Postglacial colonization history reflects in the genetic structure of natural populations of Festuca rubra in Europe

Abstract

We conducted a large‐scale population genetic survey of genetic diversity of the host grass Festuca rubra s.l., which fitness can be highly dependent on its symbiotic fungus Epichloë festucae, to evaluate genetic variation and population structure across the European range. The 27 studied populations have previously been found to differ in frequencies of occurrence of the symbiotic fungus E. festucae and ploidy levels. As predicted, we found decreased genetic diversity in previously glaciated areas in comparison with nonglaciated regions and discovered three major maternal genetic groups: southern, northeastern, and northwestern Europe. Interestingly, host populations from Greenland were genetically similar to those from the Faroe Islands and Iceland, suggesting gene flow also between those areas. The level of variation among populations within regions is evidently highly dependent on the postglacial colonization history, in particular on the number of independent long‐distance seed colonization events. Yet, also anthropogenic effects may have affected the population structure in F. rubra. We did not observe higher fungal infection rates in grass populations with lower levels of genetic variability. In fact, the fungal infection rates of E. festucae in relation to genetic variability of the host populations varied widely among geographical areas, which indicate differences in population histories due to colonization events and possible costs of systemic fungi in harsh environmental conditions. We found that the plants of different ploidy levels are genetically closely related within geographic areas indicating independent formation of polyploids in different maternal lineages.

Genetic Diversity of the Symbiotic Fungus Epichloë festucae in Naturally Occurring Host Grass Populations

“…In concordance with the presumption that diversity should decrease with the distance to the source regions, some genetic diversity appears to be lost during the colonization of exposed land following retreating ice sheet in North Europe. However, our previous study on the host grass supports the hypothesis that some of the grass individuals may have survived on nunataks in Faroe Islands (Dirihan et al, 2016;von Cräutlein et al, 2019). The present study on E. festucae in the Faroe Islands do not support the same for the symbiotic fungus.…”

“…Furthermore, we hypothesize that genetic diversity should decrease toward the edges of E. festucae range in Europe due to potential genetic drift and strong selection. We also expect to detect the highest genetic diversity near areas that remained ice-free and in glacial refugias during the last glacial maximum period, as detected in the host grass F. rubra populations (van Zijll de Jong et al, 2008;von Cräutlein et al, 2019).…”

“…We examined a total of 240 individual Epichloëinfected plants originating from 15 natural populations from six islands in the Faroe Islands (n = 71), from six populations located in two different habitat types, meadows (n = 73) and riverbanks (n = 30), in Finland, and from three populations located in two different habitat types, Mediterranean oak forest (n = 22) and semiarid oak grassland (n = 45), in Spain (n = 66). More detailed information on the plants, occurrence of associated Epichloëfungus and collection sites, including population geographic locations, coordinates, altitudes, habitat features and estimate of grazing intensity, can be found in Table 1, Dirihan et al (2016);von Cräutlein et al (2019). The frequencies of Epichloëindividuals in the studied populations ranged from 5% to 81% and the number of individual isolates varied accordingly (Table 1; Dirihan et al, 2016).…”

“…the frequencies of symbiotic plants vary among regions, and populations and habitats within regions irrespective of phenotypic and genetic variation of the host (Dirihan et al, 2016;von Cräutlein et al, 2019). The highest overall endophyte frequencies were found in Spain, where 69% of plants harbored E. festucae.…”

“…F. rubra clones have been detected to be centuries old and occupying large areas (Harberd, 1961) as selection favors the presence of one dominant genotype in a clonal population (Milgroom, 1996). Only two non-local isolates were found in the region, even though relatively high frequencies of non-local host cpDNA haplotypes were observed in the mostly clonal host grass populations (von Cräutlein et al, 2019), which can be due to reproductive differences between the species (Sullivan and Faeth, 2004). Non-local plants can have reduced probability to flower in northern latitudes preventing vertical transmission of non-local E. festucae strains (Leinonen et al, 2019).…”