Relationships of Nematodirus species and Nematodirus battus isolates (Nematoda : Trichostrongyloidea) based on nuclear ribosomal DNA sequences

Nuclear ribosomal sequence data from the internal transcribed spacers (ITS- 1 and ITS-2), 5.8S subunit, and regions of the 18S and 28S genes were used to investigate sequence diversity among geographic samples of Nematodirus b attus, and to infer phylogenetic relationships among Nematodirus species. P hylogenetic analysis of these data yielded strong support for relationships among species, depicting Nematodirus helvetianus and Nematodirus spathiger as sister-taxa and a clade of these ? species and Nematodirus filicollis. This tree is consistent with caprine bovids as ancestral hosts, with a subs equent host shift to Bovinae in N. helvetianus. Eleven of 14 N. battus sequ ences were unique, with 19 variable sites among sequences representing 5 ge ographic samples. The lowest number of variable nucleotide sites was observ ed in samples representing apparently recent introductions to the United St ates and Canada. which is consistent with a population bottleneck concomita nt with translocation. Comparison of directly sequenced polymerase chain re action products and clones revealed evidence for intraindividual variation at some of the sequence sites, and this pattern of variation and that withi n geographic samples indicates incomplete rDNA repeat homogenization within species. This pattern of variation is not conducive for inferring phylogen etic relationships among sequences representing N. battus or addressing the putative history of introduction.