It is a mutual beneficial association between Algae and Cyanobacteria. In this association fungus gets food from Algae or cyanobacteria which made food by photosynthesis. In return algae and cyanobacteria are protected by fungus filaments. The morphology, physiology and chemistry of the fungus and algae or cyanobacteria in lichens is different than individual. Lichens may be of many color, size and shape. These are more frequently present on bark, leaves, mosses, other lichens, and may survive as epiphytes in Rain forest and Temprate Woodland.Lichen is being Representing a large ecological group among fungi and especially in Ascomycota with 13,500 species. Among lichens a larger number of fungi make an association with symbiotic green algae and only 12% of all species have cyanobacteria as accessory or primary photobiont. It is possible for a small fraction of lichens to determine the photobiont at a species or strain level.
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A lichen thallus is a developed structure which made through the long-lived symbiotic growth of a filamentous fungus and uni-cellular green algae.It is still mysterious that how the symbionts recognize one another, they also resist potential attack and leads towards this structure .As the eukaryotic systems are well studied the growth of a lichen thallus must depend on the effective identification and association of these two different organisms. It is one of great challenge while studying the symbiotic relations to identify the microbes recognitions and identifications on the molecular basis.
Because of the gaps of our present understanding, problems are being faced in defining the symbiotic system that including rigid conceptual frame work of the algae and fungi taxonomy. Some lichens reproduce vegetatively and produce a large number of genetically identical symbiotic unit. Some developed by the fusion of symbiotic propagules which represent various gents. In lichens mechanistic delimitations of a single thallus is difficult.
Thallus of Lichens:
The internal structure of crustose, foliose and fruticose lichens are shown below in a diagram. 1st part is the firm fungus hyphae named as cortex which serves as protective skin. Beneath this is the portion of the green part in the form of layer named as photobiont cells. Next to it, is the layer of medulla which are loosed fungal hyphae and grey in color. Some hyphae developed in downward direction from the medulla and by its penetration it anchor the thallus firmily. Beneath the whole structure is the lower cortex. Rhizines are shown in blue color which are bundles of hyphae. These penetrate the substrate and anchor the thallus.
Thallus of Lichen
Development of Lichens:
Many ascomycetous lichens produce soredia or other diaspores which enable the joint propagation of their fungal and algal or cyanobacterial bionts. Indirect new lichen thalli is formed by reassociation of mycobiont ascospore with competible photobiont. In the foliose lichen, Xanthoria parietina, thallus development can be intermediated by an relationship of the mycobiont germ hyphae with commonly present pleurococcoid algae a wide but inadequately differentiated crust is thus shaped, increasing the chances of mycobiont interaction with the much fewer thallus-inducing Trebouxia photobiont.
Association of Ascomycetes with Algal cells
Germination of the ascospores of saxicolous crustose lichen fungi is apparently monitored by the development of a prothallus-like mycelium . observed that this purely hyphal period may remain for long periods without any significant change and Hale (1983) said that, before interaction with compatible algae, mycobionts could get food by carbohydrate-rich leachates dissolved in rain. Garty & Delarea (1987) observed germinating spores and early stages of lichenization in Caloplaca aurantia, found that thallus formation initiates with loose networks of mycobiont hyphae, trebouxioid algae and dust particles.
In a number of species of Rhizocarpon and anatomically related lichens, the development of distinct primary areoles on the vertically proceeding prothallus equals the biont interactions involved in thallus initiation. Malinowski (1911) and Gallùe (1930) observed areole formation in various Rhizocarpon species resulting from deposition of an algal cell or cells on the prothallus. The source of the areole making algae in R. geographicum was examined, but not fixed, by Armstrong & Smith (1987) in tryouts that involve the removal of portions of the thallus. Examined thallus origination and growth in an alpine population of R. geographicum and distinguished the need for comparative interpretations on other Rhizocarpon species and on saxicolous crustose lichens usually.
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Hence, Ascospore germination, thallus formation, and areole and prothallus growth in the lichen Rhizocarpon lecanorinum . The ascospore germ hyphae persist very small and do not produce a prothallus-like mycelium. However, a compact soredium-like granule produced direct from spore contact with a compatible species of Trebouxia. Diffusion of early phases involve non-trebouxioid algae are absent. It is said that thallus formation is made by the deposition of rhizocarpic acid in an initial cortical layer within the apical part of the granule. As coloration and cortex development made up this structure into a typical areole, describing that prothallus hyphae are instantaneously induced from its basal margin. More areoles developed consequently in the marginal prothallus don’t have subtending melanized hyphae a apparently stem from overgrowth by the prothallus of photobiont cells on, or in, the substratum. Apothecia reached at maturity in thalli as small as 2 mm in diameter. It is stated that the lack of diffused hyphal development in sporelings and telescoped morphogenesis of R. lecanorinum are part of a life history strategy geared to precocious, heavy investment in ascospore production. The R. lecanorinum Trebouxia symbiosis has a lot of features which make it well-suited for further studies of the life history and development of prothallus-forming crustose lichens with sexually reproducing mycobionts.
Molecular and phylogenetic Investigation:
Molecular and Phylogenetic procedures have provided strong methods to investigate the diversity and ecology of lichens. One concept said that lichens associations have been emerged out independently on several occasions and it has been also seen that loss of various symbiotic relationships have been also lost in different fungal lineages). In most cases only a small number of photobionts are serving as partners for fungi. An important fact is that lichens forming fungi are highly selective for its photobiont. It may also seen that fungi that are forming lichens from different lineages have same photobiont strains .
Cladoniaceae and Stereocaulaceae
The Cladoniaceae and Stereocaulaceae , they contain a diamorphic thallus with a foliose and crustose thallus and fructose secondry thallus also contain the hymenia that developed from the primary a fruticose secondry thalli bearing the hymenia that grows from the primary thallus. In ancient classification all the lichens were classified into one family (Cladoniaceae) that have diamorphic thallus.The recent concept of two families was given by two scients named as Jahns and co-workers.The main feature of the Cladoniaceae is the existence of podetia . However several genera does not have this character.
The Stereocaulaceae usually consists of genera with pseudopodetia that comprises ascomatal stipes that is made up of vegetative tissue (ascomatal stipes composed of vegetative, this term initially invented cephalodia. Cephalodia also present in the Sphaerophoraceae, and in many other families in the Lecanorales subord. Elemination of stereocaulaceae is due to the existence of boundry tissue between thallus and generative tissue. Deletion of the two families is artificial .
Cladiaceae and Heterodeaceae:
These two families are also in dispute .
The genus Cladia Nyl. is initially a Southern Hemisphere genus of around ten species. The genus has qualities of development of large, hollow pseudopodetia with perforations. Some said that cleadia is the subgenus or the part of Cladonia P. Browne. Monogeneric family was established . The family differs from the Cladoniaceae by following character:
•It lacks primary thallus.
Cladeceae is resemble to the Ramalinaceae C. due to the presence of the cartilage layer which is developed from the cortex, Galloway (1966) and Filson (1981) argued that the Cladiaceae probably is closer to the Ramalinaceae C. Agardh or to Bryoria Brodo & D. L. Hawksworth (Parmeliaceae) than to the Cladoniaceae, because of the cartilaginous layer being derived from the cortex. Jahns (1970b), who treated Cladia as a pseudopodetia-forming member of the Cladoniaceae, found no similarity in ascoma ontogeny between Cladia and the Parmeliaceae. In recent classifications, the family Cladiaceae is usually included in the Cladoniaceae,with few exceptions .