Phylogeny and molecular signatures for the phylum Thermotogae and its subgroups

Radhey S. Gupta, Vaibhav Bhandari

Thermotogae species are currently identified mainly on the basis of their unique toga and distinct branching in the rRNA and other phylogenetic trees. No biochemical or molecular markers are known that clearly distinguish the species from this phylum from all other bacteria. The taxonomic/evolutionary relationships within this phylum, which consists of a single family, are also unclear. We report detailed phylogenetic analyses on Thermotogae species based on concatenated sequences for many ribosomal as well as other conserved proteins that identify a number of distinct clades within this phylum. Additionally, comprehensive analyses of protein sequences from Thermotogae genomes have identified >60 Conserved Signature Indels (CSI) that are specific for the Thermotogae phylum or its different subgroups. Eighteen CSIs in important proteins such as PolI, RecA, TrpRS and ribosomal proteins L4, L7/L12, S8, S9, etc. are uniquely present in various Thermotogae species and provide molecular markers for the phylum. Many CSIs were specific for a number of Thermotogae subgroups. Twelve of these CSIs were specific for a clade consisting of various Thermotoga species except Tt. lettingae, which was separated from other Thermotoga species by a long branch in phylogenetic trees; Fourteen CSIs were specific for a clade consisting of the Fervidobacterium and Thermosipho genera and eight additional CSIs were specific for the genus Thermosipho. In addition, the existence of a clade consisting of the deep branching species Petrotoga mobilis, Kosmotoga olearia and Thermotogales bacterium mesG1 was supported by seven CSIs. The deep branching of this clade was also supported by a number of CSIs that were present in various Thermotogae species, but absent in this clade and all other bacteria. Most of these clades were strongly supported by phylogenetic analyses based on two datasets of protein sequences and they identify potential higher taxonomic grouping (viz. families) within this phylum. We also report 16 CSIs that are shared by either some or all Thermotogae species and some species from other taxa such as Archaea, Aquificae, Firmicutes, Proteobacteria, Deinococcus, Fusobacteria, Dictyoglomus, Chloroflexi and eukaryotes. The shared presence of some of these CSIs could be due to lateral gene transfers between these groups. However, no clear preference for any particular group was observed in this regard. The molecular probes based on different genes/proteins, which contain these Thermotogae-specific CSIs, provide novel and highly specific means for identification of both known as well as previously unknown Thermotogae species in different environments. Additionally, these CSIs also provide valuable tools for genetic and biochemical studies that could lead to discovery of novel properties that are unique to these bacteria.