Comparison of phylogeographical structures of a lichen‐forming fungus and its green algal photobiont in western North America

Aim Lichens comprise a symbiosis of two separate taxa that share geographical distribution but not necessarily the same evolutionary history. Comparison of phylogeographical structures of lichen symbionts provides valuable insight about the processes shaping the lichen's biogeographical pattern. In this study, we tested the extent to which the shared distribution of the widespread lichen-forming fungus Ramalina menziesii and its photobiont Trebouxia decolorans across six major ecoregions reflect parallel genetic structure and gene flow. Location Western North America. Methods Based on DNA sequences from multiple genes, we employed distance-based methods to assess co-divergence of symbiosis partners. To assess historical opportunity for co-evolution, we compare migrations among six ecoregions. Results Tests of genetic concordance between the mycobiont and the photobiont genetic variation revealed an overall significant congruent genetic structure across ecoregions. However, the photobiont and the mycobiont do not have high congruencies within ecoregions and the two taxa have different histories of migration among ecoregions. Main conclusions Congruent phylogeographical patterns in several clades between the mycobiont and the photobiont on a large spatial scale can be explained either by parallel isolation due to ecological and geographical discontinuities or by similar selective pressures on the symbionts due to common environmental conditions within each ecoregion leading to co-divergence. At the regional spatial scale, the two taxa share some degree of evolutionary history but the weak phylogeographical congruence within ecoregions and the lack of shared migration history both indicate a flexible association between mutualists. This flexibility may facilitate the widespread geographical distribution of the lichen.