Abstract An exceptional group-I intron-like insertion at position 940 of the nuclear small subunit rDNA is found in lichen mycobionts of the families Parmeliaceae and Lec- anoraceae. This shared insertion site is exceptional as it follows a G. Although several features of the self-splicing Tetrahymena intron are missing, the conserved structure of the presumed core region indicates that the new intron-like insertion, which is missing in mature transcripts, is not part of a silenced ribosomal repeat. It is unlikely that the new insertion is horizontally transferred from the adjacent po- sition 943. A phylogenetic analysis indicates congruence with lichen phylogeny and suggests that the insertion has been vertically inherited. Key words Lichen-forming fungi · SSU rDNA · Group-I intron · Insertion · 5′-splice site Introduction The polymerase chain reaction is an invaluable tool to ob- tain sequence data from fungi which are not maintained in axenic cultures or which grow extremely slowly. This is of particular importance for studies on a wide range of sym- biotic fungi, for example lichen mycobionts. Since lichen- forming fungi represent at least two-fifths of all ascomy- cetes (Hawksworth et al. 1995), lichenization is one of the most successful of fungal associations. The available phy- logenetic investigations of fungi including lichen mycob- ionts support an earlier hypothesis that lichenization is not representative for a single monophyletic group; rather, li- chen associations are found in various groups throughout the fungal kingdom (Gargas et al. 1995a; Wedin et al. 1998). In conjunction with these and other phylogenetic studies during the last decade, it was found that a high number of insertions in ribosomal DNA is characteristic for many lichen-forming fungi. Focusing on the nuclear small-subunit ribosomal DNA (SSU rDNA), Gargas et al. 1995 b) identified 17 insertion positions in eukaryotes and 11 of these are shared by li- chen-forming ascomycetes. Recently, three new insertion sites were reported from lichen mycobionts (Stenroos and DePriest 1998). Most insertions are located in conserved regions of the ribosomal RNA and it has been suggested that these regions are exposed in the tertiary structure of the mature ribosomes (Turmel et al. 1993; Gargas et al. 1995 b). Due to their putative secondary RNA structure, many insertions in the SSU rRNA gene of lichen mycobi- onts have been classified as group-I introns (Gargas et al. 1995 b). Almost all known naturally occurring group-I introns possess a U*G basepair at the 5′-splice site with U as the last base of the 5′ exon (Michel and Westhof 1990; Cech 1993). The only exceptions are two group-I introns in the cytochrome oxidase gene of the fungi Aspergillus nidulans and Podospora anserina which have a C * G basepair at the 5′-splice-site, with C as the last base of the 5′-exon (Hur and Waring 1995). In this case, it has been suggested that the C is post-translationally selected because it encodes a highly conserved amino acid. Until now, there has only been one report on insertions from the rDNA of lichen my- cobionts beginning after nucleotides other than U (Sten- roos and DePriest 1998). However, these insertions are very short and lack the typical structure of the catalytic core of group-I introns. Here we describe insertions with Curr Genet (1999) 35: 536–541 © Springer-Verlag 1999 Received: 17 July 1998 / 16 February 1999 M. Grube · B. Gutmann · U. Arup · A. de los Rios J.-E. Mattsson · M. Wedin An exceptional group-I intron-like insertion in the SSU rDNA of lichen mycobionts M. Grube () · B. Gutmann · U. Arup · A. de los Rios Institut für Botanik, Karl-Franzens-Universität Graz, Holteigasse 6, A-8010 Graz, Austria e-mail: martin.grube@kfunigraz.ac.at Fax: +43-3-1 63 80 98 83 J.-E. Mattsson Botanical Museum, Uppsala University, Villavägen 6, S-752 36 Uppsala, Sweden M. Wedin Botany Department, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Communicated by R. J. Schweyen ORIGINAL PAPER