EVOLUTION OF CYANOBACTERIA BY EXCHANGE OF GENETIC MATERIAL AMONG PHYLETICALLY RELATED STRAINS

The cyanobacterial radiation consists of several lineages of phyletica lly (morphologically and genetically) related organisms. Several of th ese organisms show a striking resemblance to fossil counterparts. To i nvestigate the molecular mechanisms responsible for stabilizing or hom ogenizing cyanobacterial characters, we compared the evolutionary rate s and phylogenetic origins of the small-subunit rRNA-encoding DNA (16S rDNA), the conserved gene rbcL (encoding D-ribulose 1,5-bisphosphate carboxylase-oxygenase large subunit), and the less conserved gene rbcX , This survey includes four categories of phyletically related organis ms: 16 strains of Microcystis, 6 strains of Tychonema, 10 strains of P lanktothrix, and 12 strains of Nostoc, Both rbcL and rbcX can be regar ded as neutrally evolving genes, with 95 to 100% and 50 to 80% synonym ous nucleotide substitutions, respectively. There is generally low seq uence divergence within the Microcystis, Tychonema, and Planktothrix c ategories both for rbcLX and 16S rDNA, The Nostoc category, on the oth er hand, consists of three genetically clustered lineages for these lo ci, The 16S rDNA and rbcLX phylogenies are not congruent for strains w ithin the clustered groups. Furthermore, analysis of the phyletic stru cture for rbcLX indicates recombinational events between the informati ve sites within this locus. Thus, our results are best explained by a model involving both intergenic and intragenic recombinations. This ev olutionary model explains the DNA sequence clustering for the modern s pecies as a result of sequence homogenization (concerted evolution) ca used by exchange of genetic material for neutrally evolving genes. The morphological clustering, on the other hand, is explained by structur al and functional stability of these characters. We also suggest that exchange of genetic material for neutrally evolving genes may explain the apparent stability of cyanobacterial morphological characters, per haps over billions of years.