Lichen Systematics: The Role of Morphological and Molecular Data to Reconstruct Phylogenetic Relationships

Abstract

It took almost a century until Schwendener’s (1867) finding that lichens belong to the fungi finally led mycologists and lichenologists to include them in the fungal system (Nannfeldt 1932; Santesson 1952). Trying to elucidate the phylogenetic relationships between lichenized and un-lichenized fungi and among lichen taxa, based solely on morphological and chemical data, has proven to be a frustrating endeavour. Lichens display few taxonomically useful characters, of which many are widely variable; the homology of character states within and between groups is difficult to assess. Often, even the interpretation of morphological characters, e.g. types of ascoma development or ascus type, proved difficult (see e.g. Henssen and Jahns 1974; Lumbsch 2000; Lumbsch et al. 2001c; Ott and Lumbsch 2001; Stenroos et al. 2002b; Lumbsch and Huhndorf 2007). In the absence of well-supported and uncontroversial phylogenetic reconstructions based on morphological data, molecular data have, therefore, gained great importance in lichen systematics. The impact of molecular data on the classification and taxonomy of lichenized ascomycetes has been summarized regularly in recent years (Lumbsch 2000, 2007; Grube and Winka 2002; DePriest 2004). This review is not an attempt to update these previous comprehensive reviews. It rather tries to shed light on the relationship between results based on molecular and morphological studies of lichens. In the late 1980s and early 1990s, morphology-based taxonomy and systematics and molecular phylogenetics of lichens more or less led their own separate lives. The first studies based on molecular data often concentrated on reconstructing phylogenetic relationships and were not so much concerned with character evolution or the reinterpretation of morphological characters in light of molecular results. Likewise, a critical evaluation of the results in light of morphological data was rarely attempted. This has changed profoundly in recent years. Most phylogenetic reconstructions of lichenized ascomycetes are now designed to test morphology-based classifications. As a result, the systematic value of morphological characters in diverse groups is now much better understood than previously and reconstructions of character evolution exist for many systematic groups. On the other hand, classical taxonomists make increasing use of molecular data because classical lichen taxonomy is riddled with problems that only independent data from molecular analyses are likely to solve. One very obvious problem that is relatively easy to solve with molecular data concerns the systematic placement of obligately sterile lichens (Stenroos and DePriest 1998; Arup and Grube 1999; Platt and Spatafora 2000; Ekman and Tonsberg 2002; Crespo et al. 2004a) or other species with doubtful systematic affinities (Printzen and Kantvilas 2004; Lucking et al. 2007; Spribille et al. 2009). Other such problems arise from the multiple description of morphologically variable species, doubtful circumscriptions of taxa and erroneous assignment of species to them, or misinterpretation of the systematic value of characters due to incorrect homology hypotheses. In all these cases, molecular analyses offer promising tools to test traditional hypotheses.