Prothallus Structure in Cladonia

Abstract

All of the taxa in the lichen genus Cladonia that were examined for this study possess a prothallus. This non-lichenized, exclusively fungal component of the thallus was found in both organic and inorganic substrates. It was usually found in contact with the primary squamule or with another part of the photosynthetic thallus such as the podetium. Most prothallus mycelial aggregations were diffuse. Some formed rhizine-like bundles, aggregations, or non-cellular masses. Most of the species had more than one prothallus form. Secreted materials, which bind the hyphae of the prothallus to one another and to particles of substrate were observed. The prothallus may provide a mode of vegetative reproduction in Cladonia lichens, and it may play a role in soil stabilization. In this study, the genus Cladonia was surveyed in order to describe the prothallus and its characteristics. The prothallus is an exclusively fungal component of the lichen thallus, in which contact with the algal photobiont is infrequent. The prothallus is contiguous with the photosynthetic lichen thallus, but it is found beneath the surface of the substrate. It has a characteristic fungal morphology, in which tubular, elongated hyphal cells ramify, anastomose, and aggregate into various configurations. Most lichens possess a prothallus, and the prothallus is conspicuous in many humicolous and saxicolous species Letrouit-Galinou & Asta 1994; Sanders 1994). All of the major morphological groups crustose, foliose, fruticose, and squamulose lichens, possess taxa with a prothallus. In most of the species of Cladonia and in the genus Cladina the prothallus is inconspicuous but in species of section Cocciferae it is macroscopically visible and conspicuous. Prothalli were present in all of the species that were studied here. Vainio (1887, 1894, 1897) described the prothallus in the species that he included in his monograph. He proposed that the prothallus (or hypothallus) might function as an anchor to the above ground thallus, or that it might translocate materials to the lichenized thallus. He also considered the potential role of the prothallus in dispersal and vegetative reproduction. Krabbe (1891) described the process by which the fungal component of Cladonia came into contact with algal cells, producing a lichen thallus. Zuckel (1896) described the wandering in Peltigera venosa (L.) Hoffm. and Solorina saccata (L.) Ach., in which the fungal mycelium forms rhizines that run horizontally through the ground, producing new thalli. Gallee (1913, 1920, 1954) applied the concept of hypothallus wanderer to questions of dispersal and reproduction in Cladonia. He suggested that the prothallus might provide the lichen with the ability to exploit new habitats outside of its immediate microenvironment. He contrasted two morphologies in the prothallus and attributed taxonomic importance to them. He distinguished between diffuse prothalli, and prothalli with thick hyphal bundles devoid of intercellular spaces. Both of these forms and their intermediates were found in the present study. Hannemann (1973) illustrated the prothallus rhizomorphs of a few Cladonia species and described them as rhizoptes or Rhizinenstriigen, (rhizine-strands), a term later used by Ott et al. (1993). Thogersen (1977) illustrated the prothallus of Cladonia macrophylla (Schaer.) Stenh., but no explanation was provided with the illustration. Jahns (1988) suggested that the prothallus might serve to attach the thallus to its substrate and in addition, might function in water and mineral uptake. Ahmadjian (1993) briefly discussed mycelial growth in Cladonia and suggested that the prothallus might be a free-living symbiont with a potential role in the establishment new lichen colonies. Ott et al. (1993) concluded that the hyphae of the prothallus are reservoirs or conduits for subsurface moisture. Hammer (1996) discussed the role of the prothallus as a soil stabilizer in lichen-dominated habitats. The genus Cladonia was surveyed here in order to describe the prothallus and its characteristics. MATERIALS AND METHODS Approximately 500 specimens in the genera Cladina, Cladonia, and Trapeliopsis were examined for the present study. The specimens were obtained from fresh collections that were not curated or from herbarium specimens at the Farlow Herbarium (FH). Connections with the photosynthetic thallus were confirmed by dissection. The speci0007-2745/96/212-217$O.OO/O This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 06:07:04 UTC All use subject to http://about.jstor.org/terms 1996] HAMMER: FUNGAL AGGREGATIONS 213 C,,rCl;t, 2---~-~ ,, I / : 9L: Y1 ?L `4 L~E~V `\?