Fungal farmers or algal escorts: lichen adaptation from the algal perspective

Abstract

Domestication of algae by lichen-forming fungi describes the symbiotic relationship between the photosynthetic (green alga or cyanobacterium; photobiont) and fungal (mycobiont) partnership in lichen associations (Goward 1992). The algal domestication implies that the mycobiont cultivates the alga as a monoculture within its thallus, analogous to a farmer cultivating a food crop. However, the initial photobiont 'selection' by the mycobiont may be predetermined by the habitat rather than by the farmer. When the mycobiont selects a photobiont from the available photobionts within a habitat, the mycobiont may influence photobiont growth and reproduction (Ahmadjian & Jacobs 1981) only after the interaction has been initiated. The theory of ecological guilds (Rikkinen et al. 2002) proposes that habitat limits the variety of photobionts available to the fungal partner. While some studies provide evidence to support the theory of ecological guilds in cyanobacterial lichens (Rikkinen et al. 2002), other studies propose models to explain variation in symbiont combinations in green algal lichens (Ohmura et al. 2006; Piercey-Normore 2006; Yahr et al. 2006) hypothesizing the existence of such guilds. In this issue of Molecular Ecology, Peksa & Škaloud (2011) test the theory of ecological guilds and suggest a relationship between algal habitat requirements and lichen adaptation in green algal lichens of the genus Lepraria. The environmental parameters examined in this study, exposure to rainfall, altitude and substratum type, are integral to lichen biology. Lichens have a poikilohydric nature, relying on the availability of atmospheric moisture for metabolic processes. Having no known active mechanism to preserve metabolic thallus moisture in times of drought, one would expect a strong influence of the environment on symbiont adaptation to specific habitats. Adaptation to changes in substrata and its properties would be expected with the intimate contact between crustose lichens in the genus Lepraria. Altitude has been suggested to influence species distributions in a wide range of taxonomic groups. This is one of the first studies to illustrate an ecological guild, mainly for exposure to rainfall (ombrophiles and ombrophobes), with green algal lichens.

Lichenized Fungi and the Evolution of Symbiotic Organization

“…Such associations can be seen both among an ecological assemblage of epiphytic macrolichens, the Nephroma-guild, and among a group of predominantly terricolous cyanolichens, the Peltigera-guild, where the photobionts with each guild are closely related Nostoc-strains. Several authors have likewise observed similar photobiont-mediated guilds in green-algal lichens (99,5).…”

“…Approximately 20 000 currently known fungal species live as lichens, mostly in species-rich lineages of Ascomycota (4). The traditional view of lichens as a mutualistic symbiosis of a fungus and one or several green algae or cyanobacteria, has always been under debate (5), but it has recently been more challenged than ever by the discovery of numerous additional microorganisms that potentially occur as obligatory participants in the symbiosis.…”

Pleistocene expansion of the bipolar lichen Cetraria aculeata into the Southern hemisphere

“…). As poikilohydric organisms, they avoid periods of unsuitable conditions, while their nature as symbiotic associations between fungi (mycobionts) and green algal or cyanobacterial photobionts probably enhances their tolerance to different environmental conditions (Piercey‐Normore & DeDuke ). Selective association with different photobionts (Piercey‐Normore & DePriest ) or modulating the composition of the photobiont communities within the thallus (Sun & Friedmann ; Casano et al .…”

The genetic structure of the cosmopolitan three-partner lichenRamalina farinaceaevidences the concerted diversification of symbionts

“…Patterns of lichen symbiotic associations within an ecological and a historical framework are highly diverse. They range from a mycobiont species that cultivates the alga species as a monoculture within its thallus (Piercey‐Normore & Deduke, ) to a fungal species that associates with more than one algal lineage [e.g. Lecanora (Blaha et al ., ) and Tephromela (Muggia et al ., , )].…”