The Endomicrobia

For a long time, the nature and identity of the endosymbionts of the larger gut flagellates was completely obscure. Recently, the cytoplasmic symbionts of the hypermastigid Trichonympha agilis and the oxymonad Pyrsonympha vertens in the gut of Reticulitermes santonensis were identified as members of the so-called 'Termite Group 1' (TG-1) (Stingl et al. 2005). Based on their 16S rRNA gene sequences, whose origin was verified by fluorescence in situ hybridization with clone-specific probes, and their peculiar ultrastructure (Fig. 3), the endosymbionts of the two protozoa were provisionally classified as two species in the candidate genus 'Endomicrobium' (Stingl et al. 2005). The symbionts are specific for their respective flagellate host and do not occur on the surface of the flagellates or freely suspended in the gut fluid.

Up to that point, 16S rRNA gene sequences of TG-1 bacteria had been retrieved only from termites of the genus Reticulitermes (Ohkuma and Kudo 1996; Hongoh et al. 2003; Yang et al. 2005). A broad survey of termites from different families, using a PCR assay with TG-1-specific primers, documented that TG-1 bacteria are present in and also restricted to the guts of all lower termites and the wood-feeding cockroach Cryptocercus punctulatus - insects that are in an exclusive, obligately mutualistic association with parabasalid and oxymonad flagellates (Stingl et al. 2005).

Only a small fraction of termites has been investigated to date, but already the present dataset indicates an enormous phylogenetic diversity among TG-1 bacteria (Fig. 6). Based on their isolated phylogenetic position and their endo-symbiotic lifestyle, the candidate phylum 'Endomicrobia' has been proposed (Stingl et al. 2005). Although there are indications for a horizontal transfer of TG-1 bacteria between hypermastigid and oxymonad flagellates, the bacterial symbionts seem to have evolved together with their flagellate hosts, which would also explain the apparently simultaneous loss of flagellates and 'Endomicrobia' in the higher termites (Stingl et al. 2005).

TG-1 clone from Cryptotermes secundus (K) TG-1 clone from Hodotermes mossambicus (H) TG-1 clone from Schedorhinotermes lamanianus (R) TG-1 clone from Neotermes cubanus (K) TG-1 clone from Kalotermes flavicollis (K) TG-1 clone from Neotermes cubanus (K) TG-1 clone from Hodotermes mossambicus (H) TG-1 clone from Reticulitermes speratus (R) TG-1 clone from Mastotermes darwiniensis (M) TG-1 clone from Neotermes cubanus (K)

'Endomicrobium pyrsonymphae' from Reticulitermes santonensis (R)

TG-1 clone from Cryptocercus punctulatus (C) TG-1 clone from Cryptocercus punctulatus (C) TG-1 clone from Cryptocercus punctulatus (C) TG-1 clone from Kalotermes flavicollis (K) TG-1 clone from Zootermopsis nevadensis (T) TG-1 clone from Kalotermes flavicollis (K) TG-1 clone from Reticulitermes speratus (R)

'Endomicrobium trichonymphae' from Reticulitermes santonensis (R)

TG-1 clone from Reticulitermes speratus (R) TG-1 clone from Kalotermes flavicollis (K) TG-1 clone from Cryptocercus punctulatus (C) TG-1 clone from Schedorhinotermes lamanianus (R) TG-1 clone from Hodotermes mossambicus (H) TG-1 clone from Cryptotermes secundus (K) Clone from a TCE-contaminated site Clone from activated sludge Clone from a contaminated subsurface site

Clone from contaminated aquifer undergoing intrinsic bioremediation Clone from rumen

Acidobacterium UA1 Acidobacterium UA2 Acidobacterium capsulatum Verrucomicrobium group OPB35 Verrucomicrobium spinosum jā€” Nitrospira cf. moscoviensis SBR1015

ā€” Nitrospira moscoviensis 0 10

Fig. 6. Phylogenetic tree of the 'Endomicrobia', illustrating the relationship between the 16S rRNA gene sequences of the cytoplasmic symbionts of Trichonympha agilis and Pyrsonympha vertens to clones of 'Termite Group 1' (TG-1) bacteria obtained from various other termites and the wood-feeding cockroach Cryptocercus punctula-tus. The candidate phylum 'Endomicrobia' is clearly separated from the most-closely related environmental clones and other bacterial phyla. The TG-1 clones from different termite families do not form separate clusters. (K Kalotermitidae, H Hodotermitidae, R Rhinotermitidae, M Mastotermitidae, T Termopsidae, C Blattodea:Cryptocercidae). Bar represents 0.1 substitutions per site. (Tree modified after Stingl et al. 2005)

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