MBE Advance Access published online on May 22, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn120
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Non-Toxic Strains of Cyanobacteria are the Result of Major Gene Deletion Events Induced by a Transposable Element
1 Austrian Academy of Sciences, Institute for Limnology, Mondseestrasse 9, 5310 Mondsee, Austria
2 University of Hawaii at Manoa, Department of Chemistry, 2545 McCarthy Mall, Honolulu, HI 96822, USA
# Corresponding author: Rainer Kurmayer, Austrian Academy of Sciences, Institute for Limnology, Mondseestrasse 9, 5310 Mondsee, Phone 0043 6232 3125 32, Fax 0043 6232 3578, E-mail: rainer.kurmayer{at}oeaw.ac.at
Received for publication March 3, 2008. Revision received April 16, 2008. Accepted for publication May 15, 2008.
Blooms that are formed by cyanobacteria consist of toxic and non-toxic strains. The mechanisms that result in the occurrence of non-toxic strains are enigmatic. All of the non-toxic strains of the filamentous cyanobacterium Planktothrix that were isolated from nine European countries were found to have lost 90% of a large gene cluster (mcy) that encoded the synthesis of the toxic peptide microcystin. Those strains still contain the flanking regions of the mcy gene cluster along with remnants of the transposable elements that are found in between. The majority of the strains still contain a gene coding for a distinct thioesterase type II (mcyT), which is putatively involved in microcystin synthesis. The insertional inactivation of mcyT in a microcystin-producing strain resulted in the reduction of microcystin synthesis by 94±2% (1 SD). Non-toxic strains that occur in shallow lakes throughout Europe form a monophyletic lineage. A second lineage consists of strains that contain the mcy gene cluster but differ in their photosynthetic pigment composition, which is due to the occurrence of strains that contain phycocyanin or large amounts of phycoerythrin in addition to phycocyanin. Strains containing phycoerythrin typically occur in deep-stratified lakes. The rare occurrence of gene cluster deletion, paired with the evolutionary diversification of the lineages of strains that lost or still contain the mcy gene cluster, needs to be invoked in order to explain the absence or dominance of toxic cyanobacteria in various habitats.
Key Words: toxicity • microcystin • mobile elements • gene loss • microevolution • ecotypes
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