![]() These flanking sequences are also included in our analyses. Between the conserved core sequences and each side of D3 are 14 relatively nonconserved bases that together correspond to a stem, H14, in the folded transcript. D3 exhibits substantial intraspecific and interspecific sequence variation ( Litvaitis et al., 1994) which is phylogenetically informative and useful for the study of population structure ( Kuzoff et al., 1998). Therefore, we chose to examine sequence variation in the approximately 150-base pair, AT-rich (usually) D3 expansion segment which lies between highly conserved core sequences ( Hancock & Dover, 1988). Expansion segments evolve at up to 10 times the rate of the gene core ( Kuzoff et al., 1998). The eukaryote 28S rRNA gene is homologous to the prokaryote 23S rRNA gene except for the presence of about a dozen expansion segments ranging in length from 10 to several hundred base pairs ( Hancock & Dover, 1988 Hancock et al., 1988). This permits the selection of particular sequences as markers for a desired level of phylogeographic resolution. The rate of sequence evolution varies across a repeating unit and even within a gene ( Hillis & Dixon, 1991). Eukaryotic rDNA consists of arrays of a unit containing spacers and associated rRNA genes ( Hillis & Dixon, 1991). We have chosen to use rDNA sequence data which could reveal deep genetic structure. Prior studies employing rapidly evolving genetic markers have revealed shallow genetic structure in Ixodes ticks ( Norris et al., 1996 Kain et al., 1999). A pattern would concomitantly suggest similar rates and mechanisms of gene flow and similar potential to vector disease agents across geographical ranges. By including several species in our analysis, we may be able to determine if there is a general pattern for tick genetic structure. ricinus complex, all are competent vectors of human diseases such as spirochaete-mediated Lyme disease ( Lane et al., 1991). scapularis (from eastern North America) ( Dennis et al., 1998). ricinus (from Europe, western Asia, and northern Africa), and I. persulcatus (from eastern Europe and Asia), I. In the present study, we examine population genetic structuring in four widespread species of hard ticks (Ixodidae): Ixodes pacificus (from western North America), I. In general, however, ticks exhibit shallow population genetic structure ( Bull et al., 1984). Consequently, population genetic structure varies according to the host species utilized ( Lampo et al., 1998). Ticks (Acari) are ectoparasites of extremely limited intrinsic vagility ( Adeyeye & Butler, 1989). The population genetic structure of parasites may depend on host habitat preferences and host mobility ( Mulvey et al., 1991). ![]() Gene flow rates are determined by habitat continuity, vagility of the species, and barriers to dispersal ( Slatkin, 1987). Population genetic structure reflects historical events such as bottlenecks, colonizations, and range expansions and it reflects also the spatial scale at which the cohesive effect of gene flow is countered by microevolutionary forces promoting genetic divergence ( Thompson, 1999). Population genetic structure is deep if both slowly and rapidly evolving loci exhibit spatial variation and shallow if only rapidly evolving loci exhibit isolation by distance. The population genetic structure of a species is the geographical distribution of its component demes and is inferred from both the spatial scale at which genotypes are observed to change and the evolutionary rates of the loci examined ( Slatkin, 1987). This indicates that cellular processes underpinning concerted evolution have homogenized populations and species for particular rDNA sequence variants. scapularis, sequence variation was not observed within localities. persulcatus, from Eurasia, has sequence variation between localities of the order of that observed between other species, suggesting a long history of population isolation and deep genetic structure. southern populations, suggesting recent divergence and hybridization. scapularis, from eastern North America has two forms of the D3 sequence that are distributed differently among northern vs. ricinus, from Europe, have no sequence variation indicating a lack of deep genetic structure. pacificus, from western North America, and I. Multiple copies of D3 were sequenced from localities across the geographical ranges of four species to investigate deep population genetic structure. The base sequence of the rDNA D3 expansion segment and flanking H14 stem varies between six species of Ixodes ticks (Acari: Ixodidae) where only 33 invariant sites occur among sequences of 123–203 bases in length.
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