Evolutionary ecology of pine-mycorrhizal interactions in the Caledonian pinewoods of Scotland

Abstract

Mycorrhizal fungi are important mutualistic symbionts of most plant species, exchanging soilbound nutrients with their host plants in exchange for photosynthetically derived sugars. Although generally beneficial, not all mycorrhizal fungi provide the same amount of benefit to their hosts, and in some contexts they can also have negative effects on their hosts. Because of this variation in potential benefit, and because different fungal species are found in different places, host populations may be under selection to modify which fungi they interact with, or how they interact with fungi, in order to increase the amount of benefit they receive from the interaction. If selection occurrs differently at different host populations, this would then also lead to local adaptation. In this thesis, I explore whether evolution in response to ectomycorrhizal (EM) fungi has occurred in the Caledonian pinewoods of Scotland, the remnant fragments of Pinus sylvestris woodland that established in Scotland 10,000 years ago. Firstly, I set up a reciprocal inoculation experiment to estimate the amount of genetic variation present amongst families of Scots pine for response to mycorrhizal inoculation, and to look for potential local adaptation of populations to local fungi. We used seedlings and soil from four populations in the Caledonian pinewoods, measuring seedling total biomass after four months. While I found that ectomycorrhizal responsiveness was highly heritable, there was no evidence that pine populations were locally adapted to fungal communities. Instead, I found a complex suite of interactions between pine population and soil inoculum. These results suggest that while Scots pine has the potential to evolve in response to mycorrhizal fungi, evolution in Scotland has not resulted in local adaptation. Long generation times and potential for rapid shifts in fungal communities in response to environmental change may preclude the opportunity for such adaptation in this species, and selection for other factors such as resistance to fungal pathogens may explain the pattern of interactions found. To explore whether there was genetic variation in the association of pine seedlings with different species of EM fungi, or whether pine defensive compounds (monoterpenes) affected the colonisation of EM fungi, we set up a reciprocal transplant experiment. Seedlings from six populations of Scots pine were grown reciprocally in grids at each of the originating populations. EM communities on each seedling were characterised using a combination of microscopy and molecular barcoding, and we used gas chromatography to characterise the monoterpene chemotype for a subset of seedlings. While there was strong evidence that mycorrhizal communities varied between sites, and among grids within sites, I found no evidence that either maternal family or monoterpene chemotype explained any variation in EM community composition. These results again suggest the importance of life history: if a seedling is too selective, it may impose penalties to competitiveness at an important life stage. Instead, a lack of selectivity may allow a seedling to gain the competitive advantage needed to outcompete conspecifics. Finally, I explored the spatial ecology of EM communities from the dataset collected previously. Although I previously found that EM community composition varied between sites and grids, approximately 30% of the variation remained unexplained. I conducted spatial correlogram analysis on EM communities at the 0 - 70 cm scale, and performed an analysis of co-occurrence of species pairs to find evidence of positive and negative interactions between fungal species. I found that most grids showed no spatial autocorrelation in EM root tips at this scale, suggesting relatively homogeneous EM communities. However, some grids were autocorrelated up to 20 cm in distance, suggesting patchy distributions of some EM fungi. I found evidence for negative interactions between species, with the two most abundant species being involved in over half of negative interactions detected. I also found evidence of positive interactions between species pairs, with most involving the genus Suillus, a group of fungi that have been previously found to host N-fixing bacteria. The presence of positive interactions with these species may suggest a facilitative effect of Suillus on other EM species. Overall, I found that while mycorrhizal traits in Scots pine had a genetic component, in natural conditions ectomycorrhizal interactions may be determined much more strongly by aspects of the ecology of individual mycorrhizal species than by host genetics. More generally, the outcomes of these interactions may depend on the life history strategy of the host, and selection pressures may be stronger in species with shorter lifespans or which occur in competition-heavy environments. Further work investigating these effects of life history should also aim to incorporate more natural conditions, as ecological interactions between fungi may mitigate genetic effects.