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Appl Environ Microbiol, January 1998, p. 310-315, Vol. 64, No. 1
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Sequence Variation of the tRNA^Leu Intron as a Marker for Genetic Diversity and Specificity of Symbiotic Cyanobacteria in Some Lichens

Per Paulsrud^* and Peter Lindblad

Department of Physiological Botany, Uppsala University, S-752 36 Uppsala, Sweden

Received 12 May 1997/Accepted 17 October 1997

We examined the genetic diversity of Nostoc symbionts in some lichens by using the tRNA^Leu (UAA) intron as a genetic marker. The nucleotide sequence was analyzed in the context of the secondary structure of the transcribed intron. Cyanobacterial tRNA^Leu (UAA) introns were specifically amplified from freshly collected lichen samples without previous DNA extraction. The lichen species used in the present study were Nephroma arcticum, Peltigera aphthosa, P. membranacea, and P. canina. Introns with different sizes around 300 bp were consistently obtained. Multiple clones from single PCRs were screened by using their single-stranded conformational polymorphism pattern, and the nucleotide sequence was determined. No evidence for sample heterogenity was found. This implies that the symbiont in situ is not a diverse community of cyanobionts but, rather, one Nostoc strain. Furthermore, each lichen thallus contained only one intron type, indicating that each thallus is colonized only once or that there is a high degree of specificity. The same cyanobacterial intron sequence was also found in samples of one lichen species from different localities. In a phylogenetic analysis, the cyanobacterial lichen sequences grouped together with the sequences from two free-living Nostoc strains. The size differences in the intron were due to insertions and deletions in highly variable regions. The sequence data were used in discussions concerning specificity and biology of the lichen symbiosis. It is concluded that the tRNA^Leu (UAA) intron can be of great value when examining cyanobacterial diversity.

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^* Corresponding author. Mailing address: Department of Physiological Botany, Uppsala University, Villavägen 6, S-752 36 Uppsala, Sweden. Phone: 46-184712814. Fax: 46-184712826. E-mail: Per.Paulsrud{at}fysbot.uu.se.

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