The Symbiotic Phenotype of Lichen-Forming Ascomycetes
Lichens are the symbiotic phenotype of lichen-forming fungi, a polyphyletic, taxonomically heterogenous assembly of nutritional specialists which acquire fixed carbon from a population of minute, extracellularly located green algal or cyanobacterial cells
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HONEGGER
CONTENTS I. A.
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peculiarities of the Lichen Symbiosis ........ B. Concentric Bodies .... . . . . . . . . . . . . . . . . . . . C. Lichen Photobionts ....................... II. Thallus Structure in Microlichens ........... A. Leprose, Microfilamentous and Microglobose Lichens. . . . . . . . . . . . . . . . . . B. Crustose Lichens ......................... C. Gelatinous Lichens ....................... III. Foliose and Fruticose Macrolichens with Internally Stratified (Hetereomerous) Thallus .................................. A. Peculiarities and Main Building Blocks ...... 1. Conglutinate Pseudoparenchyma ......... 2. Gas-Filled Plectenchyma of the Medullary and Algal Layers ...................... B. Ontogeny and Growth .................... 1. Acquisition of a Compatible Photobiont ... 2. Prethallus Formation ................... 3. Differentiation of a Stratified Thallus ..... C. Growth and Cell Turnover ................. 1. Growth Rates ......................... 2. Growth Patterns ....................... D. Cephalodiate Species and Photomorphs ..... IV. Mycobiont-Photobiont Interfaces ........... V. Lichenicolous Fungi and Fungal Endophytes of Lichen Thalli .......................... VI. Conclusions and Perspectives .............. References ..............................
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I. Introduction A. Peculiarities of the Lichen Symbiosis Lichens are the symbiotic phenotype of lichenforming fungi, a polyphyletic, taxonomically heterogenous assembly of nutritional specialists which acquire fixed carbon from a popUlation of minute, extracellularly located green algal or cyanobacterial cells; these are referred to as the photobiont. Geosiphon pyriforme, the only known fungal symbiosis with an intracellularly located cyanobacteria, is not normally considered a lichen Institute of Plant Biology. University of ZUrich, Zollikerstr. 107,8008 ZUrich, Switzerland
(see SchuBler and Kluge, Chap. 9, this Vol.). In marked contrast to fungal parasites of algae, lichen-forming fungi do not damage their photoautotrophic partner. The thalli of morphologically advanced taxa are, in fact, very elaborate culturing chambers for photobiont cells, many of which are poor competitors on their own (see below). As many lichen-forming fungi either form symbiotic propagules or disperse successfully by means of thallus fragmentation, the photobiont cells are carried along and invade areas where they would not normally occur in the free-living state. Thus, the lichen symbiosis is regarded as a mutualistic association. Lichenization is a common and successful nutritional strategy, with more than 25% of fungi being lichenized (Hawksworth et al. 1995; Honegger 1996b). Approximately 98% of lichenforming fungi are ascomycetes, or about 46% of ascomycetes are lichenized, respectively (Honegger 1997). Lichen-forming fungi are typical representatives of their fungal classes but differ from
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