Phylogenetic evidence for a clade of tick-associated trypanosomes

Abstract

Background: Trypanosomes are protozoan parasites of vertebrates that are of medical and veterinary concern. A variety of blood-feeding invertebrates have been identified as vectors, but the role of ticks in trypanosome transmission remains unclear.

Methods: In this study, we undertook extensive molecular screening for the presence and genetic diversity of trypanosomes in field ticks.

Results: Examination of 1089 specimens belonging to 28 tick species from Europe and South America led to the identification of two new trypanosome strains. The prevalence may be as high as 4% in tick species such as the castor bean tick Ixodes ricinus, but we found no evidence of transovarial transmission. Further phylogenetic analyses based on 18S rRNA, EF1-α, hsp60 and hsp85 gene sequences revealed that different tick species, originating from different continents, often harbour phylogenetically related trypanosome strains and species. Most tick-associated trypanosomes cluster in a monophyletic clade, the Trypanosoma pestanai clade, distinct from clades of trypanosomes associated with transmission by other blood-feeding invertebrates.

Conclusions: These observations suggest that ticks may be specific arthropod hosts for trypanosomes of the T. pestanai clade. Phylogenetic analyses provide further evidence that ticks may transmit these trypanosomes to a diversity of mammal species (including placental and marsupial species) on most continents.

Keywords: Amblyomma; Ixodes; Trypanosoma pestanai; Trypanosome specificity.

Fig. 1

Trypanosoma phylogeny constructed using maximum likelihood (ML) estimations based on the 18S rRNA gene sequence (1305 unambiguously aligned bp; best-fit approximation for the evolutionary model: general time-reversible [GTR]) including Trypanosoma sequences obtained in this study (in bold). The 10 major Trypanosoma clades are indicated [2]. For members of the T. pestanai clade, origin of the Trypanosoma sequences (i.e., from ticks, mammals either marsupials or placentals) is shown by black squares, while host species and geographical origin are listed next to the name of Trypanosoma species or strain. GenBank accession numbers of sequences used in analyses are shown on the phylogenetic trees. Numbers at nodes indicate percentage support of 1000 bootstrap replicates. Only bootstrap values >70% are shown

Fig. 2

Phylogenies of Trypanosoma constructed using maximum likelihood (ML) estimations based on (a) EF1-α amino acid gene sequences (171 unambiguously aligned amino acid; best-fit approximation for the evolutionary model: Jones–Taylor–Thornton [JTT] + G), (b) hsp60 amino acid gene sequences (142 unambiguously aligned amino acid; best-fit approximation for the evolutionary model: LG + G), (c) hsp85 amino acid gene sequences (203 unambiguously aligned amino acid; best-fit approximation for the evolutionary model: LG + G). Trypanosoma sequences obtained from ticks in this study are shown in bold. GenBank accession numbers of sequences used in analyses are shown on the phylogenetic trees. Numbers at nodes indicate percentage support of 1000 bootstrap replicates. Only bootstrap values >70% are shown