Lichen
The symbiosis in lichens is between a fungus and a blue-green alga or cyanobacterium. The fungal component is called the mycobiont and is an ascomycete or a basidiomycete fungus. Lichens are defined by their symbiosis with algae, which is a method of nutrition, not due to a common ancestry. All kinds of unrelated fungi form lichens. Therefore, lichens cannot be grouped easily onto one branch of a phylogenetic tree. [16] |
The fungal component of C. coralloides is an ascomycete fungus, meaning that its spores develop and are released from sacs called asci. To the right is a drawing of a cross section of the fruiting body of Caloplaca coralloides showing the eight ascospores in asci within the fertile layer of the fruiting body. There are 15,000 species of ascomycete fungi that form lichens. The fungal components of some other lichens are basidiomycetes, which are mushroom forming fungi. [17] |
The lichen symbiosis is different than other kinds of symbioses because the lichen takes on a new body shape that neither the fungus nor the alga had independently. Without the alga, the fungus would not develop the organized tissue layers present in the lichen. The alga is able somehow to “turn on” the fungal genes that control morphogenesis. The algal and fungal components of the lichen need to grow at similar rates for the lichen to survive. If one grows faster than the other, then the lichen breaks down. This requirement explains why lichens grow slowly. [18] [19] |