Exploring symbiont management in lichens

Abstract

Lichens are unique among fungal symbioses in that their mycelial structures are compact and exposed to the light as thallus structures. The myriad intersections of unique fungal species with photosynthetic partner organisms (green algae in 90% of lichens) produce a wide variety of diverse shapes and colours of the fully synthesized lichen thallus when growing in nature. This characteristic complex morphology is, however, not achieved in the fungal axenic state. Even under ideal environmental conditions, the lichen life cycle faces considerable odds: first, meiotic spores are only produced on well-established thalli and often only after achieving considerable age in a stable environment, and second, even then in vivo resynthesis requires the presence of compatible algal strains where fungal spores germinate. Many lichen species have evolved a way around the resynthesis bottleneck by producing asexual propagules for joint propagation of symbionts. These different dispersal strategies ostensibly shape the population genetic structure of lichen symbioses, but the relative contributions of vertical (joint) and horizontal (independent) symbiont transmission have long eluded lichen evolutionary biologists. In this issue of Molecular Ecology, Dal Grande et al. (2012) close in on this question with the lung lichen, Lobaria pulmonaria, a flagship species in the conservation of old growth forests. By capitalizing on available microsatellite markers for both fungal and algal symbionts, they show that while vertical transmission is the predominant mode of reproduction, horizontal transmission is demonstrable and actively shapes population genetic structure. The resulting mixed propagation system is a highly successful balance of safe recruitment of symbiotic clones and endless possibilities for fungal recombination and symbiont shuffling. The study of lichens has been for many years the almost exclusive purview of enthusiastic systematists. Although the symbiotic nature of lichens became clear already in the second half of the 19th century, most researchers have continued to focus on the fungal structures and only rarely have paid closer attention to the algal symbionts. While it has always been possible to assess relationships among the fungal partners through microscopy of their complex structures, the same cannot be said for the algae, which are smaller and usually do not develop diagnostic characters in the symbiotic stage. Culturing of lichen photobionts has been the only possibility to distinguish and classify species of microalgae and cyanobacteria isolated from lichen symbioses, but such approaches are cumbersome and, therefore, limited in their use. Consequently, although more than 18 500 species of lichenized fungi are known, the algal partners of most lichen symbioses have yet to be identified (little more than 100 species have been reported; Lü cking et al. 2009). The refinement of molecular methods and development of specific primers for algal gene loci has dramatically improved our baseline knowledge of the div...

The symbiotic playground of lichen thalli - a highly flexible photobiont association in rock-inhabiting lichens

“…To successfully colonize their habitats, lichenized fungi use different and mixed strategies to fine‐tune their associations with algae (Grube & Spribille, ). The community adaptation hypothesis suggests that lichens can vary the ratio of producers (algae) and consumers (fungi) according to local climatic conditions (Sun & Friedman, ).…”

Coalescent-Based Species Delimitation Approach Uncovers High Cryptic Diversity in the Cosmopolitan Lichen-Forming Fungal Genus Protoparmelia (Lecanorales, Ascomycota)

“…They inhabit diverse ecosystems from the arctic to the tropics and commonly form an integral part of terrestrial biodiversity [ 4 ]. Lichens are preferred model systems for ecological, evolutionary, phylogeographic and population genetic studies of symbiotic associations on the account of their wide, often cosmopolitan, distribution, intriguing eco-physiological interdependence and co-evolutionary and adaptive strategies [ 5 ]. Almost one fifth of all known fungi and half of all ascomycetes are lichenized, consisting of approximately 28,000 species worldwide [ 6 , 7 ].…”

Genetic Basis of Self-Incompatibility in the Lichen-Forming Fungus Lobaria pulmonaria and Skewed Frequency Distribution of Mating-Type Idiomorphs: Implications for Conservation

“…This is one of the best-studied lichen species [30], concerning its ecology [31]–[32], population genetics [33]–[34] and photobiont selection [35]. It has a wide geographic distribution covering major parts of Europe, Asia, North America and Africa.…”