Modeling in yeast how rDNA introns slow growth and increase desiccation tolerance in lichens

Regulation of symbiotic interactions and primitive lichen differentiation by UMP1 MAP kinase in Umbilicaria muhlenbergii

Lichens are of great ecological importance but mechanisms regulating lichen symbiosis are not clear. Umbilicaria muhlenbergii is a lichen-forming fungus amenable to molecular manipulations and dimorphic. Here, we established conditions conducive to symbiotic interactions and lichen differentiation and showed the importance of UMP1 MAP kinase in lichen development. In the initial biofilm-like symbiotic complexes, algal cells were interwoven with pseudohyphae covered with extracellular matrix. After longer incubation, fungal-algal complexes further differentiated into primitive lichen thalli with a melanized cortex-like and pseudoparenchyma-like tissues containing photoactive algal cells. Mutants deleted of UMP1 were blocked in pseudohyphal growth and development of biofilm-like complexes and primitive lichens. Invasion of dividing mother cells that contributes to algal layer organization in lichens was not observed in the ump1 mutant. Overall, these results showed regulatory roles of UMP1 in symbiotic interactions and lichen development and suitability of U. muhlenbergii as a model for studying lichen symbiosis.

The coming golden age for lichen biology

Lichens are a diverse group of organisms. They are both commonly observed but also mysterious. It has long been known that lichens are composite symbiotic associations of at least one fungus and an algal or cyanobacterial partner, but recent evidence suggests that they may be much more complex. We now know that there can be many constituent microorganisms in a lichen, organized into reproducible patterns that suggest a sophisticated communication and interplay between symbionts. We feel the time is right for a more concerted effort to understand lichen biology. Rapid advances in comparative genomics and metatranscriptomic approaches, coupled with recent breakthroughs in gene functional studies, suggest that lichens may now be more tractable to detailed analysis. Here we set out some of the big questions in lichen biology, and we speculate about the types of gene functions that may be critical to their development, as well as the molecular events that may lead to initial lichen formation. We define both the challenges and opportunities in lichen biology and offer a call to arms to study this remarkable group of organisms.

Evolutionary biology of lichen symbioses

Lichens are the symbiotic outcomes of open, interspecies relationships, central to which are a fungus and a phototroph, typically an alga and/or cyanobacterium. The evolutionary processes that led to the global success of lichens are poorly understood. In this review, we explore the goods and services exchange between fungus and phototroph and how this propelled the success of both symbiont and symbiosis. Lichen fungal symbionts count among the only filamentous fungi that expose most of their mycelium to an aerial environment. Phototrophs export carbohydrates to the fungus, which converts them to specific polyols. Experimental evidence suggests that polyols are not only growth and respiratory substrates but also play a role in anhydrobiosis, the capacity to survive desiccation. We propose that this dual functionality is pivotal to the evolution of fungal symbionts, enabling persistence in environments otherwise hostile to fungi while simultaneously imposing costs on growth. Phototrophs, in turn, benefit from fungal protection from herbivory and light stress, while appearing to exert leverage over fungal sex and morphogenesis. Combined with the recently recognized habit of symbionts to occur in multiple symbioses, this creates the conditions for a multiplayer marketplace of rewards and penalties that could drive symbiont selection and lichen diversification.