated whether strains from allopatric host populations have higher competitive ability on their native host species and show positive assortative mating. In general, strains isolated from S. latifolia outcompeted strains isolated from S. dioica on both host species, but in female hosts, heterotypic dikaryons (i.e., dikaryons composed of a haploid strain originating from S. latifolia and a haploid strain originating from S. dioica) were most successful in competition. Furthermore, the latency period was significantly shorter for heterokaryons that contained at least one strain originating from S. latifolia, compared to heterokaryons that only contained strains originating from S. dioica. The frequencies of conjugations between strains originating from S. latifolia were much higher than conjugation frequencies between strains originating from S. dioica. A significant positive correlation was detected between the relative success of strains in competition and in conjugation, suggesting that success of a strain in competition might be partly determined by its swiftness of mating. In addition, reciprocal differences within heterotypic crosses revealed a significant effect of fungal mating type, with mating type a1 being the main determinant of mating pace. The observed differences in infection success, conjugation rate, and latency period in favor of strains from S. latifolia relative to strains from S. dioica on both host species are discussed in an evolutionary context of opportunities for the maintenance of differentiation between different formae speciales upon secondary contact.

INTRASPECIFIC COMPETITION AND MATING BETWEEN FUNGAL STRAINS OF THE ANTHER SMUT MICROBOTRYUM VIOLACEUM FROM THE HOST PLANTS SILENE LATIFOLIA AND S. DIOICA

We studied intraspecific competition and assortative mating between strains of the anther smut fungus Microbotryum violaceum from two of its host species, Silene latifolia and S. dioica. Specifically, we investigated whether strains from allopatric host populations have higher competitive ability on their native host species and show positive assortative mating. In general, strains isolated from S. latifolia outcompeted strains isolated from S. dioica on both host species, but in female hosts, heterotypic dikaryons (i.e., dikaryons composed of a haploid strain originating from S. latifolia and a haploid strain originating from S. dioica) were most successful in competition. Furthermore, the latency period was significantly shorter for heterokaryons that contained at least one strain originating from S. latifolia, compared to heterokaryons that only contained strains originating from S. dioica. The frequencies of conjugations between strains originating from S. latifolia were much higher than conjugation frequencies between strains originating from S. dioica. A significant positive correlation was detected between the relative success of strains in competition and in conjugation, suggesting that success of a strain in competition might be partly determined by its swiftness of mating. In addition, reciprocal differences within heterotypic crosses revealed a significant effect of fungal mating type, with mating type a1 being the main determinant of mating pace. The observed differences in infection success, conjugation rate, and latency period in favor of strains from S. latifolia relative to strains from S. dioica on both host species are discussed in an evolutionary context of opportunities for the maintenance of differentiation between different formae speciales upon secondary contact.

The spatial distribution of plant populations, disease dynamics and evolution of resistance

Empirical studies of the interaction between the anther smut fungus Microbotryum violaceum and its host plant Lychnis alpina were combined with modelling approaches to investigate how variation in the spatial distribution of host populations influences disease dynamics and variation in resistance. Patterns of disease incidence and prevalence were surveyed in three contrasting systems of natural L. alpina populations where there is substantial variation in spatial structure, ranging from large continuous populations through to small isolated patches. Disease incidence (fraction of populations where disease was present) was highest in the continuous situation, and lowest in the most isolated populations. The reverse was true for prevalence (fraction of individuals diseased). To better understand the long‐term ecological and evolutionary consequences of differences in among population spatial structure, we developed a two‐dimensional spatially explicit simulation model in which host‐population spacing was modelled by varying the percentage of sites suitable for the host. The general patterns of disease incidence and prevalence generated in the simulations corresponded well with the patterns observed in natural populations of L. alpina and M. violaceum; i.e. the fraction of sites with disease increased while the average disease prevalence in diseased populations decreased when host populations became more connected. One likely explanation for the differences in disease incidence and prevalence seen in natural populations is that the evolution of host resistance varies as a function of the degree of fragmentation. This is supported by simulation results that were qualitatively similar to the survey data when resistance was allowed to vary, but not when hosts were assumed to be uniformly susceptible. In the former, the frequency of resistance increased markedly as host populations became more connected.

