Genomic analysis of Coccomyxa viridis, a common low-abundance alga associated with lichen symbioses.

Lichen symbiosis is centered around a relationship between a fungus and a photosynthetic microbe, usually a green alga. In addition to their main photosynthetic partner (the photobiont), lichen symbioses can contain additional algae present in low abundance. The biology of these algae and the way they interact with the rest of lichen symbionts remains largely unknown. Here we present the first genome sequence of a non-photobiont lichen-associated alga. Coccomyxa viridis was unexpectedly found in 12% of publicly available lichen metagenomes. With few exceptions, members of the Coccomyxa viridis clade occur in lichens as non-photobionts, potentially growing in thalli endophytically. The 45.7 Mbp genome of C. viridis was assembled into 18 near chromosome-level contigs, making it one of the most contiguous genomic assemblies for any lichen-associated algae. Comparing the C. viridis genome to its close relatives revealed the presence of traits associated with the lichen lifestyle. The genome of C. viridis provides a new resource for exploring the evolution of the lichen symbiosis, and how symbiotic lifestyles shaped evolution in green algae.

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.

The impact of salinity on mycorrhizal colonization of a rare legume, Galactia smallii, in South Florida pine rocklands.

OBJECTIVES: The success of restoration plantings depends on the capacity of transplanted individuals or seeds to establish and reproduce. It is increasingly recognized that restoration success depends quite heavily upon biotic interactions and belowground processes. Under stressful abiotic conditions, such as soils salinized by storm surge and sea level rise, symbiotic interactions with soil microbes such as mycorrhizae may be critically important. In this study, we investigate the impact of salinity on percent colonization of roots by arbuscular mycorrhizal fungi, in addition to the impacts of this colonization on plant fitness under saline conditions. Fifty Galactia smallii plants from an ex situ collection were subjected to a salinity treatment for 6 weeks, and 50 plants were untreated. Plants were harvested and assessed for percent colonization by arbuscular mycorrhizal fungi, nodule number, shoot and root dry biomass, and micronutrient content. RESULTS: Colonization by arbuscular mycorrhizae was higher in plants in the salinity treatment than in untreated plants; plants in the salinity treatment were also found to have a lower root:shoot ratio, and higher phosphorus and nitrogen levels. These results support the importance of arbuscular mycorrhizal fungi in restoration efforts of endangered plants in fragmented and threatened ecosystems, such as pine rocklands.

Cheaters must prosper: reconciling theoretical and empirical perspectives on cheating in mutualism.

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating.