Cyanobacteria and horizontal gene transfer

In order to evaluate the evolutionary relationships for a group of organisms it is necessary to identify a 'core' set of genes that can be further used to explore their phylogenetic relationships. Using multiple sequence comparison techniques, a cyanobacterial core phenotype was determined from 15 cyanobacterial species. The study identified 192 genes unique to cyanobacteria and absent in non-cyanobacterial species. The extent to which this set of 192 cyanobacterial signature genes was subject to horizontal gene transfer (HGT) was carefully evaluated. Homologs for the 192 "signature genes" were obtained from 15 cyanobacterial genomes. Phylogenetic trees were constructed for all genes found in at least 10 or more cyanobacterial genomes and compared for congruency with the small subunit (16S) rRNA tree. Any incongruency with the 16S rRNA tree was considered a possible HGT event. There was a low (22%) incidence of putative horizontal gene transfer among this set of cyanobacterial signature genes. In order to assess how this level of horizontal transfer of the signature genes compared to other groups of cyanobacterial genes, a whole genome phylogenetic analysis of 1713 genes from four marine cyanobacteria was undertaken. Only 10% of all the genes were probable candidates for HGT. Genes belonging to the photosynthesis/respiration and hypothetical categories had a higher incidence than any other category. The most common horizontal transfer events were between Prochlorococcus marinus sps. MIT and SS120.