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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 ( cox 1), 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.    Electronic supplementary material  The online version of this article (10.1186/s13071-018-3186-9) contains supplementary material, which is available to authorized users.

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