Date Published: April 17, 2019
Publisher: Public Library of Science
Author(s): Päivi H. Leinonen, Marjo Helander, Beatriz R. Vázquez-de-Aldana, Iñigo Zabalgogeazcoa, Kari Saikkonen, Benedicte Riber Albrectsen.
Recent work on microbiomes is revealing the wealth and importance of plant-microbe interactions. Microbial symbionts are proposed to have profound effects on fitness of their host plants and vice versa, especially when their fitness is tightly linked. Here we studied local adaptation of host plants and possible fitness contribution of such symbiosis in the context of abiotic environmental factors. We conducted a four-way multi-year reciprocal transplant experiment with natural populations of the perennial grass Festuca rubra s.l. from northern and southern Finland, Faroe Islands and Spain. We included F. rubra with and without transmitted symbiotic fungus Epichloë that is vertically transmitted via host seed. We found local adaptation across the European range, as evidenced by higher host fitness of the local geographic origin compared with nonlocals at three of the four studied sites, suggesting that selection pressures are driving evolution in different directions. Abiotic factors did not result in strong fitness effects related to Epichloë symbiosis, indicating that other factors such as herbivory are more likely to contribute to fitness differences between plants naturally occurring with or without Epichloë. Nevertheless, in the case of asymmetric symbiosis that is obligatory for the symbiont, abiotic conditions that affect performance of the host, may also cause selective pressure for the symbiont.
Variability in direction and magnitude of natural selection is a major force shaping biodiversity . As a result, natural populations encountering differing selection pressures become genetically differentiated and locally adapted [2–4]. Local adaptation is traditionally defined as higher fitness of local than nonlocal individuals in a given environment . Selective agents driving local adaptation consist of both abiotic and biotic factors, and the latter become especially apparent when local populations of closely interacting species coevolve [6,7].
Our study shows that adaptive evolution in contrasting climatic environments has resulted in local adaptation across the European range in the perennial host grass F. rubra. We found that large-scale abiotic environments did not result in strong differences in fitness between genotypes naturally occurring with or without Epichloë in the absence of high herbivory pressure. In the case of tight fitness linkage, however, it should be noted that selection against nonlocal host genotypes indirectly also decreases fitness of nonlocal symbiont genotypes and thus possibly contributing to the evolution of the symbiont. Future studies should strive for combining reciprocal transplantation experiments with reciprocal inoculations to unravel more complex interactions between host and symbiont genotypes and natural environments.