Geographic mosaic of symbiont selectivity in a genus of epiphytic cyanolichens

Abstract

In symbiotic systems, patterns of symbiont diversity and selectivity are crucial for the understanding of fundamental ecological processes such as dispersal and establishment. The lichen genus Nephroma (Peltigerales, Ascomycota) has a nearly cosmopolitan distribution and is thus an attractive model for the study of symbiotic interactions over a wide range of spatial scales. In this study, we analyze the genetic diversity of Nephroma mycobionts and their associated Nostoc photobionts within a global framework. The study is based on Internal Transcribed Spacer (ITS) sequences of fungal symbionts and tRNALeu (UAA) intron sequences of cyanobacterial symbionts. The full data set includes 271 Nephroma and 358 Nostoc sequences, with over 150 sequence pairs known to originate from the same lichen thalli. Our results show that all bipartite Nephroma species associate with one group of Nostoc different from Nostoc typically found in tripartite Nephroma species. This conserved association appears to have been inherited from the common ancestor of all extant species. While specific associations between some symbiont genotypes can be observed over vast distances, both symbionts tend to show genetic differentiation over wide geographic scales. Most bipartite Nephroma species share their Nostoc symbionts with one or more other fungal taxa, and no fungal species associates solely with a single Nostoc genotype, supporting the concept of functional lichen guilds. Symbiont selectivity patterns within these lichens are best described as a geographic mosaic, with higher selectivity locally than globally. This may reflect specific habitat preferences of particular symbiont combinations, but also the influence of founder effects.

Combined observational and experimental data provide limited support for facilitation in lichens

“…The differences between the system studied here and previous observational work merits further attention. The lichens of our study are pioneer chlorolichen species colonizing new substrata, whereas other systems for which facilitation has been inferred (Rikkinen et al , Fedrowitz et al , ) concern cyanolichens typical of old trees in mature forest ecosystems. In such ‘closed’ systems, where a larger proportion of the space available for colonization is already occupied, the presence of species with overlapping photobiont specificity could be vital for successful establishment for a newly colonizing species.…”

Lichen species identity and diversity of cyanobacterial toxins in symbiosis

“…The mycobiont internal transcribed spacer (ITS) was amplified from the same DNA extractions used for screening for the cyanobiont mcyE gene (Kaasalainen et al ., ), and then sequenced as described in Fedrowitz et al . (). All sequence data was submitted to NCBI GenBank (Supporting Information Table S2).…”

Cyanobacteria in Terrestrial Symbiotic Systems

“…As appropriate cyanobionts are not likely to be ubiquitously distributed, the local availability of cyanobionts can explain many patterns of cyanolichen species occurrence, and shared symbiont specificity may lead to facilitative interactions between different fungal hosts. For example, some spore-dispersed mycobionts appear to be facilitated by the prior establishment of other cyanolichen species that produce symbiotic propagules Rikkinen 2003Rikkinen , 2013Fedrowitz et al 2011Fedrowitz et al , 2012Belinchón et al 2014;Dal Grande et al 2014) or by bryophytes that house appropriate cyanobacteria (Cornejo and Scheidegger 2016).…”