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  • 1.
    Huttunen, Piritta
    et al.
    Finnish Forest Research Institute, Muhos Research Unit, Muhos, Finland.
    Kärkkäinen, Katri
    Finnish Forest Research Institute, Muhos Research Unit, Muhos, Finland.
    Löe, Geir
    University of Gävle, Faculty of Engineering and Sustainable Development, Department of Electronics, Mathematics and Natural Sciences, Biology.
    Rautio, Pasi
    Finnish Forest Research Institute, Rovaniemi, Finland.
    Ågren, Jon
    Department of Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
    Leaf trichome production and responses to defoliation and drought in Arabidopsis lyrata (Brassicaceae)2010In: Annales Botanici Fennici, ISSN 0003-3847, E-ISSN 1797-2442, Vol. 47, no 3, p. 199-207Article in journal (Refereed)
    Abstract [en]

    Leaf trichomes can protect plants against herbivory and drought, but can be costly to produce. Theory suggests that selection for reduced costs of resistance may result in the evolution of inducible defences. We quantified variation in tolerance to drought and defoliation, and tested the hypotheses that (a) tolerance is associated with cost, (b) leaf trichome production increases tolerance to drought, and (c) trichome production is increased in response to defoliation and drought stress in Arabidopsis lyrata (Brassicaceae). Eight maternal half-sib families were exposed to two watering regimes and four defoliation treatments in a factorial design. Tolerance to drought varied among families and was inversely related to leaf size, but was not related to trichome density. Family mean performance in the low-watering treatment tended to correlate negatively with that in the control treatment. Trichome production was not induced by defoliation or drought stress. The results suggest that there is genetic variation in tolerance to drought in the study population, that tolerance to drought is associated with a cost, and that trichome production does not increase tolerance to drought in A. lyrata.

  • 2.
    Kivimäki, Maarit
    et al.
    Finnish Forest Research Institute, Vantaa Research Unit, Finland; Population Genetics Graduate School, Department of Biology, University of Oulu, Finland.
    Kärkkäinen, Katri
    Finnish Forest Research Institute, Vantaa Research Unit, Finland.
    Gaudeul, Myriam
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden; Département Systématique et Evolution, Museum National d’Histoire Naturelle, Paris, France.
    Løe, Geir
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
    Agren, Jon
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
    Gene, phenotype and function: GLABROUS1 and resistance to herbivory in natural populations of Arabidopsis lyrata2007In: Molecular Ecology, ISSN 0962-1083, E-ISSN 1365-294X, Vol. 16, no 2, p. 453-462Article in journal (Refereed)
    Abstract [en]

    The molecular genetic basis of adaptive variation is of fundamental importance for evolutionary dynamics, but is still poorly known. Only in very few cases has the relationship between genetic variation at the molecular level, phenotype and function been established in natural populations. We examined the functional significance and genetic basis of a polymorphism in production of leaf hairs, trichomes, in the perennial herb Arabidopsis lyrata. Earlier studies suggested that trichome production is subject to divergent selection. Here we show that the production of trichomes is correlated with reduced damage from insect herbivores in natural populations, and using statistical methods developed for medical genetics we document an association between loss of trichome production and mutations in the regulatory gene GLABROUS1. Sequence data suggest that independent mutations in this regulatory gene have provided the basis for parallel evolution of reduced resistance to insect herbivores in different populations of A. lyrata and in the closely related Arabidopsis thaliana. The results show that candidate genes identified in model organisms provide a valuable starting point for analysis of the genetic basis of phenotypic variation in natural populations.

  • 3. Kärkkäinen, Katri
    et al.
    Løe, Geir
    University of Gävle, Department of Mathematics, Natural and Computer Sciences, Ämnesavdelningen för naturvetenskap.
    Agren, Jon
    Population structure in Arabidopsis lyrata: evidence for divergent selection on trichome production.2004In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 58, no 12, p. 2831-2836Article in journal (Refereed)
    Abstract [en]

    Leaf trichomes may serve several biological functions including protection against herbivores, drought, and UV radiation; and their adaptive value can be expected to vary among environments. The perennial, self-incompatible herb Arabidopsis lyrata is polymorphic for trichome production, and occurs in a glabrous and a trichome-producing form. Controlled crosses indicate that the polymorphism is governed by a single gene, with trichome production being dominant. We examined the hypothesis that trichome production is subject to divergent selection (i.e., directional selection favoring different phenotypes in different populations) by comparing patterns of variation at the locus coding for glabrousness and at eight putatively neutral isozyme loci in Swedish populations of A. lyrata. The genetic diversity (H-e) and allele number at isozyme loci tended to increase with population size and decreased with latitude of origin, whereas genetic diversity at the locus coding for glabrousness did not vary with population size and increased with latitude of origin. The degree of genetic differentiation at the glabrousness locus was much higher than that at isozyme loci. Genetic identity at isozyme loci was negatively related to geographic distance, suggesting isolation by distance. In contrast, there was no significant correlation between genetic identity at the glabrousness locus and at isozyme loci. The results are consistent with the hypothesis that divergent selection contributes to population differentiation in trichome production in A. lyrata.

