Jouni Ahlholm

University of Turku, Turku, Province of Western Finland, Finland

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Publications (12)

  • Source
    Jouni Ahlholm · Marjo Helander · Pirjo Elamo · [...] · Kari Saikkonen
    Full-text Dataset · Aug 2015
  • [Show abstract] [Hide abstract] ABSTRACT: Seedborne systemic endophytic fungi of grasses are thought to be plant mutualists, because they have been shown to improve their host's resistance against biotic and abiotic stresses. The interactions in plant-endophyte associations vary from mutualistic to parasitic with environmental conditions and the genotypes of interacting species. The possible pros and cons of endophytic fungi are expected to be most evident during the seedling establishment, where host fitness is most directly affected. If this holds true, endophytes may play a focal role in local adaptation of hosts to different environments. We examined if endophyte-infected and uninfected seeds and seedlings of two native grass species, Festuca rubra and F. ovina, differ in seed germination and seedling growth rates under greenhouse conditions. The germination of F. rubra seeds was also studied in the field. This is the first time that the effects of Epichloë endophyte on seedling establishment of fine fescues from natural populations have been experimentally evaluated. Mother plant (seed family) had a marked effect on many response variables in both grass species. Length and mean biomass of tillers of endophyte-infected (E+) F. ovina seedlings were lower, but root:shoot ratios were higher than in endophyte-free (E-) seedlings. In F. rubra, the effects of the endophyte were dependent on the habitat where the seeds were collected. The E+ seeds from river banks germinated faster than E+ seeds from meadows, and E+ seedlings from the river banks produced fewer but taller and heavier tillers than the other seedlings. Our data suggest that the effects of the endophyte infection on the seedling stage of fine fescues are dependent the species of grass, host genetic background and mother plant habitat. The germination strategy and growth form of E+ red fescue seedlings from river banks may be beneficial to surviving in the harsh conditions of that habitat.
    Article · Dec 2008 · Oecologia
  • Source
    M Helander · J Ahlholm · T N Sieber · [...] · K Saikkonen
    [Show abstract] [Hide abstract] ABSTRACT: The effect of environmental fragmentation on the species distribution and frequency of horizontally transmitted endophytic fungi in Betula pubescens and Betula pendula leaves was studied in an archipelago in southwestern Finland. The study system consisted of 14 islands, ranging in size and distance to the mainland, and five mainland sites. Endophytic fungi were grown out from surface-sterilized leaves. The frequency of endophytic fungi mainly depended on the size of the island, explaining 32-35% of the variation, and the distance to the mainland explaining 29-35% of the variation. The birch trees on the largest islands near the mainland had the highest endophyte frequencies. Fusicladium betulae, Gnomonia setacea and Melanconium betulinum were the most commonly isolated fungi. Foliar endophytes of birch trees are able to disperse to fairly fragmented areas, but their frequencies seem to depend on environmental isolation and size of the island.
    Full-text Article · Feb 2007 · New Phytologist
  • Piippa R Wäli · Jouni U Ahlholm · Marjo Helander · Kari Saikkonen
    [Show abstract] [Hide abstract] ABSTRACT: Epichloë species are systemic fungal endophytes that usually specialize in a certain group of related grass species. We examined the infection frequency of Epichloë festucae in populations of two fine fescue species (Festuca rubra and F. ovina) in natural and seminatural habitats at 86 study sites (total = 2514 plants) across Finland and northern Norway. Infection incidence varied significantly among grass species and populations. A substantial number of the F. rubra and F. ovina populations (53 out of 77 and 25 out of 30, respectively) were either endophyte-free or had very low (<20%) infection frequencies. The highest infection frequencies were found in subarctic areas. Moreover, infection incidence differed between habitats. In the area with the highest infection frequencies, we used microsatellite markers to study genetic diversity and the rates of gene flow of E. festucae among 12 F. rubra populations. Twenty out of the 25 fungal genotypes detected with four microsatellite markers were carrying multiple alleles in at least one locus, indicating multiple infections or vegetative hybridization of the fungus. One dominant genotype occurred in all 12 populations, representing 63.5% of all isolates. We found a moderate level of average genotypic variation and a low level of genetic differentiation (F st = 0.0814). There was no correlation between infection frequency and genotypic diversity. Although the existence of a dominant genotype and the detected linkage disequilibrium suggest that the fungus is mainly asexual and vertically transmitted, the multiallelic loci and variation of genetic diversity among populations indicate occasional contagious spread and sexual or parasexual recombination of the fungus in some populations. Furthermore, the genotypes carrying multiallelic loci suggest the possibility of multiple infections or hybridization of the endophyte.