Host-specific differentiation in the anther smut fungus Microbotryum violaceum as revealed by microsatellites

Abstract The anther smut fungus Microbotryum violaceum ( = Ustilago violacea) is a parasite of many species of the Caryophyllaceae. Its host specificity has been debated since early this century, when cross-inoculation experiments indicated the existence of host-specific lineages. Recently, on the basis of spore ultrastructure, all presumed host races were lumped within M. violaceum. To measure gene flow among natural populations of anther smuts from different host species, we used microsatellite variation at 5 loci among samples from 8 Silene, 2 Saponaria, 2 Dianthus and 1 Gypsophila species. Most of the 326 M. violaceum samples investigated originated from the Swiss Alps and close surroundings. Microsatellite variation revealed almost perfect isolation among anther smut fungi from different host species. In addition, differentiation was supported by the nonrandom distribution of null alleles among samples from different host species and host genera. Null alleles were most abundant in anther smut samples from non-Silene hosts. The resolution of genetic differentiation among anther smuts from different host species was highest in those from Silene species. Genetic relationships among samples as indicated by Neighbour-Joining analysis based on genetic distances are discussed with respect to host phylogeny and host ecology. One sample was identified as Ustilago gausseni because it had verrucose instead of reticulate spores and was collected from Silene italica. Neighbour-Joining analysis revealed that this sample was similar to the M. violaceum samples from other Silene host species. Therefore, our data question spore morphology as a reliable character for anther smut systematics.

INBREEDING EFFECTS ON RESISTANCE AND TRANSMISSION-RELATED TRAITS IN THESILENE–MICROBOTRYUMPATHOSYSTEM

Inbreeding in local host populations will be a common phenomenon in host–pathogen systems that are characterized by metapopulation dynamics, i.e., frequent extinction and recolonization of local host populations by small numbers of founding individuals. As an example of a pathosystem with metapopulation dynamics we investigated the impact of inbreeding in the host plant Silene alba on its interaction with the anther-smut fungus Microbotryum violaceum. Seeds from eight populations of S. alba were sampled, and five generations of sib mating resulted in 65 inbred lines, with inbreeding coefficients of f = 0, 0.25, 0.375, 0.5, and 0.59 per line. In a first experiment these lines were tested for active, biochemical resistance against fungal infection, by artificially inoculating individuals. The percentage of infected individuals differed significantly among populations, lines, and inbreeding levels, and both population-by-inbreeding level and line-by-inbreeding level interactions were significant. The most st...

The influence of plant spacing on density-dependent versus frequency-dependent spore transmission of the anther smut Microbotryum violaceum.

The anther smut fungus Microbotryum violaceum is a pollinator-transmitted plant disease. As for other vector-borne diseases, frequency-dependent transmission patterns are predicted, in contrast to the density-dependent transmission of passively spread diseases. Frequency dependence will, however, only arise if vectors compensate for varying plant spacings. To test this assumption, we set up experimental populations of the host plant, Silene latifolia, with varying disease density (number of diseased plants per plot) and frequency (proportion of plants diseased), and three different plant spacings. We measured spore deposition on healthy flowers in these plots on two dates. Spore deposition decreased considerably from the first to the second census, perhaps related to the concomitant decrease in inflorescence sizes of diseased plants. At our first census, spore deposition rates varied with disease frequency, and the effect of frequency depended on plant spacing. While spore deposition was positively frequency dependent at the 1.5-m inter-plant spacing, no effect of disease frequency was found at a spacings of 0.5 m or 3 m. Nor was there an effect of disease density on spore deposition at the first census. At the later census, on the other hand, spore deposition increased almost significantly with increasing disease density (P = 0.08). This difference in deposition pattern together with a significant decrease in spore receipt indicates changes in pollinator spectrum and/or activity. The correlation of spore numbers among flowers within plants, an indication for intra-plant moves by vectors, was significant at 0.5 m and 1.5 m but not at 3 m. Floral traits and sex of individual plants influenced the number of spores they received. On the first census date, spore deposition increased with increasing inflorescence size in female but not in male plants. On the second census date, neither sex nor number of open flowers had an effect on spore receipt. None of the experimental plants became infected, however, probably because of the unusually hot and dry weather.

Local Maladaptation in the Anther-Smut Fungus Microbotryum violaceum to Its Host Plant Silene latifolia: Evidence from a Cross-Inoculation Experiment

Local Maladaptation in the Anther-Smut Fungus Microbotryum violaceum to Its Host Plant Silene latifolia: Evidence from a Cross-Inoculation Experiment

Nonmeiotic Mechanisms of Recombination in the Anther Smut Microbotryum violaceum

Recombination in the anther smut Microbotryum violaceum (Ustilago violacea) can occur in a variety of ways, from conventional meiosis in diploid strains, to meiosis in polyploids, to several methods of parasexual recombination involving diploid, haploid, and aneuploid segregants. This article summarizes the analyses of the nonmeiotic methods of recombination that have been carried out by the author and by Dr. E. D. Garber and discusses the advantages and uses of each technique. These studies have provided a detailed "classical" map of this fungus and have pointed to features of the genetic organization of this species that make it different from genetically well-characterized Ascomycetes such as Aspergillus, Neurospora, and Saccharomyces.