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MBE Advance Access originally published online on May 22, 2008
Molecular Biology and Evolution 2008 25(8):1695-1704; doi:10.1093/molbev/msn120
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© 2008 The Authors.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nontoxic Strains of Cyanobacteria Are the Result of Major Gene Deletion Events Induced by a Transposable Element

Guntram Christiansen*, Carole Molitor*, Benjamin Philmus{dagger} and Rainer Kurmayer*

* Austrian Academy of Sciences, Institute for Limnology, Mondseestrasse 9, Mondsee, Austria
{dagger} Department of Chemistry, University of Hawaii at Manoa

E-mail: rainer.kurmayer{at}oeaw.ac.at.

Accepted for publication May 15, 2008.

Blooms that are formed by cyanobacteria consist of toxic and nontoxic strains. The mechanisms that result in the occurrence of nontoxic strains are enigmatic. All the nontoxic strains of the filamentous cyanobacterium Planktothrix that were isolated from 9 European countries were found to have lost 90% of a large microcystin synthetase (mcy) gene cluster that encoded the synthesis of the toxic peptide microcystin (MC). 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 MC synthesis. The insertional inactivation of mcyT in an MC-producing strain resulted in the reduction of MC synthesis by 94 ± 2% (1 standard deviation). Nontoxic 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

Geoffrey McFadden, Associate Editor


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