    Article · Jan 2007 · Microbial Ecology
  • Jouni U. Ahlholm · Marjo Helander · Silja Lehtimäki · [...] · Kari Saikkonen
    [Show abstract] [Hide abstract] ABSTRACT: The relationship between vertically transmitted asexual fungal grass endophytes and their hosts is considered to be mutualistic. Results from agronomic field support this line of reasoning but recent studies have shown more variable results in natural systems. We investigated how high and low nutrient and water treatments affected biomass allocation patterns of endophyte-infected and uninfected Festuca pratensis and F. rubra in greenhouse experiments over two growing seasons. Irrespective of infection status, both grass species showed improved performance on highly fertilized and watered soils. However, infected F. pratensis plants produced larger tillers than endophyte-free plants on soil low in nutrients and water in the first growing season, although they (E+) otherwise showed decreased performance on nutrient-poor soil. In low nutrient and water conditions, endophyte-infected plants produced less tillers and had lower total biomass compared to uninfected plants, and displayed a negative phenotypic correlation between seed production and vegetative growth. The latter indicates costs of reproduction when the plant shares common resources with the fungal endophyte. However, endophyte infection status (E+, E−) interacted significantly with the soil fertilisation in terms of plant growth, having a stronger positive effect on growth in infected F. pratensis plants. In F. rubra, endophyte-infected plants showed higher vegetative growth in fertilized and watered soils compared to uninfected plants. However, infected plants tended to produce fewer inflorescences. This had no effect on seed production, perhaps because seed production was partly replaced by asexual pseudovivipary. Contrary to the general assumption in the literature that fungal endophytes are plant mutualists, these findings suggest that the costs of endophytes may outweigh their benefits in resource limited conditions. However, the costs of endophyte infections appear to differ among the grass species studied; costs of endophytes were mainly detected in F. pratensis under low nutrient conditions. We propose that differences in response to endophyte infection in these species may depend on the differences in life-history strategies and environmental requirements of these two fescue and fungal species and may change during the life span of the plant.
    Article · Nov 2002 · Oikos
  • Jouni Ahlholm · Marjo Helander · Pirjo Elamo · [...] · Kari Saikkonen
    [Show abstract] [Hide abstract] ABSTRACT: We studied interactions between microfungi and herbivores sharing a host tree. In a series of experiments and field observations over a 3-year period, we compared phenotypic and genetic correlations of fungal frequencies and performance of invertebrate herbivores growing on mature half-sib progenies of mountain birches (Betula pubescens ssp. czerepanovii) in two environments, a forested river valley and an adjacent higher-elevation mountain birch woodland. We found little support for direct relation between fungal frequencies and performance of herbivore species. Instead, genetic correlations, particularly between autumnal moth (Epirrita autumnata) and rust fungus (Melampsoridium betulinum), suggest that herbivore performance may be caused by (1) genetic differences in plant quality for fungi and herbivores, or (2) genetic differences in responses to environmental conditions.
