Individuals?

In lichenology the term “thallus” or “body” is less contentious than “individual,” as a lichen thallus is not a single genetic entity. The status of “individual” has sometimes been applied loosely to physically distinct thalli, such as the stalked umbrella-shaped structure of Umbilicaria, or to discrete thalli of any lichen species. A more sophisticated approach, and one which is more meaningful biologically, is to consider a lichen “individual” as any thallus material which is genetically uniform with respect to the dominant, or fungal, symbiont. This, of course, requires genetic information which may not always be available.

Even defined according to genetic properties of the mycobiont, lichen “individuals” do not correspond to individuals of most other species. The reason is the high degree of internal thallus complexity. Not only are two or more interdependent primary symbiotic partners closely associated with one another, but there may be more than one strain of each and possibly even an array of other symbionts. In some ways lichens are as much like little communities or ecosystems (Section 1.6) as individuals.

The primary partners

Lichens are often presented as an association of two symbionts, but because they have evolved several times (Gargas et al. 1995; Lutzoni et al. 2001) few generalizations are applicable to all. Not only the nature, but also the number of participating symbionts may differ.