position of repetitive DNA sequence in the southern cattle tick genome permits chromosome identification

Fluorescent in-situ hybridization (FISH) using meiotic chromosome preparations and highly repetitive DNA from the southern cattle tick, Rhipicephalus microplus, was undertaken to investigate genome organization. Several classes of highly repetitive DNA elements were identified by screening a R. microplus bacterial artificial chromosome (BAC) library. A repeat unit of approximately 149 bp, RMR-1 was localized to the subtelomeric regions of R. microplus autosomes 1-6 and 8-10. A second repeat unit, RMR-2 was localized to the subtelomeric regions of all autosomes and the X chromosome. RMR-2 was composed of three distinct repeat populations, RMR-2a, RMR-2b and RMR-2c of 178, 177 and 216 bp in length, respectively. Localization of an rDNA probe identified a single nucleolar organizing region on one autosome. Using a combination of labeled probes, we developed a preliminary karyotype for R. microplus. We present evidence that R. microplus has holocentric chromosomes and explore the implications of these findings for tick chromosome biology and genomic research.