    Article · Sep 2002 · Ecology Letters
  • Jouni U Ahlholm · Marjo Helander · Janne Henriksson · [...] · Kari Saikkonen
    [Show abstract] [Hide abstract] ABSTRACT: We investigated whether genetic variation of a common foliar endophyte of birch trees, Venturia ditricha, is affected by environmental conditions or host genotype. Fungal samples were collected from 10 half-sibling families of mountain birch (Betula pubescens ssp. czerepanovii) grown in two environmental conditions with different daily average temperatures: a forested river valley and an adjacent open tundra (altitudinal difference 180 m). Genetic analysis of V. ditricha isolates was done using random amplified microsatellite polymerase chain reaction. We found that host genotypes, along with prevailing environmental conditions, influence the probability of infection by particular endophyte genotypes. The most susceptible host genotypes were highly infected with genetically similar endophyte genotypes, whereas the most resistant trees were poorly infected and they were infected by genetically dissimilar endophytes. Our results also showed environment-host genotype interactions, suggesting that the susceptibility of the host to a particular endophyte genotype may change in natural environments when environmental conditions are changed. It appears that a particular endophyte genotype needs to find the right host genotype for a successful infection. There are many host genotypes in natural stands; this means, from the point of view of the fungus, the environment is heterogeneous. Thus, under the influence of birch tree genotypes, genetically differentiated subgroups of the endophytic fungus may be formed in different environments.
    Article · Sep 2002 · Evolution
  • K. Saikkonen · J. Ahlholm · M. Helander · [...] · J. Tuominen
    [Show abstract] [Hide abstract] ABSTRACT: Correlative evidence suggests that pathogenic leaf rust fungus, Melampsoridium betulinum, in late summer may negatively affect folivorous insects in the following summer. Correlative association does not necessarily, however, reveal causality. Alternatively, other interconnected plant characters may determine rust densities and herbivore performance. In this study, we used birch clones and rust fungus inoculations to manipulatively test the effects of birch rust and birch genotypes on the growth performance of folivorous moth larvae of Epirrita autumnata (Lep. Geometridae) in the subsequent year. The inoculation treatment increased rust densities (three- to 60-fold) compared with natural infection levels. E. autumnata performance varied among birch clones and showed 4% lower growth performance on rust-inoculated trees. However, the larval performance did not differ between rust-treated shoots and untreated control shoots and the use of tree-specific rust densities as a covariant in statistical analyses failed to reveal any negative association between rust fungus and larval performance. As the slight difference in larval growth performance also levelled until pupation, we propose that rust infection has biologically insignificant importance to the performance of E. autumnata.
    Article · Dec 2001 · Forest Pathology
  • K. Saikkonen · J. Ahlholm · M. Helander · [...] · O. Niemelainen
    [Show abstract] [Hide abstract] ABSTRACT: We examined the occurrence of vertically via host seeds transmitted endophyte infections of 14 grass species in natural populations in Finland and totally 97 agricultural cultivars of 13 grass species. Although endophyte infections were widespread in native grass species, overall endophyte occurrence and frequencies were lower than published reports have suggested. In natural populations, 10 out of 14 grass species examined harbor fungal endophytes in their seeds. The highest species-specific mean incidences of endophyte infected plants in infected populations were found in Agrostis capillaris,Festuca arundinacea, F. ovina, F. pratensis, F. rubra and Phleum pratense (67%, 98%, 29%, 42%, 32% and 33%, respectively). Mean incidences were Dactylis glomerata, Deschampsia flexuosa, D. cespitosa and Elymus repens, and no infections were detected in Calamagrostis lapponica, C. epigejos, Alopecurus pratensis and Phalaris arundinacea. However, we detected a very high variation in infection incidences among natural populations and a large proportion of populations was, indeed, endophyte-free. This supports the ideas that 1) endophytic fungi provide selective advantage of infected grasses to their uninfected conspecifics in some habitats, and/or 2) fungi are occasionally transmitted horizontally by spores. In grass cultivars, endophyte infected seeds were detected only in F. pratensis and Lolium perenne, and endophyte frequencies were either very high or very low. Cultivars of 11 other grass species were endophyte-free.