  • 4.
    Løe, Geir
    University of Gävle, Department of Mathematics, Natural and Computer Sciences, Ämnesavdelningen för naturvetenskap.
    Divergent selection on trichome production in the perennial herb Arabidopsis lyrata: Insect herbivory and population differentiation in a putative resistance trait2005Conference paper (Other (popular science, discussion, etc.))
  • 5.
    Løe, Geir
    University of Gävle, Department of Mathematics, Natural and Computer Sciences, Ämnesavdelningen för naturvetenskap.
    Ecology and evolution of resistance to herbivory: trichome production in Arabidopsis lyrata2006Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, I examine variation in occurrence and performance of glabrous and trichome-producing plants of the perennial herb Arabidopsis lyrata. I combine field studies and genetic analysis to (1) examine the function of trichomes as a resistance character in natural populations, (2) compare the magnitude of population differentiation in trichome-production and at putatively neutral marker loci, (3) examine the molecular genetic basis of trichome-production, and (4) quantify the effects of herbivore removal on population growth and relative performance of glabrous and trichome-producing plants.

    In a survey of 30 populations of A. lyrata in Norway and Sweden, I documented spatiotemporal variation in damage from insect herbivores. With few exceptions, glabrous plants were more damaged by herbivorous insects than trichome-producing plants in polymorphic populations. Damage levels varied substantially among populations and among years. The intensity of herbivory quantified as mean leaf removal to glabrous plants was higher in polymorphic populations than in monomorphic glabrous populations.

    Within the Swedish range, populations were more strongly differentiated at the locus coding for glabrousness than at eight putatively neutral isozyme loci. This is consistent with the hypothesis that trichome production is subject to divergent selection.

    A study of the genetic basis of trichome production showed that glabrousness was associated with mutations in an orthologue to GLABROUS1, a regulatory gene known to cause glabrousness in A. thaliana. Comparative data indicate that the genetic basis of glabrousness varies among populations.

    Experimental removal of insect herbivores in a natural A. lyrata population increased population growth rate and the relative fitness of the glabrous morph. The results suggest that insect herbivory may influence both population dynamics and selection on trichome production in A. lyrata.

  • 6.
    Løe, Geir
    University of Gävle, Department of Mathematics, Natural and Computer Sciences, Ämnesavdelningen för naturvetenskap.
    Herbivore-mediated selection on trichome-production in the perennial herb Arabidopsis lyrata2008Conference paper (Other academic)
    Abstract [en]

    The evolution of a wide range of morphological and physiological characters in plants is believed to have been driven by interactions with insect herbivores. Yet, the effects of insect herbivory on population dynamics and evolutionary trajectories of putative defence traits in natural plant populations are little known. Leaf hairs, trichomes, can reduce herbivore damage, but may also contribute to tolerance against drought and UV-radiation. We conducted a four-year field experiment to determine whether insect herbivory reduces population growth and affects selection on trichome production in a natural population of the outcrossing, perennial herb Arabidopsis lyrata, which is polymorphic for trichome production and occurs in a glabrous and a trichome-producing form. In control plots, glabrous plants were more damaged by insect herbivores and had a lower population growth rate than trichome-producing plants due to a lower probability of remaining in the reproductive stage. Herbivore removal increased population growth rate, in particular in the glabrous morph. In plots from which insect herbivores had been removed, glabrous and trichome-producing plants had similar growth rates. Herbivore removal enhanced the growth rate of the glabrous morph mainly through increasing the probability of remaining in the reproductive stage, and in the trichome-producing morph mainly through increasing the probability for vegetative plants to enter the reproductive stage. The results show that insect herbivory may reduce population growth and result in selection for trichome production in A. lyrata. Spatial and temporal variation in insect herbivory could thus contribute to the maintenance of the polymorphism in trichome production.

  • 7.
    Løe, Geir
    et al.
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
    Toräng, Per
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
    Gaudeul, Myriam
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden; Dépt. Systématique et Evolution, Museum National d’Histoire Naturelle, Paris, France.
    Ågren, Jon
    Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden.
    Trichome-production and spatiotemporal variation in herbivory in the perennial herb Arabidopsis lyrata2007In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 116, no 1, p. 134-142Article in journal (Refereed)
    Abstract [en]

    Allocation theory suggests that the optimal level of resistance against herbivores should vary with the risk of herbivory if allocation to resistance is costly. The perennial herb Arabidopsis lyrata has a genetically based polymorphism for trichome production and occurs in a glabrous and a trichome-producing form. Leaf trichomes (hairs) can protect plants against insect herbivores, and may increase tolerance to drought and UV-radiation. To examine the functional significance of trichome production, we documented the frequency of glabrous plants and damage by insect herbivores in 30 A. lyrata populations in Sweden and Norway. The proportion of glabrous plants ranged from 0.10 to 0.71 (median=0.44) in polymorphic populations; 7 of 12 populations in Norway and 14 of 18 populations in Sweden were monomorphic glabrous, i.e. with fewer than 5% trichome-producing plants. The mean proportion of the leaf area removed by herbivores varied substantially among populations and years. With few exceptions, glabrous plants were more damaged than trichome-producing plants in polymorphic populations. The intensity of herbivory quantified as the mean damage to glabrous plants tended to be higher in polymorphic populations than in populations monomorphic for the glabrous morph and was higher in Sweden than in Norway. In Norway, both the magnitude of herbivore damage and the frequency of trichome-producing plants tended to decrease with increasing altitude. The results indicate that leaf trichomes contribute to resistance against herbivorous insects in A. lyrata, and suggest that herbivore-mediated selection contributes to the maintenance of the polymorphism in trichome production.

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