Molecular Phylogenetic Analyses Indicate That the Ixodes Ricinus Complex Is a Paraphyletic Group

Abstract

The Ixodes ricinus species complex is a group of ticks distributed in almost all geographic regions of the world. Lyme borreliosis spirochetes are primarily transmitted by tick species within this complex. It has been hypothesized that the Lyme vector ticks around the world are closely related and represent a monophyletic group. This implies that vector competence in ixodid ticks for Lyme agents might have evolved only once. To test this hypothesis, we used a molecular phylogenetic approach. Two fragments of mitochondrial 16S ribosomal deoxyribonucleic acid were sequenced from 11 species in the I. ricinus complex and from 16 other species of Ixodes. Phylogenetic analysis using Bayesian methodology indicated that the I. ricinus complex is not a monophyletic group unless 3 additional Ixodes species are included in it. The known major vectors of Lyme disease agents in different areas of the world are not sister taxa. This suggests that acquisition of the ability to transmit borreliosis agents in species of Ixodes may have multiple origins.

Assessment of intraspecific mtDNA variability of European Ixodes ricinus sensu stricto (Acari: Ixodidae)

“…On the base of the low number of substitutions within its members, Xu et al (2003) hypothesised that the I. ricinus complex is the most recently evolved group of ticks in the genus Ixodes (Xu et al, 2003). In addition, our results showed a high A + T content in the DNA fragments examined.…”

“…Only sample 277 (from Neuchâtel, Switzerland) and 642 (from Smaland, Sweden) shared the same haplotype for the four genes considered. Two factors might explain the I. ricinus s.s. homogeneity: the first one is the recent evolution of this species (Xu et al, 2003) and the second is the host vagility (the ability to move from place to place) throughout the country.…”

“…The Ixodes ricinus complex forms a group composed of 14 species distributed in different geographic regions of the world. Despite this geographical delocalisation, it includes ticks which are closely related (Acari: Ixodidae) (Xu et al, 2003). A number of organisms of this complex is involved in the transmission of various diseases to animals and humans such as Lyme borreliosis, tick-borne encephalitis, ehrlichiosis, and babesiosis.…”

“…Whereas phylogenetic relationships between the members of the I. ricinus complex have been analysed in the past (Caporale et al, 1995;Xu et al, 2003), in contrast little is known about the genetic structure of the species I. ricinus s.s.. This kind of information is potentially important to understand the epidemiology and evolutionary dynamics of the disease and the vector.…”

“…the ovule), they do not undergo meiotic recombination (Simon et al, 1994). A few studies have demonstrated that their sequences are useful to distinguish closely related tick species and are thus appropriate for distinguishing populations within a species (Caporale et al, 1995;Xu et al, 2003). This is particularly the case with the control region, which appears to be subject to very weak selective constraints and evolves quite rapidly (Li, 1997).…”