Photobionts in Lichens: Possible Indicators of Phylogenetic Relationships?

A compilation of records of photobionts that are associated in lichens of the order Lecanorales shows that mycobionts, when seen at the levels of suborders, families, and genera, are strongly selective towards their photobionts. The recently proposed split of the genus Trebouxia into two genera, Asterochloris and Trebouxia, is reflected by the choice of mycobionts. While Asterochloris is mostly found in the suborder Cladoniineae, Trebouxia is mainly confined to the Lecanoriineae. The symbiosis with coccoid green algae other than Trebouxia or Asterochloris might indicate the basal position of a lichen family within Lecanorales. For instance, Dictyochloropsis and Coccomyxa are confined to the Lecanoralean families Biatoraceae (Lecanorineae), Megalosporaceae (cf Teloschistineae), Peltigeraceae and Nephromataceae (Peltigerineae), and Catillariaceae (fam. inc. sed.). The most important cyanobacterial photobionts in Lecanorales are Nostoc and Scytonema as primary and accessory photobionts, while Gloeocapsa and Stigonema are more frequent as accessory photobionts in a few suborders and families of Lecanorales. The co-occurrence of cyanobacteria with Trentepohlia as a primary photobiont in lichen symbioses is not known from families of lichenized ascomycetes. This may reflect differences of ecological preferences of both photobiont groups and of the mycobiont groups that belong to different natural relationships. These examples show that the knowledge of photobionts is valuable information for systematics of lichens and therefore, exploration of photobiont diversity in lichens needs to be expanded. Within the last years, lichen photobionts have attracted increasing interest that has led to the unraveling of their diversity and evolutionary relationships using microscopy, ultrastructure, and molecular techniques (Btidel 1992; Ettl & Giirtner 1995; Friedl 1995; Friedl & Biudel 1996; Melkonian & Peveling 1988; Tschermak-Woess 1988). In the green algal genus Trebouxia, the most common photobiont, the recently demonstrated congruence of morphological and ultrastructural characters with rDNA sequence data resulted in a revised systematic concept of the genus (Friedl & Rokitta 1997). However, the question whether photobionts are important markers for the evolution and systematics of lichens, has been controversially discussed (Ahmadjian 1993). The capability of lichen forming fungi to specifically select their appropriate partners among the algal taxa common in subaerial habitats may be an important phylogenetic trait. From a closer examination of the distribution of algal taxa symbiotic in lichens it may become obvious that only a single algal species or a very narrow spectrum of algal taxa may cause transformation of a fungus into a lichen thallus. Chodat (1913), an early pioneer in the study of lichen algae, felt that his investigations indicated specificity of lichen-forming fungi with respect to their algal symbionts. Later, observations of Jaag (1933) showed that lichen species belonging to the same genus (e.g., Cladonia, Parmelia, Solorina and Umbilicaria) always contain the same certain type of 'gonidia. ' He thought that this phenomenon may also hold true at the level of lichen families. In the following, however, the investigators of lichen photobionts stated that specificity of lichen-forming fungi with respect to their algal symbionts is rather poor. Wang-Yang and Ahmadjian (1972) found "different species or strains of algae in morpholog1 This paper was presented as part of a symposium entitled "Bridging the Gap Between Phylogeny and the Classification of Lichen-forming Ascomycetes" organized by Frangois Lutzoni in Montreal, Canada at the August, 1997, ABLS annual meeting. 0007-2745/98/392-397$0.75/0 This content downloaded from 157.55. 39.249 on Wed, 03 Aug 2016 05:52:48 UTC All use subject to http://about.jstor. org/terms 1998] RAMBOLD ET AL.: PHOTOBIONTS IN LICHENS 393 ically identical lichen thalli and conversely different subspecies of a particular lichen may contain the same type of algal symbiont. " The authors felt that the contribution of knowledge of photobionts to lichen systematics might be poor. Recently, Ahmadjian (1993) stated that "it is unlikely that taxonomists can find much value in the types of photobionts that occur in lichens, at least at the species