Genetic Variation in a Fungal Pathogen: Response to Host Defensive Chemicals

Abstract

Abstract. -In response to infection by shoot infecting pathogens, Scots pine releases cortical resin into affected tissues. The resin contains a mixture ofmonoterpene compounds (a-pinene, tl-pinene, 3-carene, tl-myrcene, limonene and tl-phellandrene) that retard the growth ofa range of pathogens. The proportion of each monoterpene in the resin shows substantial variation among trees within a population. Thus pathogens on different trees encounter quite different monoterpene environments. To investigate the evolutionary response of pathogens to the chemically heterogeneous environment provided by Scots pine, isolates of the ascomycete canker pathogen Crumenulopsis sororia were collected from trees within a range of natural Scots pine populations. Growth rates ofthese isolates were measured in the presence and absence offive host monoterpenes. Substantial heritable variation for growth rate in the presence and absence of monoterpenes, and for monoterpene tolerance was recorded, suggesting the potential for the evolution ofchemically specialized pathogen subpopulations on different trees within a wood. However, genetic correlations between growth rates in different monoterpene environments and between tolerance of different monoterpenes were either positive or non-significant, and there was no evidence of "tradeoffs" in performance under different monoterpene regimes. The results suggest that, on its own, the presence of monoterpene variability within Scots pine will not lead to disruptive selection on the C. sororia population. The relationship between defensive chemical diversity and pest resistance is discussed in the light of these results.

Variation in pathogen aggressiveness within a metapopulation of the Cakile maritima–Alternaria brassicicola host–pathogen association

“…For example, a purple pigment in the stem epidermis of wild soybean ( Glycine soja ) plants is correlated with resistance to the agromyzid fly Ophiomyia centrosematis (Chiang & Norris, 1984); the anthocyanidin malvadin was identified as the source of the purple colour in the stem epidermis, which was underlain by lignins and polyphenols which further contributed to insect resistance. It has been hypothesized that pathogens or herbivores may be subject to disruptive selection when they interact with chemically diverse host populations (but see Ennos & Swales, 1991). For example, selection for adaptation to the chemical phenotype of one host individual could result in reduced fitness on an alternative chemical phenotype, preventing the evolution of herbivores with high fitness on all plants (Dolinger et al ., 1973).…”

Effects of Genetic Diversity in Experimental Stands of Solidago Altissima -- Evidence for the Potential Role of Pathogens as Selective Agents in Plant Populations

“…However the trend, indicated by the exceptional family, of a genetic trade-off between pathogen level and plant size was not significant when the means of all families were correlated [genetic correlations in the sense of Via (1984)]. The evidence in the literature for fitness costs of plant resistance to pathogens seems to be relatively weak (Burdon 1987;De Nooij & Van Damme 1988;Parker 1990;Ennos & Swales 1991;Marquis & Alexander 1992).…”

Genetic correlations in resistance to morpholine and piperidine fungicides inPyrenophora teres populations

“…Variation in correlation coefficients in resistance to DMI fungicides among populations was observed in a previous study [Peever and Milgroom, 1993]. These results have demonstrated that cross-resistance relationships can be quite variable among populations and among different combinations of SBI fungicides, even for fungicides with identical (Ennos and Swales, 1991). Confidence intervals not including 0 and 1 are significantly different from 0 and 1 at the 5% probability level.…”

“…Ninety five percent confidence intervals for the correlation coefficients were estimated using the method of Ennos and Swales [1991] as described in Peever and Milgroom [1993].…”