    Article · Jun 2000 · Ecography
  • J U Ahlholm · M L Helander · J Savolainen
    [Show abstract] [Hide abstract] ABSTRACT: Environmental variation, such as an increase of mean temperature due to the greenhouse effect, as well as the genetic factors may affect the allergenicity of pollen and thus, the prevalence of allergies. The connection between these factors and the allergen content of pollen is poorly understood. To evaluate the role of environmental and genetic factors on the allergenicity of birch pollen. Mountain birch (Betula pubescens ssp. czerepanovii (Orl.) Hämet-Ahti) pollen was studied using SDS-PAGE and IgE-immunoblotting. Pollen samples were collected from the trees of 10 half-sib families. The study trees from each family were reared in two tree line gardens where the daily mean temperatures were different during the growing season. The quantitative analysis of band intensities suggested that the responses of the major birch pollen allergen, Bet v 1, were stronger in the samples collected from the garden with higher daily mean temperature. Half-sib families and individual trees differed in their Bet v 1 content. Our results show that both genetic and environmental factors have an effect on the amount of Bet v 1. This suggests that breeding for trees low in allergen content may be possible.
    Article · Dec 1998 · Clinical & Experimental Allergy
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    M L Helander · J Savolainen · J Ahlholm
    [Show abstract] [Hide abstract] ABSTRACT: To determine the effects of anthropogenic pollution on water-soluble proteins and specifically allergens in birch (Betula pendula and B. pubescens) pollen, we analyzed extracts of pollen from the pollution gradient around a factory complex (emitting sulfur oxides and heavy metals) by sodium dodecyl sulfate (SDS)--polyacrylamide gel electrophoresis (PAGE) and IgE immunoblotting. In addition, tree density-associated shading of the tree habitat, and quantity and quality of proteins and allergens in pollen of the two birch species were studied. The two studied birch species gave identical allergen profiles even though their protein profiles differed. Distance from the factory did not affect the amount of birch pollen major allergen, Bet v 1 (17 kDa), or of two other strong allergens (23 and 36 kDa). Trees growing in shaded places had significantly stronger responses to Bet v 1 and to the 23-kDa allergen than trees growing in open or half-open environments. Thus, we propose that combined heavy metal and sulfur dioxide pollution does not have an important effect on birch pollen allergens. Instead, other factors, e.g., shading and soil properties of the tree habitat, as well as the genetic background of the tree, may have a stronger influence on the quantity and relative composition of allergens.
    Full-text Article · Jan 1998 · Allergy
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    Eija Yli‐Panula · Jouni Ahlholm
    [Show abstract] [Hide abstract] ABSTRACT: Pollen of birch (Betula) and grass (Poaceae) are the main cause of seasonal allergy in northern European countries. Allergic symptoms may also occur outside the local pollen season. From the point of view of allergic sufferers, it is important to know how long pollen retains its sensitising power. The maintenance of antigenic activity under different experimental conditions was studied over a period of one year. Pollen of birch (Betula pendula Roth., B. pubescens Ehrh.) and timothy (Phleum pratense L.) grass was collected in the summer of 1995 in south‐western Finland. Pollen samples were incubated in petri dishes in separate desiccators, at 20°C at relative humidities of 23%, 33%, 43%, and 54%, which coincides with typical values in Finnish homes. Control pollen samples were stored under room conditions. The samples were analysed with igG‐ELISA (enzyme‐linked immunosorbent assay) using monoclonal (specific to Bet ν 1) and polyclonal birch pollen antibodies and Phl p 5 antibody, with SDS‐PAGE (sodium dodecyl sulphate‐polyacrylamide gel electrophoresis) and IgE‐immunoblotting using sera from birch pollen allergic subjects. Birch and grass pollen antigenic activity remained high in all treatments during the entire study period. Different relative humidities had no detectable effect on the antigen concentrations.
    Full-text Article · Jan 1998 · Grana