Interspecific and geographical variation in the sequence of rDNA expansion segment D3 of Ixodes ticks (Acari: Ixodidae).

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. Multiple copies of D3 were sequenced from localities across the geographical ranges of four species to investigate deep population genetic structure. Two species, I. pacificus, from western North America, and I. ricinus, from Europe, have no sequence variation indicating a lack of deep genetic structure. One species, I. scapularis, from eastern North America has two forms of the D3 sequence that are distributed differently among northern vs. southern populations, suggesting recent divergence and hybridization. I. 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. With the exception of I. scapularis, sequence variation was not observed within localities. This indicates that cellular processes underpinning concerted evolution have homogenized populations and species for particular rDNA sequence variants.