Phylogenetic study of Aliinostoc species (Cyanobacteria) using pc-igs, nifH and mcy as markers for investigation of horizontal gene transfer

Abstract

Selection of genes that have not been horizontally transferred for prokaryote phylogenetic studies is regarded as a challenging task. Internal transcribed spacer of ribosomal genes (16S–23S ITS), microcystin synthetase genes (mcy), nitrogenase (nifH) and phycocyanin intergenic spacer (PC-IGS) are among the most used markers in cyanobacteria. The region of the ribosomal genes has been considered stable, whereas the nifH, mcyG and PC-IGS may have undergone horizontal transfer. To investigate the occurrence of horizontal transfer of nifH, mcyG and PC-IGS, phylogenetic trees of Aliinostoc strains Ay1375 and Me1355 were generated and compared. Phylogenetic trees based on the markers were mostly congruent for PC-IGS, indicating a common evolutionary history among ribosomal and phycocyanin genes with no evidence for horizontal transfer of PC-IGS. Phylogenetic trees constructed from the nifH and 16S rRNA genes were incongruent. Our results suggest that nifH has been transferred from one cyanobacterium to another. Moreover, the low non-synonymous/synonymous mutation ratio (Ka/Ks) was consistent with an ancient origin of the mcyG.