Scale-dependent co-occurrence patterns of closely related genotypes in a lichen species complex

Abstract

Abstract The ‘competition-relatedness’ hypothesis postulates that co-occurring taxa should be more distantly related, because of lower competition. This hypothesis has been criticized for its dependence on untested assumptions and its exclusion of other assembly forces beyond competition and habitat filtering to explain the co-existence of closely related taxa. Here we analyzed the patterns of co-occurring individuals of lichenized fungi in the Graphis scripta complex, a monophyletic group of species occurring in temperate forests throughout the Northern Hemisphere. We generated sequences for three nuclear ribosomal and protein markers (nuLSU, RPB2, EF-1) and combined them with previously generated sequences to reconstruct an updated phylogeny for the complex. The resulting phylogeny was used to determine the patterns of co-occurrences at regional and at sample (tree) scales by calculating standard effect size of mean pairwise distance (SES.MPD) among co-occurring samples to determine whether they were more clustered than expected from chance. The resulting phylogeny revealed multiple distinct lineages, suggesting the presence of several phylogenetic species in this complex. At the regional and local (site) levels, SES.MPD exhibited significant clustering for five out of six regions. The sample (tree) scale SES. MPD values also suggested some clustering but the corresponding metrics did not deviate significantly from the null expectation. The differences in the SES.MPD values and their significance indicated that habitat filtering and/or local diversification may be operating at the regional level, while the local assemblies on each tree are interpreted as being the result of local competition or random colonization.