Changes in Photobiont Dimensions and Numbers During Co-Development of Lichen Symbionts

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Abstract

The development of the changing interactions between symbionts that occurs in a growing lichen, from the lobe tips to the more mature regions of the thallus, has not been specified precisely nor has it been investigated in detail. Such work that has been done has been reviewed by various authors, e.g. Nienburg (1926), Jahns (1973), Henssen and Jahns (1974), Greenhalgh and Anglesea (1979), Hill (1981). These developmental processes, which are of key importance to the lichen symbiosis, have two major features. Firstly, the overall structure is primarily composed of fungal tissues originating from hyphae which aggregate in some kind of process of continuous morphogenesis. The vegetative thallus that has evolved, at least in the more advanced species, does not appear to have any obvious counterpart in non-lichenized fungi. Secondly, the developmental sequence only leads to a functionally competent lichen thallus if both the fungus and the photobiont develop in a co-ordinated tandem, termed here balanced co-development. The balance of the co-development involves the co-ordination of the expression of two separate genomes. Anatomically, it involves growth of the fungus by hyphal aggregation and of the photobiont, which is usually unicellular or in some cases filamentous, by aplanospore formation or cell fission