Phylogeny and evolution of rhipicephaline and hyalommine ticks (Acari: Ixodidae)

Abstract

I studied the phylogeny and evolution of 25 species of ticks from the subfamily Rhipicephalinae with four molecular markers and morphology. I also studied 5 members of the subfamily Hyalomminae, which molecular analyses have shown arose within the Rhipicephalinae, and two members of Haemaphysalinae which were used as an outgroup. All taxa were studied with morphological characters and 12S rRNA gene sequences. Sequences for the other three genes, cytochrome oxidase c I, internal transcribed spacer 2 and 18S rRNA, were not available for all taxa. My initial analyses were with mitochondrial 12S rRNA gene sequences. I found that the subfamily Rhipicephalinae was paraphyletic with respect to the Hyalomminae (Hyalomma), but that the Hyalomma spp. studied were monophyletic. The species of Rhipicephalus and Boophilus studied were closely related but the monophyly of these genera could not be established.Because many parts of the 12S phylogeny were poorly resolved I combined mitochondrial cytochrome c oxidase I (COI) and 12S sequences. The phylogenetic resolution was markedly improved compared with separate analyses of each gene. My most controversial result was that the genus Rhipicephalus was apparently paraphyletic unless species of Boophilus were included in it. The species of Rhipicephalus most closely related to Boophilus spp. were ticks of the subgenus R. (Digineus) and members of the R. pravus species group.Next I sequenced the internal transcribed spacer 2 (ITS2) to obtain phylogenetic information from a nuclear gene. The phylogenies from ITS2 were mostly consistent with results from the mitochondrial genes. In addition, the ITS2 provided support for the monophyly of Boophilus spp., the monophyly of the ornate spp. of Rhipicephalus, and for Anocentor nitens arising within the genus Dermacentor. The genus Rhipicephalus was again found to be paraphyletic without the inclusion of Boophilus spp. A repeat sequence was found in almost all ticks studied. By mapping the presence of the second copy of the repeat onto the phylogeny from the ITS2 gene it appears that there have been many independent gains and losses of that copy. I inferred a putative secondary structure for the region comprising the repeat. It seems that each copy folds into a distinct and almost identical stem-loop complex. Gains and losses of one of the copies do not seem to impair the function of the ITS2 in these ticks.Phylogenetic inferences from the ITS2 and the mitochondrial genes, 12S rRNA and COI, were informative but no single set of characters produced a fully resolved tree. The three genes (12S, COI and ITS2) were then combined with 18S rRNA gene sequences and morphological characters and analysed separately and together in a total evidence analysis. The total evidence dataset had 33 taxa and 3303 characters. Analyses of this dataset led to robust hypotheses about the phylogeny of the Rhipicephalinae and Hyalomminae. I conclude that: (i) the genus Rhipicephalus is paraphyletic and should be revised to include Boophilus; (ii) the subgenus R. (Rhipicephalus) is paraphyletic without the inclusion of the subgenus Digineus and the genus Boophilus; (iii) Hyalomma should only comprise two sub-genera because the subgenus Hyalomma is paraphyletic without the inclusion of the sub genus Hyalommasta; (iv) A. nitens arose within the genus Dermacentor and thus should not retain generic status, and; (v) the subfamily Hyalomminae (Hyalomma) arose within the Rhipicephalinae, so the Rhipicephalinae should be revised to include Hyalomma. The phylogeny from my total evidence analysis was used to map life history and morphological features. It appears that the evolution of a truncated life cycle (one- or two- hosts) has occurred at least three times in the species studied here. Omateness (patterning on the scutum) appears to have evolved on three separate occasions (in N. monstrosum, in the R. pulchellus group and in the Dermacentor spp.) whereas there has been one reversal (in A. nitens). Mapping the geographic distribution of rhipicephaline and hyalommine taxa onto the phylogeny indicated that the Rhipicephalinae s.l. probably evolved at least 50 million years ago. The centre of diversity of most of the subfamily appears to have been Afrotropical, with secondary radiations of species to Palearctic, Nearctic, Oriental and Neotropical regions.Analyses of molecular and morphological characters together provided the greatest resolution of phylogeny in the Rhipicephalinae and Hyalomminae so far.