alignment_Schneider_et_al_2016_MolEcol_DryadDNA sequence alignment (in FASTA format) used for construction of phylogenetic trees. Includes data of four non-coding loci, namely the internal transcribed spacer region (ITS), which consists of two parts and includes the strongly conserved 5.8S rRNA region, the nuclear small ribosomal subunit gene (SSU), the nuclear large ribosomal subunit gene (LSU), and the mitochondrial small ribosomal subunit gene (mtSSU), and four protein-coding genes, namely DNA-directed RNA polymerase II subunit RPB1 (RPB1), DNA-directed RNA polymerase II subunit RPB2 (RPB2), DNA replication licensing factor MCM7 (MCM7), and translation elongation factor 1 alpha (EF1a).RAxML_MLtree_Schneider_et_al_2016_MolEcol_DryadA phylogenetic tree in Newick format constructed using RAxML 7.4.2 based on the alignment included in this data package.MrBayes_allcompat_consensus_Schneider_et_al_2016_MolEcol_DryadA consensus phylogenetic tree in Nexus format constructed using MrBayes 3.2.1 based on the alignment included in this data package.BEAST_MCC_run1_Schneider_et_al_2016_MolEcol_DryadA maximum clade credibility (MCC) phylogenetic tree in Nexus format constructed using BEAST 1.8.1 based on the alignment included in this data package.BEAST_MCC_run2_Schneider_et_al_2016_MolEcol_DryadA maximum clade credibility (MCC) phylogenetic tree in Nexus format constructed using BEAST 1.8.1 based on the alignment included in this data package.StarBEAST_MCC_run1_Schneider_et_al_2016_MolEcol_DryadA maximum clade credibility (MCC) phylogenetic species tree in Nexus format constructed using *BEAST 1.8.1 based on the alignment included in this data package.StarBEAST_MCC_run2_Schneider_et_al_2016_MolEcol_DryadA maximum clade credibility (MCC) phylogenetic species tree in Nexus format constructed using *BEAST 1.8.1 based on the alignment included in this data package.

Large, architecturally complex lichen symbioses arose only a few times in evolution, increasing thallus size by orders of magnitude over those from which they evolved. The innovations that enabled symbiotic assemblages to acquire and maintain large sizes are unknown. We mapped morphometric data against an eight-locus fungal phylogeny across one of the best-sampled thallus size transition events, the origins of the Placopsis lichen symbiosis, and used a phylogenetic comparative framework to explore the role of nitrogen-fixing cyanobacteria in size differences. Thallus thickness increased by >150% and fruiting body core volume increased ninefold on average after acquisition of cyanobacteria. Volume of cyanobacteria-containing structures (cephalodia), once acquired, correlates with thallus thickness in both phylogenetic generalized least squares and phylogenetic generalized linear mixed-effects analyses. Our results suggest that the availability of nitrogen is an important factor in the formation of large thalli. Cyanobacterial symbiosis appears to have enabled lichens to overcome size constraints in oligotrophic environments such as acidic, rain-washed rock surfaces. In the case of the Placopsis fungal symbiont, this has led to an adaptive radiation of more than 60 recognized species from related crustose members of the genus Trapelia. Our data suggest that precyanobacterial symbiotic lineages were constrained to forming a narrow range of phenotypes, so-called cryptic species, leading systematists until now to recognize only six of the 13 species clusters we identified in Trapelia.