title: Monophyletic origin of beta-division proteobacterial endosymbionts and their coevolution with insect trypanosomatid protozoa Blastocrithidia culicis and Crithidia spp.
abstract: Some trypanosomatid protozoa (order Kinetoplastida) are well known to harbor bacterial endosymbionts. Their phylogenetic positions and evolutionary relationships with the hosts were deduced by comparing the rRNA gene sequences. Earlier, we observed that these symbionts from three Crithidia spp. are identical and are closely related to Bordetella bronchiseptica. 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 approximately 97% identity and form a distinct group, branching off the B. bronchiseptica lineage in the beta-division of Proteobacteria. Comparison of their secondary structures in the stem regions suggests compensatory mutations of the symbiont sequences, contributing to their biased base transitions 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 transcribed spacer region. Comparisons of the host rRNA gene sequences suggest that the symbiont-containing Crithidia and Blastocrithidia are more akin to each other than to other trypanosomatids. The evidence suggests that Blastocrithidia and Crithidia symbionts descend from a common 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 Kinetoplastibacterium crithidii.
authors.affiliation: Department of Microbiology and Immunology, Finch University of Health Sciences/Chicago Medical School, IL 60064.
authors.forename: Y
authors.initials: Y
authors.lastname: Du
authors.forename: D A
authors.initials: DA
authors.lastname: Maslov
authors.forename: K P
authors.initials: KP
authors.lastname: Chang
mesh_terms: D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D003421:Crithidia / Q000235:genetics; D017931:DNA Primers / Q000737:chemistry; D004269:DNA, Bacterial / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics*; D005798:Genes, Bacterial; D017125:Genes, Protozoan; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D012329:RNA, Bacterial / Q000235:genetics; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D013559:Symbiosis; D014351:Trypanosomatina / Q000235:genetics
chemical_list: D017931:DNA Primers; D004269:DNA, Bacterial; D004275:DNA, Ribosomal; D012329:RNA, Bacterial; D012337:RNA, Ribosomal, 18S