MONOPHYLETIC ORIGIN OF BETA-DIVISION PROTEOBACTERIAL ENDOSYMBIONTS AND THEIR COEVOLUTION WITH INSECT TRYPANOSOMATID PROTOZOA BLASTOCRITHIDIA-CULICIS AND CRITHIDIA SPP

Some trypanosomatid protozoa (order Kinetoplastida) are well known to harbor bacterial endosymbionts. Their phylogenetic positions and evolu tionary relationships with the hosts were deduced by comparing the rRN A gene sequences. Earlier, we observed that these symbionts from three Crithidia spp. are identical and are closely related to Bordetella br onchiseptica. We have now sequenced the genes of another endosymbiont and the host protozoan Blastocrithidia culicis. The 16S rRNA genes of the Blastocrithidia and Crithidia symbionts share approximate to 97% i dentity and form a distinct group, branching off the B. bronchiseptica lineage in the beta-division of Proteobacteria. Comparison of their s econdary structures in the stem regions suggests compensatory mutation s of the symbiont sequences, contributing to their biased base transit ions from G to A and C to T. Two putative genes encoding tRNA(Ile) and tRNA(Ala) are highly conserved in the otherwise variable internal tra nscribed spacer region. Comparisons of the host rRNA gene sequences su ggest that the symbiont-containing Crithidia and Blastocrithidia are m ore akin to each other than to other trypanosomatids. The evidence sug gests that Blastocrithidia and Crithidia symbionts descend from a comm on ancestor, which had presumably entered an ancestral host and thence coevolved with it into different species. We therefore propose naming the symbionts Kinetoplastibacterium blastocrithidii and Kinetoplastib acterium crithidii.