HETEROGENEITY OF THE INTERNAL TRANSCRIBED SPACER (ITS-2) REGION WITHIN INDIVIDUAL DEER TICKS

To determine whether nuclear rDNA sequences provide a useful means for assessing the structure of populations of Ixodes ticks, we compared v ariability among copies of an internal transcribed spacer (ITS-2) sequ ence within individual ticks to the variability between ticks. At leas t 4% of the nucleotides comprising this sequence vary among the copies present within individual ticks. ITS-2 diversity in each of two ticks is nearly half as great as that reported between ticks from geographi cally disparate populations. Because individual ticks retain ancestral polymorphism, ITS-2 variation does not accurately reflect descent rel ationships among these ticks. Sequencing single copies of PCR-amplifie d ITS-2 therefore does not permit assessment of the phylogenetic relat ionships among the I. ricinus-like ticks in eastern North America. We recommend caution in future analyses, and emphasize the importance of procedures designed to ensure that the many paralogous copies of the r DNA cistron have been sufficiently homogenized by concerted evolutiona ry processes. Such precautionary measures will make certain that phylo genetic trees based on these gene sequences reflect the phyletic relat edness of the biological species.