The Amblyomma maculatum Koch, 1844 (Acari: Ixodidae) group of ticks: phenotypic plasticity or incipient speciation?

BACKGROUND: The goal of this study was to reassess the taxonomic status of A. maculatum, A. triste and A. tigrinum by phylogenetic analysis of five molecular markers [four mitochondrial: 12S rDNA, 16S rDNA, the control region (DL) and cytochrome c oxidase 1 (cox1), and one nuclear: ribosomal intergenic transcribed spacer 2 (ITS2)]. In addition, the phenotypic diversity of adult ticks identified as A. maculatum and A. triste from geographically distinct populations was thoroughly re-examined. RESULTS: Microscopic examination identified four putative morphotypes distinguishable by disjunct geographical ranges, but very scant fixed characters. Analysis of the separated mitochondrial datasets mostly resulted in conflicting tree topologies. Nuclear gene sequences were almost identical throughout the geographical ranges of the two species, suggesting a very recent, almost explosive radiation of the terminal operational taxonomic units. Analysis of concatenated molecular datasets was more informative and indicated that, although genetically very close to the A. maculatum - A. triste lineage, A. tigrinum was a monophyletic separate entity. Within the A. maculatum - A. triste cluster, three main clades were supported. The two morphotypes, corresponding to the western North American and eastern North American populations, consistently grouped in a single monophyletic clade with many shared mitochondrial sequences among ticks of the two areas. Ticks from the two remaining morphotypes, south-eastern South America and Peruvian, corresponded to two distinct clades. CONCLUSIONS: Given the paucity of morphological characters, the minimal genetic distance separating morphotypes, and more importantly the fact that two morphotypes are genetically indistinguishable, our data suggest that A. maculatum and A. triste should be synonymized and that morphological differences merely reflect very recent local adaptation to distinct environments in taxa that might be undergoing the first steps of speciation but have yet to complete lineage sorting. Nonetheless, future investigations using more sensitive nuclear markers and/or crossbreeding experiments might reveal the occurrence of very rapid speciation events in this group of taxa. Tentative node dating revealed that the A. tigrinum and A. maculatum - A. triste clades split about 2 Mya, while the A. maculatum - A.triste cluster radiated no earlier than 700,000 years ago.

Molecular characterization of Anaplasma marginale in ticks naturally feeding on buffaloes.

Anaplasma marginale is the most prevalent pathogen transmitted by ticks in cattle in tropical and subtropical regions of the world. However, the tick species involved in the transmission of A. marginale in buffaloes in Brazil have not been identified. The objective of the present study was to determine the presence of A. marginale in ticks parasitizing water buffaloes. A total of 200 samples of Rhipicephalus microplus, Dermacentor nitens, Amblyomma cajennense, and Amblyomma maculatum were collected and tested by conventional and quantitative PCR for the presence of the msp1a and msp5 genes. In the present study, 35 ticks (17.5%) were positive for A. marginale DNA by qPCR analysis. The positive ticks belonged to four different species: R. microplus (22.2%), A. cajennense (13.8%), A. maculatum (16.0%), and D. nitens (10.0%). Individuals of the three developmental stages (larvae, nymphs, and adults) of R. microplus and A. cajennense were found to be positive for A. marginale, only nymphs and adults of A. maculatum were found to be positive, and finally, only adults of D. nitens were positive for A. marginale. Our results suggest that R. microplus, A. cajennense, A. maculatum, and D. nitens ticks may be involved in the transmission of A. marginale in buffaloes. However, while A. marginale PCR positive ticks were recorded, this does not indicate vector competence; only that the ticks may contain a blood meal from an infected host. Additionally, the results show that the strains of A. marginale from buffaloes and cattle are phylogenetically related.